Incidence, Risk Factors and Clinical Outcomes of Inoculum Effect against β-lactams Including Ampicillin-Sulbactam and Cefazolin among  Methicillin-Susceptible Staphylococcus aureus(MSSA) Strains Isolated from Patients  with Bacteremia

doi:10.21203/rs.3.rs-4902408/v1

Download PDF

Research Article

Incidence, Risk Factors and Clinical Outcomes of Inoculum Effect against β-lactams Including Ampicillin-Sulbactam and Cefazolin among Methicillin-Susceptible Staphylococcus aureus(MSSA) Strains Isolated from Patients with Bacteremia

https://doi.org/10.21203/rs.3.rs-4902408/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this latest preprint version

Background Some type of staphylococcal β-lactamases could destroy also β-lactams other than penicillin, including cefazolin (Cz) and ampicillin-sulbactam (SAM) especially in the presence of higher inoculum, which called inoculum effect (InE). We aimed to investigate the incidence, risk factors, simple definiton methods and clinical implication of InE against different β-lactams including SAM and Cz in MSSA strains isolated from patients with bacteremia.

Methods All patients with MSSA bacteremia between 2016 and 2018 at our hospital were included. MSSA strains isolated from blood cultures of those patients were tested via the disk diffusion and broth microdilution methods at standard (10⁵/µl) and high (10⁷/µl) inoculum concentrations. The presence and type of β-lactamases were determined by nitrocefin and PCR plus DNA sequencing. InE was defined as a 4-fold or greater increase in MIC values at high inoculum concentrations. The geometric mean MIC and zonal diameter were compared between the strains with and without InE or β-lactamases. Patient data were obtained retrospectively from the hospital database, and risk factors for Cz IE or SAM IE or mortality were analyzed via univariate analysis.

Results A total of 52 patients with MSSA bacteremia were included in the study. Eighty-five percent of the 52 MSSA strains were β-lactamase positive, all of which were classified as type A. Thirteen (25%), 20 (38.5%), and 2 (3.8%) of the 52 MSSA strains were InE against Cz, SAM, and ceftriaxone, respectively, and no InE was observed against cefuroxime or cefotaxime. The β-lactams most affected by the high inoculum were SAM and Cz, with 2.94- and 2.20-fold increases in the MIC, respectively. Compared with MIC testing, a cefazolin zone diameter of < 28 mm was found to be 100% sensitive to both standard and high inoculum to define the CzInE. Among the β-lactams tested, only penicillin G, SAM and Cz were significantly affected by β-lactamase positivity. The mortality rate in patients infected with MSSA strains showing SAMInE and treated with SAM was significantly greater than that in those not treated with SAM (37.5% vs 68%, p = 0.044, OR 7.8, 95% CI 1.23–49.68).

Conclusions Among Cz, cefuroxime, cefotaxime and ceftriaxone, SAM is the β-lactam most affected by the type A β-lactam of MSSA strains, followed by Cz, and this effect becomes more prominent with increasing inoculum. A cefazolin zone diameter of < 28 mm could be used as a screening method to define Cz InE. SAM treatment of patients infected with MSSA strains harboring SAMInE may increase mortality.

Staphylococcus aureus

MSSA

bacteremia

inoculum effect

cefazolin

ampicillin-sulbactam

Staphylococcus aureus is one of the most important causative agents of community- and hospital-acquired bacteraemia and endocarditis worldwide. In recent years, the incidence of community-acquired and hospital-acquired S. aureus bacteremia (SAB) has increased due to the aging of the population, increased use of intravascular devices, surgical procedures, and increased number of immunocompromised patients. SAB is associated with high morbidity, mortality, and healthcare costs (1).

Currently, approximately 90% of methicillin-sensitive S. aureus (MSSA) strains are resistant to penicillin G (PG), which is related to the production of β-lactamases called penicillinases. There are four different types of staphylococcal β-lactamases, called types A, B, C and D, which are encoded by different blaz genes. Anti-staphylococcal penicillins (ASPs), such as nafcillin and oxacillin, have been recommended as first-line antibiotics for the treatment of MSSA bacteremia for several years (2). However, after the publication of several recent studies showing that cefazolin (Cz) could be more effective but less nephrotoxic than ASPs, Cz became one of the first-choice agents for the treatment of MSSA bacteremia and other deep-seated MSSA infections, including bacteremia, endocarditis, bone and joint infection, deep-seated abscesses, osteomyelitis or pneumonia (3). Additionally, Cz and ampicillin-sulbactam (SAM) have been used as the first-line agents for the treatment of MSSA infections for years in countries where ASPs are not available, such as Türkiye, Argentina, and Japan.

In vitro effect of some β-lactam agents used for the treatment of MSSA infections could be decreased by the size of the bacterial inoculum, which called “inoculum effect” (InE). Presence of InE and its clinical impact were most studied for Cz, while some studies have suggested that treatment failure may occur with Cz therapy of deep-seated MSSA infections caused by the strains showing Cz IE, some have reported no such effect. Although activity of ASPs repeatedly reported to be not affected by high inocula, other agents used for the treatment of MSSA including SAM has not been evaluated adequatly in this concept (2). We aimed to investigate the incidence, risk factors, simple definiton methods and clinical implication of InE against different β-lactams including SAM and Cz in MSSA strains isolated from patients with bacteremia.

Study population

All patients who were admitted to Istanbul University, Istanbul Faculty of Medicine Hospital, between 2016 and 2018 and were diagnosed with MSSA bacteremia, with MSSA growth in blood culture and available stored strains were included in the study. The clinical and laboratory information of each patient was recorded on a preprepared form.

Antimicrobial susceptibility testing

MSSA isolates were identified via classical methods. Disk diffusion susceptibility tests for penicilline G, cefoxitin, cefazoline, linezolid, clindamycin, erythromycin, trimethoprim-sulfamethoxazole, ciprofloxacin, fusidic acid, rifampicin, gentamicin, vancomycin and teicoplanin were performed via methods defined by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) (4) at standard (105 CFU/µl) and high (107 CFU/µl) inocula, and breakpoints of EUCAST were used for the interpretation of the results if available. The minimal inhibitor concentrations (MICs) of Cz, SAM, cefuroxime, cefotaxime, and ceftriaxone for the MSSA strains were determined at standard and high inoculum concentrations via the broth microdilution (BMD) method defined by the Clinical and Laboratory Standards Institute (CLSI) (5). After the MIC values of the control MSSA strain for SAM were not found within valid ranges via the microdilution method, the MIC values were redetermined via the SAM Etest®. A 4-fold or greater increase in MIC values at high inoculum concentrations was defined as an InE, and if a susceptible strain at a normal inoculum level became resistant at a relatively high inoculum level, it was defined as a “pronounced InE” (2).

The S. aureus ATCC 29213 strain, which is known to produce a type A β-lactamase, and the blaZ gene-negative S. aureus ATCC 25923 strain were used as control strains.

Detection of β-lactamase

The presence of β-lactamases in the isolates was qualitatively detected via the nitrocefin disc assay, the PG disc diffusion method, and the PG edge test and then confirmed via PCR plus sequencing (6).

The β-lactamase gene was identified via the primers F, 5'-CAA AGA TGA TAT AGT TGC TTA TTC-3' and R, 5'-CAT ATG TTA TTG CTT GCA CCA C-3', which were designed to amplify a 355-bp region within the blaZ gene structure. The amount of PCR products was confirmed by gel electrophoresis for all of the MSSA strains and then analyzed by DNA sequencing (6).

DNA sequencing

DNA sequencing was performed via the Sanger method with an Applied Biosystem Abi3500 device. The BLAST network service of the National Library of Medicine, National Center for Biotechnology Information (NLM, NCBI) database, was used to define the sequences. The classification of the β-lactamases of each strain relies on the amino acid content of the 128 and 216 residues encoded by blaz, which are threonine and serine, lysine and asparagine, threonine and asparagine, and alanine and serine, respectively, for bla types A, B, C and D (6).

Statistical analysis

Statistical analysis of the study data was conducted via IBM SPSS Statistics for Windows, Version 21.0 (Statistical Package for the Social Sciences, IBM Corp., Armonk, NY, USA). For the analysis of continuous variables, Student's t test was used when the distribution was normal, and the Mann‒Whitney U test was used when the distribution was abnormal; for the analysis of categorical variables, the χ2 test or Fisher's exact test was utilized. Risk is presented as the odds ratio (OR). A p value < 0.05 was considered to indicate statistical significance.

A total of 52 patients with MSSA bacteremia and 52 MSSA strains isolated from those patients were evaluated in the present study. Forty-four of the 52 strains (84.6%) were β-lactamase positive according to the nitrocefin disk, PG disk diffusion and PG edge test methods, and the results were confirmed via PCR (Fig. 1). The sequencing of the PCR products revealed that 35 of the 44 β-lactamase-positive isolates were suitable and that all 35 strains carried type A β-lactamases.

In disk diffusion and MIC susceptibility testing, 44 (85%), 8 (15%), 8 (15%), 6 (11.5%), 6 (11.5%), 5 (9.6%), and 2 (3.8%) of the 52 MSSA strains were found to be resistant to PG, erythromycin, fusidic acid, clindamycin, rifampicin, ciprofloxacin, and trimethoprim-sulfamethoxazole, respectively, and all of the strains were susceptible to cefoxitin, Cz, gentamycin, and linezolid. In MIC testing, all of the strains were found to be susceptible to SAM, cefazolin, cefuroxime, ceftriaxone and cefotaxime. Only the cefoxitin geometric mean (GM) zone diameter was lower in the high inoculum than in the standard inoculum, but no InE was detected for cefoxitin, and the zone diameter of the other antimicrobials, including cefazolin, did not significantly differ between the high inoculum and the standard inoculum. In the MIC test, thirteen (25%) and 20 (38.5%) of the 52 MSSA strains presented an InE against Cz (3 of the 13 strains presented pronounced InE), and SAM (two of the 20 strains presented pronounced InE), respectively. Two (3.8%) strains presented an InE against ceftriaxone, and none of the strains presented any InE against cefuroxime or cefotaxime. All of the GM MICs of the tested β-lactams increased significantly with increasing inoculum concentration (p < 0.001). SAM had the greatest effect, with a 2.94-fold increase in the MIC, followed by Cz, cefuroxime, ceftriaxone, and cefotaxime, with 2.20-, 1.27-, 1.21- and 1.20-fold increases in the MIC, respectively, at high inoculum concentrations.

All of the β-lactamase-negative strains were susceptible to PG, but all of the β-lactamase-positive strains were resistant. The GM zone diameters of the 52 MSSA strains for PG and Cz were significantly lower for the β-lactamase-positive strains than for the β-lactamase-negative strains, both at standard and high inoculum rates (< 0.001). Among the β-lactams tested, only the MICs of SAM and Cz were significantly affected by the presence or absence of β-lactamases; the GM MIC values of SAM against strains with β-lactamase were 23 times greater than those against strains without β-lactamase, and this difference became even more pronounced at higher inocula, reaching 61 and GM MIC values of Cz against strains with β-lactamase that were 1.66 and 3.86 times greater than those without β-lactamase at standard and high inocula, respectively. The MICs of cefuroxime, ceftriaxone and cefotaxime were not affected by the presence of the β-lactamase (Table 1 and Fig. 2).

While SAM InE was strongly related to the increase in only the MIC of SAM (p < 0.001), CzInE was strongly related to the decrease in the PG and Cz zone diameters (p < 0.001 for both) and the increase in the Cz MIC (p < 0.001). The mean Cz zone diameters of the strains with CzInE were significantly lower than the zone diameters of the strains without CzInE, both at standard (27.94 ± 3.09 versus 23.85 ± 1.99, p < 0.001) and high (28.26 ± 2.63 versus 24.39 ± 2.33, p < 0.001) inoculum (Table 2); the zone diameters of the other tested antimicrobials, including cefuroxime, ceftriaxone, cephotaxime, vancomycin, teicoplanin, linezolid, clindamycin, erythromycin, trimethoprim-sulfamethoxazole, ciprofloxacin, fusidic acid, rifampicin and gentamicin, did not decrease with beta-lactamase positivity or SAMInE or CzInE.

The effects of inoculum, β-lactamase positivity, SAMInE and CzInE on the zone diameters or MICs of the tested antimicrobials against the MSSA strains are given in Table 3.

Compared with MIC testing, a Cz zone diameter of < 28 mm was found to be 100% susceptible with 100% negative predictive value in both standard and high inoculums to define the CzInE, but the specificity and positive predictive values of that zone diameter were 44%-36% and 37%-35.8% at standard and high inoculums, respectively.

When comparing the patients infected with strains showing or not showing SAMInE or CzInE, sex; mean age; presence of comorbidities, including DM, HT, CRF or immunosuppression; CCI; health-care or community-acquired infection; Pitt bacteremia score; and admission serum/blood test (WBC, Hb, platelet, AST, ALT, creatinin, albumin, CRP, PCT) results were not different between the groups (p > 0.005). However, the rate of β-lactamase positivity (24/24 vs 20/28, p = 0.005) was significantly greater, and the rates of neutropenia (4/24 vs, 1/28, p = 0.169) and mortality (6/24 vs 2/28, p = 0.123) were greater in patients infected with strains showing SAM InE or CzInE than in patients infected with strains not showing any InE.

The clinical and laboratory characteristics of 52 patients with MSSA bacteremia are presented in Table 4. While 24 cases of bacteraemia were community-acquired (46.2%), 20 were hospital-acquired (38.5%), and 7 were healthcare-associated (13.5%). Catheter-related infections, endocarditis, bone-joint infections, soft tissue infections and intra-abdominal infections were the sources of bacteremia in 20 (38.5%), nine (17.3%), eight (15.4%) and five (9.6%) of the patients, respectively.

Table 1

Comparison of the zone diameters and MIC values of β-lactamase-positive and β-lactamase-negative MSSA strains at standard and high inoculum concentrations.
Antimicrobials	Total Geometric Mean (Minimum-Maximum) (MIC50-MIC90)		β-lactamase negative Geometric Mean (Minimum-Maximum)	βlactamase positive Geometric Mean (Minimum-Maximum)	p
Penicillin G, zone, mm Standard inoculum High inoculum*	15.44 (9–35) 16.25 (0–35)	p = 0.430	33.35 (32–35) 33.45 (31–35)	13.42 (9–23) 13.11 (0–25)	< 0.001 < 0.001
Cefoxitin, zone ,mm Standard inoculum High inoculum	27.85 (23–33) 26.70 (19–30)	p = 0.011	29.074 (27.0–33.0) 27.720 (26.0–30.0)	27.637 (23.0–31.0) 26.528 (19.0–30.0)	0.147 0.133
Cefazolin, zone, mm Standard inoculum High inoculum	26.72 (20–34) 27.12 (18–35)	p = 0.252	32.10 (30–34) 31.52 (26–35)	25.84 (20–32) 26.39 (18–32)	< 0.001 < 0.001
Vancomycin, zone, mm Standard inoculum High inoculum	15.34 (13–17) 16.02 (14–19)	p = 0.001	14.855 (14.0–16.0) 15.484 (15.0–17.0)	15.438 (13.0–17.0) 16.119 (14.0–19.0)
Teicoplanin, zone, mm Standard inoculum High inoculum	14.53 (12–18) 15.07 (13–17)	p = 0.005	14.350 (13.0–16.0) 14.74 (14–16)	14.57 (12–18) 15.13 (13–17)
Linezolid Standard inoculum High inoculum	26.69 (21–32) 26.75 (20–31)	p = 0.836	25.85 (23–31) 26.65 (23–30)	26.84 (21–32) 26.77 (20–31)
Fusidic acid, zone, mm Standard inoculum High inoculum	25.57 (11–30) 25.35 (10–31)	P = 0.882	25.38 (21–30) 23.50 (12–28)	25.61 (11–30) 25.71 (10–31)
Rifampin, zone, mm Standard inoculum High inoculum	27.95 (23–31) 27.80 (21–35)	P = 0.809	27.20 (25–30) 27.10 (21–30)	28.09 (23–31) 27.93 (21–35)
Cotrimoxazole, zone, mm Standard inoculum High inoculum	25.09 (12–30) 26.14 (10–33)	p = 0.049	24.97 (21–29) 27.19 (25–31)	25.12 (12–30) 25.96 (10–33)
Gentamycin, zone, mm Standard inoculum High inoculum	22.91 (19–26) 22.71 (19–26)	P = 0.446	22.53 (19–26) 22.31 (19–24)	22.97 (20–26) 22.79 (19–26)
Clindamycin, zone, mm Standard inoculum High inoculum	25.03 (21–30) 25.41 (21–30)	P = 0.300	24.43 (22–27) 24.56 (22–27)	25.15 (21–30) 25.57 (21–30)
Erythromycin, zone, mm* Standard inoculum High inoculum	22.27 (0–30) 22.27 (0–31)	P = 1.000	18.5 (0–27) 19.25 (0–27)	22.96 (0–30) 22.82 (0–31)
Ciprofloxacin* Standard inoculum High inoculum	24.39 (0–30) 23.92 (0–30)	P = 0.245	26.02 (22–30) 26.07 (23–29)	24.07 (0–30) 23.52 (0–30)
Ampicillin-sulbactam, MIC, µg/ml Standard inoculum High inoculum	0.36 (0.016-4)(0.5–1.35) 1.06 (0.023-12) (2–6)	p < 0.001	0.025 (0.016–0.032) 0.033 (0.023–0.125)	0.58 (0.064-4) 2.01 (0.25-12)	0.017 < 0.001
Cefazolin, MIC, µg/ml Standard inoculum High inoculum	0.787 (0.5-2) (1-1.7) 1.73 (0.5–16) (1–8)	p < 0.001	0.545 (0.5-1) 0.77 (0.5-1)	0.84 (0.5-2) 2 (1–16)	< 0.001 < 0.001
Cefuroxime, MIC, µg/ml Standard inoculum High inoculum	0.79 (0.5-2) (1–1) 1 (0.5-2) (1–2)	p < 0.001	0.77 (0.5-2) 1.19 (1–2)	0.79 (0.5-2) 0.97 (0.5-2)	0.795 0.172
Cefotaxime, MIC, µg/ml Standard inoculum High inoculum	2.35 (1–4) (2–4) 2.83 (2–4) (3–4)	p < 0.001	2.18 (2–4) 3.36 (2–4)	2.38 (1–4) 2.74 (2–4)	0.328 0.132
Ceftriaxone, MIC, µg/ml Standard inoculum High inoculum	3.02 (1–8) (4–4) 3.64 (2–16) (4-6.8)	p = 0.001	3.36 (2–8) 4.36 (2–16)	2.97 (1–8) 3.53 (2–8)	0.467 0.329

Table 2

Effects of SAMInE¹ and CzInE² on the antimicrobial susceptibilities of the 52 MSSA strains at standard and high inoculum concentrations.
Tested Antimicrobials		SAMInE¹			CzInE²
Tested Antimicrobials		Negative	Positive	p	Negative	Positive	p
PG zone diameter (mean ± SD)	Standard inoculum	18.47 ± 9.17	13.95 ± 2.63	0.320	18.54 ± 7.99	11.31 ± 2.16	< 0.001
PG zone diameter (mean ± SD)	High inoculum	17.19 ± 11.35	14.75 ± 4.62	0.806	18.62 ± 8.93	9.15 ± 6.9	< 0.001
Cefazolin zone diameter (mean ± SD)	Standard inoculum	27.63 ± 3.71	25.8 ± 2.35.	0.550	27.95 ± 3.09	23.85 ± 1.99	< 0.001
Cefazolin zone diameter (mean ± SD)	High inoculum	27.56 ± 3.43	26.85 ± 2.32	0.493	28.26 ± 2.63	24.39 ± 2.33	< 0.001
Cefazolin zone diameter < 28 mm, n (%)	Standard inoculum				22 (56)	13 (100)	0.005
Cefazolin zone diameter < 28 mm, n (%)	High inoculum				25 (64)	13 (100)	0.011
SAM, MIC, (mean ± SD)	Standard inoculum	0.67 ± 0.77	0.64 ± 0.45	0.569	0.59 ± 0.49	0.86 ± 1.01	0.565
SAM, MIC, (mean ± SD)	High inoculum	1.31 ± 1.31	4.24 ± 3.38	< 0.001	2.30 ± 2.86	2.85 ± 2.58	0.187
CZ MIC, (mean ± SD)	Standard inoculum	0.91 ± 0.48	0.83 ± 0.37	0.723	0.79 ± 0.31	1.12 ± 0.65	0.178
CZ MIC, (mean ± SD)	High inoculum	2.42 ± 3.21	3.35 ± 4.01	0.550	1.24 ± 0.49	7.39 ± 4.72	< 0.001

4 SAMInE: Inoculum effect against ampicillin–sulbactam

² CzInE: Inoculum effect against cefazolin

³ MIC: Minimal inhibitor concentration

Table 3

Effects of inoculum, β-lactamase production, SAmInE and CzInE on the susceptibility of MSSA to antimicrobials.
Antimicrobials Affected by	Penicillin G (zone diameter)	Cefazolin (zone diameter)	Cefazolin (MIC)	Cefoxitin (zone diameter)	Others¹ (zone diameter)	SAM (MIC)	Cefuroxime (MIC)	Cephotaxime (MIC)	Ceftriaxone (MIC)
Inoculum²	Ø	Ø	↑↑	↓↓	Ø	↑↑↑	↑	↑	↑
Inoculum effect³			↑↑			↑↑↑	Ø	Ø	↑
β-lactamase	↓↓↓	↓↓↓	↑↑	Ø	Ø	↑↑↑	Ø	Ø	Ø
SAM InE⁴	Ø	Ø	Ø	Ø	Ø	↑↑↑	Ø	Ø	Ø
Cz InE⁵	↓↓↓	↓↓↓	↑↑↑	Ø	Ø	Ø	Ø	Ø	Ø
¹Linezolid, clindamycin, erythromycin, trimethoprim- sulfamethoxazole, ciprofloxacin, fusidic acid, rifampicin, gentamicin, vancomycin and teicoplanin

² Significant zone diameter decrease or MIC increase at high inoculum concentrations.

³ A Fourfold or greater increase in MIC values at high inoculum concentrations.

⁴ SAMInE: Inoculum effect against ampicillin-sulbactam

⁵ CzInE: Inoculum effect against cefazolin

Cz (23/52, 44.2%) was the most common definitive treatment for MSSA infection, followed by SAM (15/52, 28.5%). Vancomycin and combinations of vancomycin were de-escalated in 17 of 22 patients after the culture resulted in MSSA. The mean total duration of treatment was 22.5 days in those receiving Cz and 20.8 days in those receiving noncefazolin treatment.

Mortality was observed in 8/52 patients (15.4%). Compared with surviving patients, deceased patients had higher CCIs (5.75 vs 3.29, p = 0.050) and Pitt scores (3.25 vs 1.21, p = 0.078), lower serum albumin levels (2.69 mg/dL vs 3.26 mg/dL, p = 0.99), more frequent infections with the strains showing InE against the antibiotics used for treatment (50% vs 13.6%, p = 0.041, OR 6, 95% CI 1.17–30.73) and against SAM used for treatment (37.5% vs 68%, p = 0.044, OR 7.8, 95% CI 1.23–49.68), and causative MSSA strains of deceased patients presented higher Cz MIC values at higher inocula (5.68 µg/mL vs 2.25 µg/mL, p = 0.054), CzIE (50% vs 20%, p = 0.096) and SAM InE (62.5% vs 34%, p = 0.235).

Table 4

Clinical and Laboratory Findings of the Total Cohort and Surviving and Deceasing Patients
Feature	Total cohort (n = 52)	Deceased (n = 8)	Survived (n = 44)	p value
Gender, female, n (%)	19	4 (50%)	15 (34%)	0.443
Age, average ± SD	58.4 ± 18.3	64.87 ± 21.12	57.22 ± 17.8	0.282
Source of bacteremia Communityacquired/ Health-care acquired	22 22	3 5	19 17	0.705
Fever (°C), average ± SD	38.1 ± 0.9	37.63 ± 1.08	38.18 ± 0.88	0.117
Charlson comorbidity index, mean ± SD	3.94 ± 2.61	5.75 ± 2.81	3.59 ± 2.46	0.050
Pitt bacteremia score, mean ± SD	1.54 ± 2.7	3.25 ± 4.36	1.21 ± 2.25	0.078
Blood leukocyte count (103/µL), mean ± SD	13 054.7 ± 8940	9 825 ± 5416	13 655 ± 9376	0.270
Blood hemoglobin value (gr/dl), mean ± SD	10.62 ± 2.24	10.59 ± 1.97	10.62 ± 2.31	0.967
Blood platelet count (103/µl), mean ± SD	202 910 ± 108 349.7	141 214 ± 78 856	212 953 ± 10 984	0.105
Serum ALT level (U/lt), mean ± SD	36.5 ± 43.5	39.58 ± 26.35	36.08 ± 45.84	0.857
Serum creatinine value (mg/dl), mean ± SD	1.9 ± 2.1	1.37 ± 1.09	2.01 ± 2.28	0.620
Serum procalcitonin value (ng/ml) mean ± SD	2.9 ± 1.2	40.43 ± 36.49	18.09 ± 24.69	0.521
Serum CRP level (mg/lt), mean ± SD	179 ± 106	163.64 ± 112	181.98 ± 106.36	0.659
Serum albumin value (gr/dl), mean ± SD	19.8 ± 26.7	2.69 ± 0.45	3.26 ± 0.74	0.099
Strain positive for Blaz, n (%)	44 (84.5%)	7 (87.5)	37 (84)	1.000
Cefazolin MIC value in standard inoculum, µg/mL, mean ± SD	0.875 ± 0.44	0.875 ± 0.23	0.875 ± 0.47	0.485
Cefazolin MIC value in the high inoculum, µg/mL, mean ± SD	2.78 ± 3.55	5.68 ± 5.32	2.25 ± 2.91	0.054
CzInE, n (%)	13 (25)	4 (50)	9 (20)	0.096
CzInE in patients treated with Cz, n (%)	4 (7.7)	1 (12.5)	3 (6.8)	0.514
SAM MIC, standard inoculum, µg/mL, mean ± SD	0.66 ± 0.66	0.76 ± 0.67	0.64 ± 0.66	0.636
SAM MIC, high inoculum, µg/mL, mean ± SD	2.44 ± 2.71	3.33 ± 3.69	2.27 ± 2.51	0.301
Inoculum effect of ampicillin-sulbactam, present, n(%)	20 (38.4)	5 (62.5)	15(34)	0.235
SAM InE in patients treated with SAM, n(%)	6 (7.7)	3 (37.5)	3 (6.8)	0.044 (OR7.8, 95%CI; 1.23–49.68)
InE against the drug used for definite treatment, n (%)	10	4 (50)	6 (13.6)	0. 041 (OR 6.0, 95%CI; 1.17–30.73)
ICU admisision	7	6	1	< 0.001

In this study, we found that the drugs whose activities were most affected by the inoculum effect were SAM (35% of the strains) and Cz (25% of the strains). Although the GM MICs of all cephalosporins tested were significantly lower for the higher inoculum than for the standard inoculum (p < 0.001), no InE was detected against cefuroxime or cefotaxime, and only a limited percentage (3.8% of the strains) of InE was detected against ceftriaxone. The increase in the GM MIC was also greatest for SAM, which was 2.94 times greater at high inoculum rates, followed by Cz (2.20 times), cefuroxime (1.27 times), ceftriaxone (1.21) and cefotaxime (1.20 times). Similar to our findings, in a recent Korean study including 302 MSSA isolates of bacteremia, InE was found to be more common (43% and 66% for piperacillin-tazobactam (TZP) and SAM, respectively) and prominent (MICs were nearly 24 and 9 times higher for TZP and SAM, respectively, at higher inoculum rates) in β-lactam-β-lactamase inhibitor (BL/BLI) combinations (7). Additionally, pronounced SAMInE (9.6%) was observed more frequently than pronounced CzInE (5.8%), and no significant increase in MICs at high MSSA concentrations was observed with cefotaxime or ceftriaxone in an in vitro study of 52 MSSA isolates from blood cultures (8).

There have not been any previous reports about the proportion of CzInE or SAMInE from Türkiye. The rate of InE against Cz varied significantly across the studies. The proportion of CzInE in MSSA strains has been reported to be a median of 14.4% and ranges from 0–54.5% and varies significantly from country to country, with the highest proportion being in South American countries (36.0–54.5%), followed by Asian countries (5.8–21.8%), North American countries (0–18.7%) and European countries (2.5–11.0%) (9). The different proportions of InE could be related to the definition of InE, the prevalence and type of β-lactamase among MSSA strains, the hydrolyzing capacity of the prevalent MSSA β-lactamases in the region, β-lactam consumption rates and antimicrobial stewardship efforts in the community, the presence of resistance-carrying genes on mobile genetic elements, and preventive infection control efforts to decrease the spread of resistance genes in health care facilities (10).

In some studies such as ours, CzInE defined a 4-fold or greater increase in MIC values at high inoculum concentrations and pronounced InE as an MIC of ≥ 16 µg/ml with a high inoculum (2, 11, 12), whereas others defined it as an MIC of ≥ 16 µg/ml with a high inoculum (6). In one study from South Korea that used the same definitions, the proportions of CzInE and pronounced CzInE were reported to be 57.5% and 20%, respectively (12); in another study, the proportions of CzInE and pronounced CzInE were reported to be 20% and 4% CzInE (11), respectively, which are quite similar to our rates of 25% and 5.7%, respectively. The definition of InE that we used is more sensitive than the latter definition is. Assessment of the clinical implications of InE should include a more sensitive definition of InE to avoid the risk of falsely rejecting possible associations by using a less specific metric (13).

Some studies have suggested that CzInE is related to either type A or type C β-lactamases of MSSA and that TZP InE is related to type C β-lactamases (7, 14). In our study, all of the tested MSSA strains were found to carry type A β-lactamases. While previous studies from various countries generally reported that all four enzyme types were present, albeit in different proportions, type C was reported to be present in 94% of the MSSA strains with β-lactamases in Japan. As a plasmid-mediated and antibiotic-inducible β-lactamase, the spread of the same clones carrying the same enzyme is quite logical and possible, especially in communities with high antibiotic consumption, such as Türkiye. Widespread dissemination of β-lactamase-associated resistance genes between strains of bacteria has occurred several times in our country, such as the spread of OXA-48 among Klebsiella pneumoniae strains (15) and the spread of CTX-M among E. coli strains (16).

Although all of the strains in our study were found to carry type A β-lactamases, which have repeatedly been shown to be associated with a relatively high rate of CzInE, CzInE accounted for only 25% of our strains, suggesting that the type of β-lactamase is not the only reason for CzInE. Consistent with this, a recent study from Latin America including 690 bloodstream MSSA isolates with whole-genome sequencing revealed that the allotype rather than the type of β-lactamase could be a more accurate tool for identifying strains with a likelihood of exhibiting CzInE, and the authors reported that particular amino acid residues (E112A and G145E substitutions) were highly associated with allotypes that exhibited CzInE (17). Another study reported that single nucleotide polymorphisms of the type A blaZ gene at codons 226 and 229 (Ser226Pro and Cys229Tyr) were closely associated with CzInE (18). However, either those specific allotypes or those specific SNPs could also be determined in strains without CzInE. Clearly, CzInE is a multifactorial phenomenon, and further studies analyzing the mechanism of CzIE are still needed.

Although the mechanisms of CzInE and SAM InE have not been fully defined, both are clearly related to the β-lactamases of MSSA. We found that while the GM MICs of cefuroxime, ceftriaxone, and cefotaxime were not affected by the presence or absence of β-lactamase, only the MICs of SAM and Cz were significantly affected by the presence or absence of β-lactamase; the GM MIC values of SAM against strains with β-lactamase were 23 times greater than those without β-lactamase, and this difference became even more pronounced at higher inocula, with the 61 and GM MIC values of Cz against the strains with β-lactamase being 1.66 and 3.86 times greater than those without β-lactamase at standard and high inocula, respectively. Additionally, the GM zone diameters of the 52 MSSA strains subjected to PG and cefazoline were significantly lower for β-lactamase-positive strains than for β-lactamase-negative strains, both at standard and high inoculum rates (< 0.001). However, the interaction of β-lactamases with InE seems different between SAMInE and Cz InE. Cz InE is repeatedly reported to be related to either hyperproduction or better performance of type A and type C BlaZ on cefazoline (2); SAMInE probably results from the decreased inhibition of the β-lactamase inhibitor sulbactam against MSSA β-lactamase (7). In accordance with that suggestion, in our study, we observed that while SAMInE was strongly related to the increase in only the minimum inhibitory concentration (MIC) of SAM (p < 0.001), CzInE was strongly related to the decrease in the PG and Cz zone diameters (p < 0.001 for both) and the increase in the Cz MIC (p < 0.001). and high (28.26 ± 2.63 versus 24.39 ± 2.33, p < 0.001) inoculum. The fact that SAMInE is specific to SAM and does not affect susceptibility to other antimicrobial agents indicates that the mechanism of SAMInE is specific to SAM and is not caused by excessive production of β-lactamase. It is suggested that if the presence of InE against one β-lactamase inhibitor is shown, then InE and decreased activity of other β-lactamase inhibitors could be foreseen. In accordance with this suggestion, TZP is frequently affected by MSSA β-lactamases (7). In an in vitro study, clavulanic acid and tazobactam were 93 and 11 times more active than sulbactam against the β-lactamases of MSSA strains (19).

The mean Cz zone diameter of the strains with CzInE was significantly lower than that of the strains without CzInE, both in standard and high inoculum, in our study (27.94 ± 3.09 versus 23.85 ± 1.99, p < 0.001). A Cz zone diameter of < 28 mm was found to be 100% sensitive for the definition of CzInE among type A β-lactamase-carrying MSSA strains in our study. In a recent study from Australia, it was shown that type A and type C β-lactamases of MSSA could be defined by calculations using zone diameters of cefazolin, cephalothin, and oxacillin, with a sensitivity and specificity of 88.6% and 96.6%, respectively (20). These results show that both the InE and β-lactamase types can be predicted via simple disk diffusion tests and that the zone diameter of Cz can be used as a screening test to define the InE of Cz and to define the necessity of further testing.

We found that the rate of β-lactamase positivity (26/26 vs 18/26, p = 0.004) of the infected MSSA strains was significantly higher and that the rates of neutropenia (4/26 vs. 1/26 p = 0.350), admission to the ICU (6/26 vs. 1/26, p = 0.99), and mortality (6/26 vs. 2/26, p = 0.248) were higher in patients infected with the strains showing InE against SAM or Cz than in patients not infected. In another study, metastatic cancer, recent close contact with a chronically ill patient, and resistance to clindamycin and erythromycin among the causative MSSA strains (7) were found to be risk factors for CzInE. As neutropenia frequently develops in patients with metastatic cancer, the incidence, underlying mechanism and clinical implications of CzInE among patients with cancer or neutropenia should be evaluated in further studies. We did not find any associations between SAM or CzInE and other comorbidities or test results of the patients or with the susceptibility or resistance of the strains to certain antibiotics.

It is unknown whether being less active against the β-lactamase of MSSA and having a more frequent InE among MSSA strains than Cz in SAM will affect the treatment effectiveness of the two agents.

In our study, the mortality rate in patients infected with MSSA strains showing SAMInE and treated with SAM was higher than that in those not treated with SAM (37.5% vs 68%, p = 0.044, OR 7.8, 95% CI 1.23–49.68), and the causative MSSA strains of deceased patients presented higher SAM InE (62.5% vs 34%, p = 0.235) than surviving patients did. We could find only one study analyzing the clinical impact of SAMInE. In that study, which included 302 patients with MSSA bacteremia, the mortality rates of the SAM InE-positive (n = 27) group, who received empirical β-lactam/β-lactamase inhibitors, were significantly greater than those of the negative (n = 23) (32.4% vs 5.6%, p = 0.04) group, and the mortality rate of the SAM InE-positive (n = 28) group, who received definitive β-lactam/β-lactamase inhibitors for the treatment of MSSA bacteremia, was also significantly greater than that of the negative (n = 14) group (26.1% vs. 8.3%, p = 0.38) (7).

There are also several studies comparing the effectiveness of SAM or other BL/BLI combinations and other first-choice regimens, including either Cz or ASPs. In a retrospective cohort study of 478 patients with MSSA bacteremia, mortality was found to be similar between patients treated with cloxacillin and cefazoline, but it was nearly two times (OR 2.68, p = 0.08) greater among patients treated with BL/BLI combinations, including TZP (n = 32), ampicillin-clavulanate (n = 28) and SAM (n = 1) (21). In a retrospective cohort study of our group including 127 MSSA bacteremia patients, the mortality rate of patients treated with Cz (2/30, 6.6%) was lower than that of patients treated with SAM (9/47, 19%); however, the difference did not reach statistical significance (p = 0.082) (22).

In the study of Uda et al., another BL/BLI in combination with TZP for definitive therapy of MSSA bacteremia was found to be associated with treatment failure for MSSA bacteremia (OR = 17, p = 0.003) (23).

In a USA study including more than 400 MSSA bacteremia cases, while no difference in mortality was observed between ASP and cefazolin or fluoroquinolones, higher mortality was observed with TZP than with ASP/cefazolin (HR, 0.10; 95% CI, 0.01–0.78), suggesting that TZP may not be as effective as monotherapy for MSSA bacteremia (24).

Finally, in a recent retrospective observational study from Japan comparing the clinical efficacy of SAM (41 patients) with that of Cz (30 patients) in patients with MSSA bacteremia, the mortality rate did not differ between the groups (25).

All of these data suggest that, owing to the lower effectiveness of sulbactam against the β-lactamases MSSA and SAMInE, SAM treatment of MSSA bacteremia and other higher inoculum infections could be less effective than other options, including Cz. SAM is a frequent replacement for first-line antimicrobials for MSSA bacteremia, including ASP and Cz, especially in the case of the unavailability of those agents, such as those used here in Türkiye, Argentina, and previously in Japan, or if those agents could not be used owing to adverse effects. Therefore, additional studies are urgently needed on this subject, and the limited evidence along with our findings support the hypothesis that the BL/BLI of SAM or TZP could have decreased activity against some β-lactamases of MSSA, which could lead to decreased activity of those agents during treatment.

In our study, the mortality rate in patients infected with MSSA strains showing CzInE and treated with Cz was greater than that in those not treated with Cz (12.5% vs. 6.8%, p = 0.514); additionally, the mortality rate of patients infected with the strains showing CzInE (4/13, 30%) was greater than the mortality rate of patients infected with the strains not showing CzInE (4/39, 10%) (p = 0.096), but the differences were not statistically significant in either of the findings, and the numbers were insufficient to reach a definite conclusion. Cz is the most studied drug for the presence and clinical consequences of InE in MSSA strains that cause bacteremia, but the results of these studies are conflicting. In a recent meta-analysis of 23 observational studies, CzInE was defined in 0%-55% of the cases, and the mortality rate of serious infections caused by MSSA did not differ significantly between the strains with and without CIE. However, the quality of the included studies was low (26). A well-designed study without limitations is urgently needed to answer the question about the clinical impact of CzInE.

As cefuroxime was found to be highly effective in vitro against MSSA strains, with a GM MIC similar to that of Cz and was shown to be unaffected by InE in our study and other studies (27, 28), it could be evaluated as an alternative agent to SAM or cefazoline in the case of InE against those antimicrobials. In a recent study of 268 patients with MSSA bacteremia who were empirically treated with a mean of 3 days of either ASP flucloxaciline, cefuroxime or ceftriaxone, the duration of bacteremia or SAB-related mortality did not differ between the groups (29). Therefore, it is important to conduct comparative studies with Cz and cefuroxime in such MSSA infections. However, as the β-lactamases of MSSA are not constitutive but inducible with exposure to β-lactam antibiotics, close monitoring of the susceptibility of all antibiotics used along with rational antimicrobial usage efforts is of utmost importance (30).

Our study has several limitations, including its retrospective and observational design and the small number of patients in each group for some comparisons. However, our study contributes to the current limited knowledge concerning the incidence and clinical consequences of SAMInE in patients with MSSA bacteraemia. Additionally, for the first time, we found a disk diffusion test zone diameter breakpoint for the screening of CzInE.

In conclusion, InE is more frequently encountered against SAM than Cz is among MSSA strains causing bacteremia, probably because of the decreased activity of sulbactam against some of the type A β-lactamases of the MSSA strains. SAM treatment of patients infected with MSSA strains harboring SAMInE may increase mortality. Additional studies that provide stronger evidence are needed concerning the incidence and clinical consequences of InE among MSSA strains of deep-seated infections caused by β-lactam agents, including not only Cz but also BL/BLI. A cefazolin zone diameter of < 28 mm could be used as a screening method to define Cz InE.

Competing interests

The authors have no relevant financial or nonfinancial interests to disclose.

Ethics approval and consent to participate
This study was performed in accordance with the principles of the Declaration of Helsinki. Approval was granted by the Clinical Research Ethics Committee of Istanbul University, Istanbul Medical Faculty.

Funding

This study was supported by the Istanbul University Scientific Research Projects Coordination Unit.

Author Contribution

Moumperra Chral Oglou wrote the main manuscript text and contributed in all stages of study, has isolated and cultured all the bacteria strains, did the antimicrobial disk and MIC susceptibility method alongside with Serap Şimşek Yavuz. Serap Şimşek Yavuz constructed the hypothesis of research and guided the study, Gülşen Günel and Zerrin Aktaş performed the PCR method, Elif Norton and Füsun Can performed the b-lactamase sequencing method, Haluk Eraksoy edited the final manuscript.

Data availability

Not applicable.

Lam JC, Stokes W. The Golden Grapes of Wrath - Staphylococcus aureus Bacteremia: A Clinical Review. Am J Med. 2023 Jan;136(1):19-26. doi: 10.1016/j.amjmed.2022.09.017.
Lenhard JR, Bulman ZP. Inoculum effect of β-lactam antibiotics. J Antimicrob Chemother. 2019 Oct 1;74(10):2825-2843. doi: 10.1093/jac/dkz226.
Weis S, Kesselmeier M, Davis JS, et al. Cefazolin versus anti-staphylococcal penicillins for the treatment of patients with Staphylococcus aureus bacteraemia. Clin Microbiol Infect. 2019 Jul;25(7):818-827. doi: 10.1016/j.cmi.2019.03.010. 4)https://www.eucast.org/fileadmin/src/media/PDFs/ EUCAST_files/Disk_test_documents/2024_manuals/Manual_v_12.0_EUCAST_Disk_Test_2024.pdf)
CLSI. Methods for Dilution Antimicrobial Susceptibility Tests f or Bacteria That Grow Aerobically; Approved St andard—Ninth Edition. CLSI document M07-A9. Wayne, PA: Clinical and Laboratory Standards Institute; 2012.). 6) Nannini EC, Stryjewski ME, Singh K V., et al. Inoculum effect with cefazolin among clinical isolates of methicillin-susceptible Staphylococcus aureus: Frequency and possible cause of cefazolin treatment failure. Antimicrob Agents Chemother. 2009;53(8):3437-3441. doi:10.1128/AAC.00317-09
Song KH, Jung SI, Lee S, Park S, Kim ES, Park KH, Park WB, Choe PG, Kim YK, Kwak YG, Kim YS, Jang HC, Kiem S, Kim HI, Kim HB; Korea INfectious Diseases (KIND) study group. Inoculum effect of methicillin-susceptible Staphylococcus aureus against broad-spectrum beta-lactam antibiotics. Eur J Clin Microbiol Infect Dis. 2019 Jan;38(1):67-74. doi: 10.1007/s10096-018-3392-6.
Saeki M, Shinagawa M, Yakuwa Y, Nirasawa S, Sato Y, Yanagihara N, Takahashi S. Inoculum effect of high concentrations of methicillin-susceptible Staphylococcus aureus on the efficacy of cefazolin and other beta-lactams. J Infect Chemother. 2018 Mar;24(3):212-215. doi: 10.1016/j.jiac.2017.10.021. Epub 2017 Dec 1. PMID: 29198902.
Lo CK, Sritharan A, Zhang J, Li N, Zhang C, Wang F, Loeb M, Bai AD. Clinical significance of cefazolin inoculum effect in serious MSSA infections: a systematic review. JAC Antimicrob Resist. 2024 May 6;6(3):dlae069. doi: 10.1093/jacamr/dlae069.
Bush K, Bradford PA. Epidemiology of β-Lactamase-Producing Pathogens. Clin Microbiol Rev. 2020 Feb 26;33(2):e00047-19. doi: 10.1128/CMR.00047-19.
Livorsi DJ, Crispell E, Satola SW, Burd EM, Jerris R, Wang YF, Farley MM. Prevalence of blaZ gene types and the inoculum effect with cefazolin among bloodstream isolates of methicillin-susceptible Staphylococcus aureus. Antimicrob Agents Chemother. 2012 Aug;56(8):4474-7. doi: 10.1128/AAC.00052-12.
Lee S, Kwon KT, Kim HI, Chang HH, Lee JM, Choe PG, Park WB, Kim NJ, Oh MD, Song DY, Kim SW. Clinical implications of cefazolin inoculum effect and β-lactamase type on methicillin-susceptible Staphylococcus aureus bacteremia. Microb Drug Resist. 2014 Dec;20(6):568-74. doi: 10.1089/mdr.2013.0229.
George CRR, Lahra MM, Nguyen T, Gatus B. Disc Test for Detecting Staphylococcus aureus Strains Producing Type A and Type C β-Lactamases. Microbiol Spectr. 2023 Aug 17;11(4):e0022023. doi: 10.1128/spectrum.00220-23.
Mossman AK, Svishchuk J, Waddell BJM, Izydorczyk CS, Buckley PT, Hilliard JJ, Al-Ghalith G, Zheng L, Lynch AS, Mody CH, Lisboa LF, Gregson DB, Parkins MD. Staphylococcus aureus in Non-Cystic Fibrosis Bronchiectasis: Prevalence and Genomic Basis of High Inoculum β-Lactam Resistance. Ann Am Thorac Soc. 2022 Aug;19(8):1285-1293. doi: 10.1513/AnnalsATS.202108-965°C.
Mairi A, Pantel A, Sotto A, Lavigne JP, Touati A. OXA-48-like carbapenemases producing Enterobacteriaceae in different niches. Eur J Clin Microbiol Infect Dis. 2018 Apr;37(4):587-604. doi: 10.1007/s10096-017-3112-7.
Jafari A, Falahatkar S, Delpasand K, Sabati H, Sedigh Ebrahim-Saraie H. Emergence of Escherichia coli ST131 Causing Urinary Tract Infection in Western Asia: A Systematic Review and Meta-Analysis. Microb Drug Resist. 2020 Nov;26(11):1357-1364. doi: 10.1089/mdr.2019.0312.
Carvajal LP, Rincon S, Echeverri AM, Porras J, Rios R, Ordoñez KM, Seas C, Gomez-Villegas SI, Diaz L, Arias CA, Reyes J. Novel Insights into the Classification of Staphylococcal β-Lactamases in Relation to the Cefazolin Inoculum Effect. Antimicrob Agents Chemother. 2020 Apr 21;64(5):e02511-19. doi: 10.1128/AAC.02511-19.
Lee SH, Park WB, Lee S, Park S, Kim SW, Lee JM, Chang HH, Kwon KT, Choe PG, Kim NJ, Kim HB, Oh MD. Association between Type A blaZ Gene Polymorphism and Cefazolin Inoculum Effect in Methicillin-Susceptible Staphylococcus aureus. Antimicrob Agents Chemother. 2016 Oct 21;60(11):6928-6932. doi: 10.1128/AAC.01517-16.
Payne DJ, Cramp R, Winstanley DJ, Knowles DJ. Comparative activities of clavulanic acid, sulbactam, and tazobactam against clinically important beta-lactamases. Antimicrob Agents Chemother. 1994 Apr;38(4):767-72. doi: 10.1128/AAC.38.4.767.
) George CRR, Lahra MM, Nguyen T, Gatus B. Disc Test for Detecting Staphylococcus aureus Strains Producing Type A and Type C β-Lactamases. Microbiol Spectr. 2023 Aug 17;11(4):e0022023. doi: 10.1128/spectrum.00220-23.
Paul M, Zemer-Wassercug N, Talker O, et al. Are all beta-lactams similarly effective in the treatment of methicillin-sensitive bacteraemia? Clin Microbiol Infect. 2011; 17(10): 1581-6.
Basaran S, Simsek-yavuz S, Çopur B, Çagatay AA, Öncül O, Özsüt H, Eraksoy H. Analysis of risk factors for mortality in methicillin-sens,t, ve Staphylococcus aureus bacteremia: Cefazolin is associated with better ourcome. J Istanbul Faculty of Medicine 2019; 131-138. https://doi.org/10.26650/IUITFD.2019.0009 .
Uda A, Onuma K, Shigemura K, Kitagawa K, Yan Y, Osawa K, Yano I, Miyara T. Impact of Cefazolin Shortage on Clinical Outcomes of Adult Patients with Bacteremia Caused by Methicillin-Susceptible Staphylococcus aureus in a Tertiary Care University Hospital. Antibiotics (Basel). 2021 Oct 14;10(10):1247. doi: 10.3390/antibiotics10101247.
Beganovic M, Cusumano JA, Lopes V, LaPlante KL, Caffrey AR. Comparative Effectiveness of Exclusive Exposure to Nafcillin or Oxacillin, Cefazolin, Piperacillin/Tazobactam, and Fluoroquinolones Among a National Cohort of Veterans With Methicillin-Susceptible Staphylococcus aureus Bloodstream Infection. Open Forum Infect Dis. 2019 Jun 6;6(7):ofz270. doi: 10.1093/ofid/ofz270.
Hirai, J.; Asai, N.; Hagihara, M.; Kishino, T.; Kato, H.; Sakanashi, D.; Ohashi, W.; Mikamo, H. Comparative Effectiveness of Ampicillin/Sulbactam versus Cefazolin as Targeted Therapy for Bacteremia Caused by Beta-Lactamase-Producing Methicillin-Sensitive Staphylococcus aureus: A Single-Center Retrospective Study. Antibiotics 2022, 11, 1505. https://doi.org/10.3390/antibiotics11111505
Lo CK, Sritharan A, Zhang J, et al. Clinical significance of cefazolin inoculum effect in serious MSSA infections: a systematic review. JAC Antimicrob Resist. 2024 May 6;6(3):dlae069. doi: 10.1093/jacamr/dlae069.
Nannini EC, Stryjewski ME, Singh K V., et al. Determination of an inoculum effect with various cephalosporins among clinical isolates of methicillin-susceptible Staphylococcus aureus. Antimicrob Agents Chemother. 2010;54(5):2206-2208. doi:10.1128/AAC.01325-09
Skov R, Frimodt-Moller N, Espersen F. In vitro susceptibility of Staphylococcus aureus toward amoxycillin-clavulanic acid, penicillin-clavulanic acid, dicloxacillin and cefuroxime. APMIS. 2002;110(7-8):559-564. doi:10.1034/j.1600-0463.2002.11007807.x
D T P Buis, T W van der Vaart, J M Prins, J T M van der Meer, M J M Bonten, E Sieswerda, C H van Werkhoven, KCE Sigaloff, Comparative effectiveness of β-lactams for empirical treatment of methicillin-susceptible Staphylococcus aureus bacteraemia: a prospective cohort study, Journal of Antimicrobial Chemotherapy, Volume 78, Issue 5, May 2023, Pages 1175–1181, https://doi.org/10.1093/jac/dkad057
Alexander, J.A.N., Worrall, L.J., Hu, J. et al. Structural basis of broad-spectrum β-lactam resistance in Staphylococcus aureus. Nature 613, 375–382 (2023). https://doi.org/10.1038/s41586-022-05583-3.

No competing interests reported.

Download PDF

Version 1

posted

You are reading this latest preprint version

Incidence, Risk Factors and Clinical Outcomes of Inoculum Effect against β-lactams Including Ampicillin-Sulbactam and Cefazolin among Methicillin-Susceptible Staphylococcus aureus(MSSA) Strains Isolated from Patients with Bacteremia

Status:

Version 1

Abstract

Figures

Introduction

Materials and methods

Study population

Antimicrobial susceptibility testing

Detection of β-lactamase

DNA sequencing

Statistical analysis

Results

4 SAMInE: Inoculum effect against ampicillin–sulbactam

Discussion

Declarations

Competing interests

Ethics approval and consent to participate
This study was performed in accordance with the principles of the Declaration of Helsinki. Approval was granted by the Clinical Research Ethics Committee of Istanbul University, Istanbul Medical Faculty.

Funding

Author Contribution

Data availability

References

Additional Declarations

Status:

Version 1

Incidence, Risk Factors and Clinical Outcomes of Inoculum Effect against β-lactams Including Ampicillin-Sulbactam and Cefazolin among Methicillin-Susceptible Staphylococcus aureus(MSSA) Strains Isolated from Patients with Bacteremia

Status:

Version 1

Abstract

Figures

Introduction

Materials and methods

Study population

Antimicrobial susceptibility testing

Detection of β-lactamase

DNA sequencing

Statistical analysis

Results

4 SAMInE: Inoculum effect against ampicillin–sulbactam

Discussion

Declarations

Competing interests

Ethics approval and consent to participate This study was performed in accordance with the principles of the Declaration of Helsinki. Approval was granted by the Clinical Research Ethics Committee of Istanbul University, Istanbul Medical Faculty.

Funding

Author Contribution

Data availability

References

Additional Declarations

Status:

Version 1

Ethics approval and consent to participate
This study was performed in accordance with the principles of the Declaration of Helsinki. Approval was granted by the Clinical Research Ethics Committee of Istanbul University, Istanbul Medical Faculty.