Demographic and patient characteristics
A total of 4577 patients were included in the study. Of these, 438 (9.6%) patients, developed an SSI. The SSI risk was 9.3% (376/4054) in the general surgical department, 26.5% (31/117) in the department of urology and 7.6% (31/406) in paediatric surgery. The median age of the patients who developed an SSI was 45 years (interquartile range 31 – 60 years) and 239 (54%) patients were female.
From 382 (87%) of the 438 patients, a wound swab was taken for microbiological analysis. No wound swabs were taken in 13% (56/438) of the cases due to; surveillance team missing the opportunity to take a swab at change of wound dressing or at a relaparotomy for an SSI, missing the diagnosis of an SSI until the attention was drawn to the clinical signs and, wound swabs taken but accidentally sent to a private laboratory for analysis.
Characteristics of surgical wounds
The proportion of patients who developed an SSI was 5.2% (135/2589) for wounds classified as clean, 10.1% (66/655) for a clean contaminated wound, 12.1% (104/859) for contaminated and 27.4% (130/475) for dirty wounds.
We found 352 isolates in 327 (86%) of the 382 swabs. Most were monoculture, but two different species were cultured from each of 25 swabs. Isolates from clean wounds accounted for 29% (103/352) of isolates, clean contaminated wounds for 14% (48), contaminated wounds for 26% (91), and dirty wounds for 31% (110) of isolated microorganisms.
Aetiology of surgical site infections
Gram-negative microorganisms constituted 81% (286/352) of the isolates. The five most common microorganisms were Escherichia coli (139, 39%), Pseudomonas aeruginosa (49, 14%), Klebsiella pneumoniae (35, 10%), Staphylococcus aureus (33, 9%) and Acinetobacter baumannii (23, 6%), accounting for approximately 79% of the isolated organisms (Table 1).
Table 2 describes the pathogens by type of surgery. At least one isolate was found in 187 patients (11%) of the 1646 gastrointestinal and other abdominal surgeries, in 12 (8%) of 140 genitourinary and prostate surgeries, 38 (4%) of 907 breast surgeries, 44 (5%) of 866 hernia and scrotal surgeries, 25 (16%) of 157 limb amputations, 9 (3%) of 307 thyroid surgeries and 12 (2%) of 554 other soft tissue surgeries. The ratio between Gram-negative and Gram-positive organisms differed by type of surgery (Table 4, p=0.002). For gastro-intestinal and genito-urinary surgery, Staphylococcus spp. constituted < 16% of the isolates, whereas Staphylococcus spp. constituted > 25% of the isolates for hernia, breast, soft tissue and thyroid surgery. Despite this difference, Gram-negative organisms constituted ≥ 66% of isolates for all types of surgery and as much as 87% for gastro-intestinal surgery.
There was a difference between type of wound and the ratio between Gram-negative and Gram-positive organisms (Table 4), and S. aureus was only cultured from superficial SSI (Table 3). Conversely, all isolates from organ-space SSI were Gram negative. A much higher proportion of organ/space infections than other types of infections were not cultured, mostly due to the surveillance team missing the opportunity to take a wound swab during a relaparotomy to drain abscesses.
Antimicrobial susceptibility patterns
The majority of bacterial isolates were MDR, ranging from 23% to 86% for Proteus spp. and E. coli isolates, respectively (Table 1). Acquired resistance to commonly used antibiotics ranged from 1% to meropenem in E. coli to 95% to ampicillin, ciprofloxacin and trimethoprim-sulphamethoxazole in E. coli (Figure 1A).
Among E. coli and K. pneumoniae isolates, 60% (82/139) and 74% (26/35), respectively were resistant to third generation cephalosporins. Sixty-one percent (50/82) and 65% (17/26) of the cefotaxime-resistant E. coli and K. pneumoniae, respectively were ESBL producing.
Meropenem resistance in this study was mainly found in Pseudomonas spp. (15 of 49 isolates) and Acinetobacter baumannii (6 of 23 isolates) (Table 1, Figure 1B). Four P. aeruginosa isolates harboured the vim gene, encoding Verona integron-encoded metallo-β-lactamase. One A. baumannii expressed OXA-23. K. pneumoniae showed no resistance to meropenem (Figure 1A).
Fifteen percent (5/33) of S. aureusisolates were methicillin resistant (MRSA). Overall, the MRSA prevalence was 1% (5/438) in the cultured wounds. Resistance of S. aureus against other antibiotics ranged from 6% to gentamycin and clindamycin to 42% to tetracycline. Among coagulase-negative staphylococci, the percentage resistance ranged from 23% to clindamycin to 81% to tetracycline. Neither S. aureus nor coagulase negative staphylococci showed resistance to linezolid (Figure 1C).
The sensitivity pattern of the MDR organisms, including those for which specific resistance mechanisms such as ESBL or carbapenemases were detected, did not give any indication of an outbreak of one or more clones (data not shown).