Patients in the ICU are at a higher risk of developing bacterial infections due to various factors such as weak immune systems, prolonged hospital stays and invasive medical procedures [53][54]. Thus, these patients may require antibiotics and other treatments to manage these bacterial infections and prevent complications, which will help to improve their health.
Serratia marcescens is the bacterium that able to survive on environmental surfaces and medical equipment, making it a common cause of healthcare-associated infections, especially in settings like the ICU[55][56]. In this study, 101 isolates of S. marcescens were collected from ICU patients, which present a challenge in our hospitals . Many studies have reported the spread of these bacteria in ICU [57].
Many factors assist in the risk of S. marcescens infections in ICU patients include compromised immune systems, invasive procedures, prolonged hospital, and use of medical devices (such as catheters or ventilators). In general, multidrug resistant S. marcescens isolates are collected from clinical settings. High levels of antibiotic resistance among Serratia species belong to many activity of intrinsic, acquired, and adaptive resistance elements.
Intrinsic resistance is known as naturally occurred in microorganism which affect the activity of antimicrobials e.g., active efflux of toxic compounds and decreased membrane permeability. Acquired resistance refers to resistance occurred due to acquisition of antibiotic resistance genes commonly by gene mutation or plasmid horizontal gene transfer. Horizontal gene transfer is considered the most important mechanism causes multiple antibiotic resistances [10] [13]. Lastly, adaptive resistance, also called phenotypic resistance, is linked to expeditious transcriptome adjusting in response to stress conditions or environmental stimuli. One other hand, adaptive resistance is known as a non-inheritable phenotype, which can revert after the triggering signal removal. pH, temperature, oxidative stress, and disinfectants exposure have been shown to be linked with gene regulation and higher bacterial survival in S. marcescens [58][59].
Bacterial exposure to antibiotics is known to be the main factor for antibiotics resistances. However, a recent analysis of S. marcescens isolates caused an outbreak in 1969 in Spanish neonatal ICU showed that these isolates carried disinfections and antibiotics resistance genes that were non commercialized by the time of their original isolation [60]. Furthermore, another study compared between strains collected during the 1940s and the beginning of the twenty-first century showed that resistance to different classes of antibiotics already existed in ancient S. marcescens isolates [61] , which suggested additional factors unrelated to drug therapy assist in evolution of resistance genes. In 2018, analysis of 32 Serratia spp. genomes conclude that strains isolated from environments represent an underestimated reservoir for antibiotic resistance determinants [10], which suggested these resistance genes are inherited [62].
Thus, clinical management of S. marcescens infections present a big challenge because its intrinsic resistance to different classes of antibiotics such as ampicillin, cephalosporins, aminoglycoside macrolides, and peptides [12]. In our study, the isolates showed high resistance profile to many antibiotics used, which present a serious problem to patients in ICU.
Cefepime or carbapenems are commonly used to treat S. marcescens infections [17][63]. Furthermore, aminoglycoside amikacin is found to be effective to treat these infections. However. an increasing resistance to gentamicin and tobramycin has been reported [64] [65]. In this study, high percentage of the isolates showed resistance to gentamicin and tobramycin in addition to more than half of the isolates were not suspectable to amikacin. Furthermore, most of the isolates were resistant to fluroquinolones (ciprofloxacin).
The extensive and misuse of β-lactams led to the emergence of resistance against these antibiotics. Many mechanisms are responsible for this resistance including target site modification (mutation or expression of alternative PBPs), downregulation of porin which results in reduction in cell, efflux pump and modifying enzymes (β- lactamase) [66]. β- lactamase assists in enzyme-mediated resistance, which produced by both Gram-positive and Gram-negative bacteria. Furthermore, these enzymes are encoded chromosomally or on extrachromosomal elements, which degrade the β-lactam ring . Extended-spectrum β-lactamases (ESBLs) are a group of bacterial enzymes that can be rapidly transferred via plasmid exchange [67] causing resistance to a broad range of β-lactams.
TEM, SHV, CTX β-lactamase genes presence in the bacteria results in β- lactam resistance (penicillins and cephalosporins). Enterobacteriaceae members including S. marcescens possess blaCTX-M and blaSHV and these genes express classical class A β - lactamases, which hydrolyze first generation penicillins and cephalosporins and encoded on plasmid [68] . In our study, both genes were found in most of the isolates beside blaTEM . Furthermore, the main cause of antibiotics resistance in S. marcescens is the production of chromosomally encoded β-lactamases, which involve extended-spectrum β-lactamases, AmpC-type cephalosporinase, and carbapenemases. Carbapenemases are the most versatile family of β-lactamases able to hydrolyze carbapenems and many other β-lactams including penicillins, cephalosporins, and monobactams [69] . Generally, bacteria carrying the blaKPC and/or ESBLs genes usually possess other resistance genes, which confer resistance to several classes of antimicrobials [70][71]. Carbapenems are considered as main antibiotics used to treat S. marcescens infections because these bacteria showed resistance to broad-spectrum cephalosporins [72]. However, Many studies have reported the emergence of carbapenem-resistant strains around the world, which are caused by production of Ambler class A (KPC and SME), loss of porins associated with AmpC overexpression or class B metallo-β-lactamases (MβLs; IMP and VIM) [73][74]. In this study, the most of our isolates possessed blaIMP, blaNDM1, blaVIM1 and blaKPC and were resistant to imipenem and meropenem.
carbapenem-resistant S. marcescens isolates were shown in this study, which present as a serious concern in nosocomial environment. It was reported that S. marcescens is intrinsically resistant to polymyxins [75], but in this study, only 25% of the isolates were resistance to colistin.
In addition, fluoroquinolones resistance occurs by modification off target sites DNA gyrase and topoisomerase IV or by plasmid-mediated quinolone resistance (PMQR) determinant qnr, qepA, aac(6’)-Ib-cr, and oqxAB . Furthermore, the aac(6')-Ib gene encodes aminoglycoside-modifying enzymes and confers resistance to tobramycin, kanamycin, and amikacin, which is the most prevalent gene. The aac(6')-Ib-cr variant gene can induce resistance against aminoglycoside and fluoroquinolone simultaneously. In our study, isolates with aac(6’)- Ib-cr were resistant for both fluoroquinolone and/or aminoglycoside antibiotics as were found in many other studies [76][77]. In addition to beta lactam, Quinolones are widely used to treat many infections, which are synthetic antibiotics used against Gram-negative bacteria such as Enterobacteriaceae [78]. Fluoroquinolones have broad-spectrum intrinsic activity greater than quinolones. It was reported that Qnr genes are often coexisted on the same resistance plasmids as extended spectrum β-lactamase , which explain the co-resistance to β-lactams and fluoroquinolones [79]. In present study, all qnrs-positive isolates produced CTX-M1, VIM, SHV and TEM as in appendix 1 . Previous study was found disruption of the ompA gene caused to decrease MICs of chloramphenicol, aztreonam, and nalidixic acid [80],which indicates the role of this gene in antibiotic resistance. Furthermore, Integrons have an essential role in the spread of antibiotic resistance, especially in Gram-negative bacteria, 39% from the isolates in current study had the int1 gene. Furthermore, resistance phenotype-genotypic correlation and dendrogram phylogenetic analysis were performed for the isolates. In this study showed S. marcescens in ICU associated with the high frequency of resistance genes and correlated with resistance phenotype for the antibiotics tested , while dendrogram phylogenetic analysis showed most of the isolates were closely related in their resistance in spite of their huge diversity of sources.
Many studies have shown the spread of highly resistance bacteria in Iraqi hospitals [81][82][83][84][85].Form this study, it shows the emergence of highly resistance S. marcescens in Iraqi patients from ICU, which can present a higher risk for effective treatment and control the spread of these isolates. Thus, finding alternative therapy need to be considered like phage therapy, Furthermore, our study has many limitations like size samples and should collect many samples from different hospitals cross Iraq. In addition, plasmid isolation should be considered to get actual distribution of resistance genes between chromosomes and plasmids.