Antibiotic Susceptibility Pattern and Biofilm Formation in Coagulase Negative Staphylococci

Introduction Coagulase negative staphylococci (CoNS) are commensals of non-sterile sites in humans and become pathogenic mostly when the host is immunocompromised by prior diseases or invasive surgical or related procedures [1]. Slime or biofilm production by CoNS has been identified as an important factor in the pathogenesis of infections as bacteria organized in biofilms are protected from the action of antibiotics and the immune system [2]. Biofilm is ascribed the most important virulence factor of S. epidermidis as it enables attachment and persistence of the bacteria on foreign materials [3,4]. Other studies have indicated a correlation between antibiotic resistance and slime expression. For instance, insertion of a certain transposon influences both biofilm formation and the expression of methicillin resistance in S. epidermidis [4]. In another study methicillin resistance was found to be significantly higher in slime positive isolates (81%) than in slime negative isolates (57%) [5]. Due to the frequent recovery of CoNS in clinical infections their antibiotic susceptibility profile as well as their biofilm forming ability was investigated in this study.


Introduction
Coagulase negative staphylococci (CoNS) are commensals of non-sterile sites in humans and become pathogenic mostly when the host is immunocompromised by prior diseases or invasive surgical or related procedures [1]. Slime or biofilm production by CoNS has been identified as an important factor in the pathogenesis of infections as bacteria organized in biofilms are protected from the action of antibiotics and the immune system [2]. Biofilm is ascribed the most important virulence factor of S. epidermidis as it enables attachment and persistence of the bacteria on foreign materials [3,4]. Other studies have indicated a correlation between antibiotic resistance and slime expression. For instance, insertion of a certain transposon influences both biofilm formation and the expression of methicillin resistance in S. epidermidis [4]. In another study methicillin resistance was found to be significantly higher in slime positive isolates (81%) than in slime negative isolates (57%) [5]. Due to the frequent recovery of CoNS in clinical infections their antibiotic susceptibility profile as well as their biofilm forming ability was investigated in this study.

The Study
One were isolated from various clinical specimens obtained from wards and outpatient departments of 8 medical centres from the states of Lagos and Ogun in Nigeria. The isolates were identified by cultural and biochemical characteristics. Identification to species level was performed with the VITEK-2 automated system (bioMérieux, Marcy l'Etoile, France). Susceptibility to 19 different antimicrobial agents (benzylpenicillin, oxacillin [methicillin], gentamicin, tobramycin, levofloxacin, moxifloxacin, erythromycin, clindamycin, linezolid, teicoplanin, vancomycin, tetracycline, tigecycline, fosfomycin, nitrofurantoin, fusidic acid, mupirocin, rifampicin and trimethoprimsulfamethoxazole) was tested with the VITEK-2 automated system. The biofilm-forming capacity of one S. lugdunensis strain and five S. epidermidis strains obtained in this study was investigated by a method previously described [6].
The results of the antibiotic susceptibility test showed that the β-lactam antibiotics were less effective, particularly the penicillins against CoNS. The emergence of large clusters of methicillin resistant CoNS in this study portend further therapeutic dilemma. These strains have been shown to spread within and between hospitals [7,8]. The 41% of CoNS which were methicillin resistant in this present study is encouragingly lower than the proportion (60-70%) seen in some hospitals in Europe [9]. Mupirocin's excellent activity against the CoNS is important as it will continue to be the drug of choice for decontaminating the nasal cavity of carriers of staphylococci [10].
The resistance pattern in S. intermedius and S. xylosus as seen in this present study is of major concern. Of particular interest is the 100% resistance of these two species to antibiotics (methicillin, linezolid, teicoplanin, vancomycin, tigecycline with the exception of mupirocin) that produced 100% activity against other species. The present study showed that organisms that lacked the ability to form biofilms were more susceptible to antibiotics. It is important to observe also that two of the five strains of S. epidermidis that failed to form biofilms represented the antibiotic sensitive clone. Our finding supports an earlier work that methicillin resistance in CoNS was higher in species that produce slime [5]. It is worthwhile to note that several of the isolates in this study could be contaminants and not real pathogens.
In conclusion, further studies are required to determine the epidemiological implication of diversified resistance in coagulase negative staphylococci.