Detecção da resistência à oxacilina e perfil de sensibilidade de Staphylococcus coagulase negativos isolados em um hospital escola

Abstract

For many years coagulase negative Staphylococcus (CoNS), skin commensals, were regarded as mere contaminants. However, in recent decades, they emerged as important agents of nosocomial infections, particularly in immunocompromised patients and patients with biomaterials. This is attributed to increasing antimicrobial resistance by these microorganisms, and oxacillin resistance was the main mechanism presented by CoNS. Thus, this study aimed to characterize this resistance, as well as the susceptibility of 160 isolates of CoNS obtained from patients admitted to HUSM, and also compare phenotypic tests used in laboratory detection of oxacillin resistance with genotypic detection of mecA gene, considered the gold standard. The antimicrobial susceptibility tests were performed by the qualitative technique of disk diffusion (CLSI 2009), as well as by semi-quantitative method (MicroScan ®). The phenotypic detection of resistance was performed by using cefoxitin and oxacillin disk. The genotypic identification of mecA gene was performed by PCR. Together with the molecular detection of mecA gene was performed the detection of gene icaD, responsible for biofilm formation. S. epidermidis (62%) was the main species identified among the samples selected for this study, and the prevalent clinical material was blood (38%). Due to the fact that they are skin commensals, an evaluation of clinical significance of the selected samples was performed. It presented that 48% of the isolates were considered more likely to be the etiologic agents of infections. The adult and neonatal ICUs represent prevalent clinical units in isolation of CoNS, with 33% and 29% of the isolates, respectively. Regarding the antimicrobial susceptibility, a high rate of resistance among the isolates was observed. These were grouped into seven prevalent susceptibility profiles, in which three were characterized by resistance to most of the antimicrobials tested. All strains were susceptible to linezolid, teicoplanin and tigecycline, in total 59 isolates (36.8%). All isolates were susceptible to vancomycin. Through the results of phenotypic tests, 89% and 94% of the isolates were characterized as oxacillin resistant by using cefoxitin and oxacillin disks, respectively. The molecular technique revealed that 79% of isolates carried mecA gene. A susceptibility of 96% to cefoxitin disk and 95% to oxacillin disk could be found from the statistical analysis when compared to the gold standard. Biofilm formation was observed in 45% of samples tested, in which S. epidermidis has been identified as prevalent in positive biofilm samples (74%). The phenotypic methods used in this study are equivalent for detecting oxacillin resistance when compared to the gold standard, and the use of oxacillin disk together with cefoxitin disk should be encouraged, since the oxacillin disk can identify other resistance mechanisms not mediated by mecA. In relation to susceptibility of the isolates, there was a high resistance to first-choice antimicrobial used for the treatment of CoNS.