ABSTRACT
In the present study, 100 strains of coagulase-negative staphylococci were isolated from 57 males and 43 females, their ages ranged from eighteen years to fifty years. Cases were admitted in General Surgery, Haemodialysis Unit, Gynaecology and Obstetric Departments and Outpatient Clinics of Zagazig University Hospitals. The objective of the present study was to evaluate the most commonly used susceptibility test methods for detection of methicillin resistance among coagulase negative staphylococci [CoNS] using oxacillin disk diffusion and oxacillin salt agar screen methods and to compare the results of these methods to PCR detection of the mec A gene for 100 clinical isolates of CoNS. Typing, of isolated CoNS into methicillin resistant coagulase-negative staphylococci [MRCoNS] strains and methicillin sensitive coagulase-negative staphylococci [MS CoNS] strains was done using the disk diffusion method: seventy two [72%] strains of the total CoNS isolates were susceptible to oxacillin while the remaining twenty eight [28%] strains were resistant. On using oxacillin salt agar screen it was revealed that 69 [69%] strains of the total 100 CoNS isolates were susceptiblandto oxacillin while the remaining 31 [31%] strains were resistant. The DNA extracted from 28 oxacillin resistant CoNS strains and 12 oxacillin susceptible CoNS were amplified by polymerase chain reaction. Then, analysis of the amplified products by agarose gel electrophoresis for detection of the mecA gene. The gel electrophoresis revealed that 4 strains of MRCoNS strains as detected by disk diffusion were negative by PCR and out of MSCoNS we found that 3 strains were PCR positive. The sensitivities of the disk diffusion method and oxacillin salt agar screen method at 48 hr in relation to PCR results were 88.9% and 100% respectively. While, the specificities of the disk diffusion method and oxacillin salt agar screen method at 48 hr in relation to PCR results were 69.2% and 92.3% respectively. We concluded that oxacillin salt agar screen method at 48 hr by the current National Committee for Clinical Laboratory Standards [NCCLS] methodology for determination of methicillin susceptibility to CoNS can be considered reliable and cost-effective alternative to PCR assay [for detection of mec A gene] for identification of MRCoNS isolates