PHENOTYPIC DETECTION OF B-LACTAMASE PRODUCING STAPHYLOCOCCUS AUREUS CLINICAL ISOLATES.

More than 90% of Staphylococcal Aureus isolates produce penicillinase, regardless of the clinical setting. However, penicillin remains the treatment of choice for penicillin-susceptible Staphylococcus aureus. A number of phenotypic methods for the detection of ?-lactamase production in Staphylococcus species have been investigated and compared to detection of the blaZ gene by PCR. All phenotypic methods had a sensitivity of less than 72%, so phenotypic tests with high sensitivity should be adopted to detect ?-lactamse production in S. aureus. This would guide the treatment of cases of serious infections requiring penicillin therapy. The present study aims to identify prevalence of S. aureus resistance to penicillin and to evaluate the nitrocefin reaction in detecting ?-lactamse production in S. aureus, compared to detection of blaZ- gene presence by real time PCR in S. aureus isolates. The present study was conducted on one hundred clinical isolates of S. aureus that were collected from various clinical specimens. Antimicrobial susceptibility was made for cefoxitin and penicillin. Nitrocefin disks were used to detect ?-lactamase production. Real time PCR was used to detect blaZ gene. Real time PCR results showed that 100% of S. aureus isolates were positive for blaZ gene presence. Out of 100 staph aureus isolates 97 (97%) were resistant to penicillin by disk diffusion and gave positive reaction by nitrocefin test. Penicillin disk and nitrocefin test has a sensitivity of 97% and positive predictive value of 100% to detect positive ?-lactamase producer S. aureus strains. In conclusion, the high sensitivity of phenotypic methods like penicillin susceptibility and nitrocefin reaction would facilitate more routine testing of the ?-lactamse production in S. aureus clinical isolates.