Phenotypic determination of methicillin-resistant Staphylococcus aureus in Aminu Kano Teaching Hospital, Kano, Nigeria

Background: Phenotypic detection of methicillin-resistant Staphylococcus aureus(MRSA) has been problematic ever since its discovery in the early 1960s. The emergence of low-level resistant MRSA clones acquired in the community has only added to these difficulties. In 2005, the Clinical and Laboratory Standards Institute (CLSI) published zone diameter (10), breakpoint guidelines for cefoxitin. However, a number of technical issues remain regarding the use of cefoxitin as a predictor for methicillin resistance. Materials and Methods: In these studies, 252 nonduplicate samples of staphylococcal isolates were collected from various clinical samples obtained from patients attending Aminu Kano Teaching Hospital. The isolates were subcultured and identified using standard bacteriological procedure according to CLSI (13). Antibiotic susceptibility testing was done using a modified form of the Kirby–Bauer method. Methicillin resistance was screened using disk-diffusion method with cefoxitin 30 μg and oxacillin 1 μg. Results: High percentage of the isolates were recovered from patients of age groups of 1–9 years and <1 year with 45.2% and 23.4%, respectively. About 77% isolates were obtained from blood culture followed by wound (11.5%) and ear swab (6.7%). MRSA prevalence of 20.6% and 25.8% was obtained in this study using cefoxitin (30 μg) and oxacillin (1 μg), respectively. High prevalence of MRSA was obtained from people of the old age group which may be due to used and misused of antibiotics. From the 252 isolates obtained in this study, 84.1%, 77.4%, and 77.0% were found to be susceptible to ciprofloxacin, gentamicin, and clindamycin, respectively. The least susceptible was found 49.2%, 52.0%, and 62.7% in erythromycin, co-trimoxazole, and tetracycline, respectively. Conclusion: The study revealed that routine phenotypic screening of MRSA gives a better result when both oxacillin and cefoxitin were used, especially in resource-limited areas where molecular analysis is not available.