Full manuscript: Comparative Analysis of AMR Index Between Methicillin-Resistant and Methicillin-Sensitive Staphylococcus aureus in Tertiary Health Facilities of Northwestern Nigeria

Abstract

Antimicrobial resistance (AMR) in Staphylococcus aureus poses a significant public health challenge, particularly in resource-limited settings. The methicillin-resistant S. aureus (MRSA) strain is particularly notorious for multidrug resistance compared to methicillin-sensitive S. aureus (MSSA). In clinical microbiology, the AMR Index has been particularly useful in evaluating resistance trends in high-priority pathogens, such as S. aureus. This study aimed to determine the AMR index of various clinical bacterial isolates and compare the index between resistant and susceptible strains. A cross-sectional hospital-based study was conducted using S. aureus isolates recovered from clinical samples processed in selected tertiary hospitals of Northwestern Nigeria. Phenotypic identification and antimicrobial susceptibility testing were performed using the modified Kirby-Bauer disk diffusion method according to standard CLSI guidelines. Methicillin resistance was determined using cefoxitin disc diffusion, and isolates were categorized into MRSA and MSSA groups. The AMR index was calculated as the ratio of the number of antibiotics to which an isolate was resistant to the total number tested. The result of this study indicated MRSA isolates demonstrated higher resistance indices for five antibiotics: gentamicin (0.2202 vs. 0.1872), ciprofloxacin (0.4107 vs. 0.3227), erythromycin (0.3036 vs. 0.2709), tetracycline (0.2679 vs. 0.2291), and clindamycin (0.2321 vs. 0.1798), compared to MSSA. This pattern underscores the broader multidrug-resistant nature of MRSA. Conversely, MSSA exhibited higher resistance to chloramphenicol (1.2808 vs. 0.9940) and trimethoprim/sulfamethoxazole (0.4039 vs. 0.3631) than MRSA, which may be attributed to local antimicrobial usage patterns or the acquisition of specific resistance determinants through horizontal gene transfer. Overall, the elevated resistance indices in MRSA emphasize the clinical challenge it poses and reinforce the need for judicious antibiotic use, robust antimicrobial stewardship, and continuous surveillance. The AMR index remains a practical and informative metric for tracking resistance trends and guiding targeted interventions in healthcare settings. The study highlights notable differences in the antimicrobial resistance profiles of MRSA and MSSA in Northwestern Nigeria. Continuous surveillance and targeted antimicrobial stewardship interventions are essential to mitigate the burden of AMR in healthcare settings.