In Vitro Evaluation of Antimicrobial Drug Activity and Resistance-Associated Genetic Markers in Staphylococcus Aureus: A Comparative Study of Antibiotic Susceptibility and vanA/vanB Genes in Swat, Pakistan

Abstract

Background: Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of hospital infection and vancomycin is a first-line treatment. Vancomycin resistance, usually driven by vanA and vanB genes of enterococcal origin, is rare but rising, and local data from Swat, Pakistan, are scarce.

Methods: In this cross-sectional study carried out at a tertiary care hospital in Swat from February to July 2026, a total of 145 non-duplicated laboratory confirmed samples of Staph aureus were selected for further antimicrobial susceptibility testing by disk diffusion according to CLSI standards. MRSA was defined by cefoxitin resistance. Vancomycin-resistant isolates were screened by PCR for vanA and vanB. Proportions are reported with Wilson 95% confidence intervals, and vancomycin resistance in MRSA and non-MRSA isolates was compared with Fisher's exact test.

Results: Most isolates came from pus (42.76%) and blood (28.97%). Forty-seven isolates (32.41%; 95% CI 25.3 to 40.4) were MRSA. Resistance exceeded 79% for ampicillin, aztreonam, ertapenem, tobramycin, tetracycline, and moxifloxacin, whereas susceptibility remained high for linezolid (98.62%), tigecycline (95.17%), and vancomycin (92.41%). Eight isolates (5.52%; 95% CI 2.8 to 10.5) were vancomycin-resistant, and all eight were MRSA, giving a vancomycin resistance rate of 17.02% (8/47) in MRSA, against none in the 98 non-MRSA isolates (p < 0.001). All eight carried vanA, and five (62.5%) also carried vanB.

Conclusion: One in three S. aureus isolates in this Swat hospital was MRSA, and resistance to conventional drugs was widespread. Vancomycin resistance was confined to MRSA and was linked to vanA, with frequent vanB co-detection that needs sequencing to confirm. MIC-based vancomycin testing, molecular surveillance, and stricter vancomycin stewardship are advisable.