Prior Antibiotic Exposure and Risk of Nasal Methicillin-Resistant Staphylococcus aureus Colonisation in a Nigerian Tertiary Hospital

Background: Methicillin-resistant Staphylococcus aureus (MRSA) remains an important cause of healthcare-associated infections, yet evidence linking specific antibiotic exposure patterns with MRSA colonisation in sub-Saharan Africa is limited. We investigated the association between prior antibiotic exposure, classified using the World Health Organization (WHO) Access, Watch and Reserve (AWaRe) framework, and nasal MRSA colonisation among hospitalised patients in Nigeria.

Methods: We conducted a cross-sectional study among 400 consecutively admitted patients at Babcock University Teaching Hospital, Ogun State, Nigeria, between June 2023 and June 2024. Nasal swabs were collected within 48 hours of admission and cultured for Staphylococcus aureus. MRSA was identified using cefoxitin susceptibility testing according to Clinical and Laboratory Standards Institute guidelines. Antibiotic exposure during the preceding 30 days was obtained from medical records and patient interviews and classified according to the WHO AWaRe framework. Associations between antibiotic exposure and MRSA colonisation were evaluated using odds ratios (ORs) with 95% confidence intervals (CIs).

Findings: Thirty-six participants were colonised with MRSA, giving a prevalence of 9.0% (36/400). Exposure to Watch antibiotics was strongly associated with MRSA colonisation (OR 8.99, 95% CI 2.12–38.14), whereas Access antibiotic exposure showed no significant association (OR 0.99, 95% CI 0.47–2.08). Among individual antibiotic classes, fluoroquinolones (OR 6.74, 95% CI 3.33–13.65), cephalosporins (OR 5.85, 95% CI 2.19–15.64), and penicillins (OR 2.35, 95% CI 1.17–4.72) were associated with increased MRSA colonisation. A clear dose–response relationship was observed, with MRSA prevalence increasing from 1.6% among patients without exposure to fluoroquinolones or cephalosporins to 42.9% among those exposed to three or more high-risk antibiotic classes (χ² for trend = 28.45; p<0.0001). Patients exposed to combined cephalosporin and fluoroquinolone therapy had the highest odds of MRSA colonisation (OR 50.83, 95% CI 10.26–251.9).

Interpretation: Recent exposure to WHO Watch antibiotics, particularly fluoroquinolones and cephalosporins, was strongly associated with nasal MRSA colonisation among hospitalised patients. The observed dose–response relationship and markedly increased risk associated with combined high-risk antibiotic exposure support antimicrobial stewardship strategies that prioritise optimisation of Watch antibiotic prescribing. These findings provide evidence to inform antibiotic stewardship and infection prevention programmes in resource-limited healthcare settings.