Multidrug resistance including borderline-oxacillin resistance and spa typing of Staphylococcus aureus from milk and milk products of Tigray region, Northern Ethiopia.

Abstract

BACKGROUND: Antimicrobial resistance (AMR) is one of the greatest global public health threats, and Staphylococcus aureus is well known for its remarkable ability to acquire resistance. Livestock-associated methicillin-resistant S. aureus (MRSA) is an emerging pathogen epidemiologically linked to foods of animal origin. This study aimed to (i) assess the antimicrobial resistance profile of S. aureus isolates from dairy foods, (ii) determine the presence of MRSA, and (iii) investigate the genetic characteristics of resistant strains by spa typing. RESULTS: A total of 160 genetically confirmed S. aureus isolates were obtained from bulk milk (n = 100) and dairy products (n = 60). Antimicrobial susceptibility testing was performed using the Kirby-Bauer disk diffusion method, and PCR was used to detect the mecA and mecC genes. Spa typing was applied to characterize resistant strains. Overall, 61 (45%) isolates were multidrug-resistant (MDR), showing 35 distinct resistance patterns. Among these, 61% were resistant to five or more antibiotic classes. MDR strains were more common in milk (69%) than in milk products (30%). Thirty-one isolates were classified as borderline oxacillin-resistant S. aureus (BORSA), characterized by cefoxitin resistance and susceptibility to amoxicillin-clavulanic acid, but lacking both mecA and mecC. The highest resistance rates were observed for penicillin G (69.4%), followed by streptomycin (53.1%) and erythromycin (41.9%). Mastitis in cows, antibiotic misuse, and poor hygiene during dairy processing were implicated as sources of MDR strains. Spa typing of MDR and BORSA isolates identified 20 spa types, including three novel sequences. The most prevalent were t314, t458, and t6218. No consistent correlation was observed between spa types and resistance profiles. Of the spa types detected, 12 were unique to milk, four to dairy products, and seven were shared, indicating multiple contamination routes. CONCLUSIONS: This study demonstrated the widespread occurrence of MDR S. aureus and BORSA with diverse spa types in dairy foods. The detection of resistant S. aureus with heterogeneous genetic backgrounds in dairy products highlights a significant public health risk, as resistance genes may spread through the food chain. Strengthened monitoring and control measures are urgently needed to limit the dissemination of AMR.