Molecular characterization of antibiotic-resistant Staphylococcus aureus in an intensive pig production system in KwaZulu-Natal, South Africa.

Abstract

The increase in antibiotic resistance in food animals and food of animal origin has been attributed to the extensive use of antibiotics during animal husbandry giving rise to multidrug-resistant bacteria. Staphylococcus aureus is a major threat in veterinary medicine, the agricultural sector and public health because of its zoonotic potential. Despite significant research on S. aureus in food animals in other parts of the world, in-depth studies outside healthcare facilities are limited in South Africa. This study characterized the molecular epidemiology of antibiotic resistant S. aureus from farm-to-fork in an intensive pig production chain in the uMgungundlovu district, Kwa-Zulu Natal, South Africa. A total of 333 samples collected along a pig production chain on the farm (faecal, litter and slurry samples) during transport (truck samples) and at the abattoir (caeca, carcass swabs, carcass rinsate and retail meat samples) were investigated for the presence S. aureus using selective media and biochemical tests. Confirmation was done by using PCR targeting the nucA gene. Antibiotic susceptibility patterns were investigated by the Kirby Bauer disk diffusion according to CLSI guidelines against the WHO-AGISAR recommended panel of antibiotics. Selected resistance and virulence genes were detected using PCR. REPPCR was used to evaluate the molecular relatedness of isolates across the pig production chain. Of the 333 samples, 141 (43%) yielded staphylococci isolates. After molecular confirmation, 97(69%) isolates were confirmed S. aureus and 44(31%) as other staphylococcal species. Isolates displayed resistance to erythromycin (85%), clindamycin (85%), penicillin-G (81%), tetracycline (79%), doxycycline (77%), vancomycin (69%), ampicillin (61%), trimethoprim/sulfamethoxazole (57%), rifampicin (57%), teicoplanin (52%), linezolid (51%), chloramphenicol (51%), nitrofurantoin (47%), moxifloxacin (33%), cefoxitin (20%), ciprofloxacin (15%), tigecycline (10%), levofloxacin (8%), gentamicin (8%), and amikacin (2%). Multidrug resistance (MDR) was recorded in 84% (80/97) of isolates with 56 different antibiograms. Resistance genes

ermC, blaZ, tetK, tetM, msrA, aac’6, mecA were evident in 82%, 73%, 58%, 28%, 15%, 5%, and 53% respectively and not all resistance phenotypes were genotypically confirmed. The hla (39%), hld (23%), seb (3%), sed (2%), etb (1%), LukS/F-PV (30%) and tst (11%) virulence genes encoding hemolysin, cytotoxins, staphylococcal enterotoxins (sea and seb), exfoliative toxins, PVL pore-forming toxin and toxic shock syndrome toxin-1 were detected. Genetic fingerprinting revealed the diversity of MRSA isolates in the pig production chain with the major REP-types constituting isolates from different sources within the farm, suggesting transmission within the farm environment with no evidence of transmission across the production chain. This study highlights the phenotypic and genotypic diversity of the virulence and resistance profiles of S. aureus isolated across the pig production chain. Resistance to antibiotics used as growth promoters was evident and the high prevalence of MDR isolates with elevated MAR index values >0.2, specifically at farm level indicates exposure to environments of high antibiotic use, necessitating antibiotic stewardship and proper infection control measures in pig husbandry and intensive pig production.