Molecular detection of antibiotic resistance genes in Staphylococcus aureus isolated from poultry farms of Kathmandu Valley, Nepal

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BACKGROUND: Antimicrobial resistance (AMR) has emerged as a critical global health challenge and is currently addressed through a "One Health" approach that investigates its occurrence in humans, animals, and the environment, as well as the transmission pathways linking these reservoirs. In commercial poultry farms, antibiotics are routinely used to treat bacterial infections that are economically significant as well as to promote weight gain. However, data on AMR in Staphylococcus aureus (S. aureus) of poultry origin remain scarce in Nepal. Antimicrobial resistance in S aureus, particularly the emergence of methicillin-resistant (MRSA) and vancomycin-resistant (VRSA) strains in both humans and poultry, poses a significant public health threat worldwide requiring immediate attention. This study aimed to detect the prevalence of multidrug-resistant S. aureus and the distribution of tetA, tetB, ermA, ermB, and ermC genes among S. aureus from litter and soil of poultry farms of Kathmandu Valley. METHODS: Litter and soil samples were processed using serial dilution and spread plate techniques to isolate S. aureus. Antibiotic susceptibility testing was assessed by a modified Kirby-Bauer disc diffusion method. DNA was extracted from the isolates, and polymerase chain reaction (PCR) was performed to detect resistance genes (tetA, tetB, ermA, ermB, and ermC). RESULTS: A total of 32 S. aureus were obtained, comprising 20 isolates from litter and 12 isolates from the soil of poultry farms. A total of 59.3% (19/32) of S. aureus isolates were multidrug-resistant (MDR); from which 65% (13/20) were from litter and 50% (6/12) from soil. A Chi-square test revealed no statistically significant association between MDR isolates and the source (litter vs. soil) (p > 0.05). The antibiotic susceptibility results showed high degree of resistance towards erythromycin (68.8%) followed by tetracycline (59.4%). In PCR analysis, the majority of isolates (96.8%) showed the tetA gene, while none of the isolates showed the tetB gene. Only 15 and 11 isolates out of 32 showed the ermB (46.8%) and ermC (34.3%) genes, respectively, while all isolates tested negative for ermA. Additionally, 5 S. aureus isolates carried both ermB and ermC genes. Chi-square analysis showed no significant association between AMR gene occurrence and the source of S. aureus isolates. CONCLUSION: The high incidence of multidrug-resistant S. aureus and detection of antibiotic resistance genes in poultry litter and soil highlight a significant risk of transmission to farm workers, butchers, and consumers, as well as possible environmental contamination through manure application. These findings underscore the urgent need for strict antimicrobial stewardship, improved biosecurity, and policy measures to prevent the spread of resistant S. aureus from poultry farms to the wider community.

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