Investigation of high-level aminoglycoside resistance in Enterococcus spp. isolated from Caspian horse feces in Guilan province, northern Iran

Background and Aim: Due to the widespread use of antimicrobials in veterinary medicine and the increase in livestock production, it seems that the risk of spreading antibiotic resistance in human societies is more related to animals and the veterinary field. In this study, antibiotic resistance, high-level aminoglycoside resistance and frequency of aminoglycoside resistance associated genes among Enterococcus spp. isolated from Caspian horse feces were studied. Materials and Methods: One year period during 2021-2022, Enterococcus spp. were isolated from a total of 140 Caspian horse fecal samples and identified by biochemical tests. The disk diffusion and determination of minimum inhibitory concentration by broth microdilution method were used to examine the antibiotic resistance of isolates and PCR method was used to determine the frequency of four aminoglycoside resistance associated genes. The data were analyzed using SPSS software and the Chi-square test. Results: A total of 100 Enterococcus spp. was isolated from equine fecal samples. Antibacterial susceptibility testing revealed that 72% of isolates were resistant to at least one tested aminoglycoside and 38% of isolates showed high-level gentamicin and/or streptomycin resistance phenotype. Bifunctional gene aac(6′)-Ie-aph(2″)-Ia was the most common resistance associated gene and detected in 73.61% (53/72) isolates. The frequency of aph (3´)-IIIa, aph(2'')-Ib and ant(6)Ia was 69.44%, 48.61% and 70.83% respectively, among aminoglycoside resistant isolates. Frequencies of aminoglycoside resistance genes were higher in biofilm-positive isolates (P<0.05). Conclusion: Our findings demonstrate the presence of multi aminoglycoside resistance encoding genes in Enterococcus spp., isolated from equine normal microflora in northern Iran and support the ongoing concern about proper monitoring of antimicrobial use in veterinary.