503 ISSN 0372-5480 Printed in Croatia VETERINARSKI ARHIV 93 (5), 503-512, 2023 DOI: 10.24099/vet.arhiv.1814 Phenotypic and genotypic investigation of antimicrobial resistance and extended-spectrum beta-lactamase production among Escherichia coli isolated from bovine mastitis Fatemeh Aflakian1 , Niloufar Mohseni1, Maryam Hafiz1, Helia Nikoueian1, Mahdi Askari Badouei1 and Abolfazl Rafati Zomorodi2,3* 1Department of Pathobiology, School of Veterinary Medicine, Ferdowsi University of Mashhad, Mashhad, Iran. 2 Department of Bacteriology and Virology, Shiraz Medical School, Shiraz University of Medical Sciences, Shiraz, Iran. 3Student Research Committee, Shiraz University of Medical Sciences, Shiraz, Iran. AFLAKIAN, F., N. MOHSENI, M. HAFIZ, H. NIKOUEIAN, M. A. BADOUEI, A. R. ZOMORODI: Phenotypic and genotypic investigation of antimicrobial resistance and extended-spectrum beta-lactamase production among Escherichia coli isolated from bovine mastitis. Vet. arhiv 93, 503-512 2023. ABSTRACT Escherichia coli has been one of the most highlighted pathogens isolated from clinical cases of mastitis. Unfortunately, the deficiency of appropriate antimicrobial stewardship and overuse of antimicrobial agents have increased antimicrobial resistance in animal isolates. The current study aimed to demonstrate the prevalence of extended-spectrum beta-lactamase (ESBL) production, resistance genes, and multi drug resistant (MDR) among E. coli isolates from cases of clinical mastitis. For this purpose, 40 E. coli isolates were collected from clinical mastitis milk samples from five farms in Mashhad, Iran. The antimicrobial susceptibility testing (AST) was applied to 15 antimicrobial agents of veterinary and human medicine interest. Also, ESBL production was evaluated using a double- disc synergy test (DDST). The distribution of 20 resistance genes was sought among E. coli isolates by six multiplex- PCR and three uniplex-PCR assays. The highest sensitivity was identified against imipenem and amikacin (100%). On the other hand, the highest resistance was observed for tetracycline and trimethoprim-sulfamethoxazole (70% - 72.5%), respectively. According to the AST and DDST tests, one isolate was confirmed as ESBL-producing and MDR. In addition, the most frequent resistance genes were blaTEM and AmpC (100% each). The qnrA encoding resistance to quinolones was similarly prevalent and detected in 50% of the isolates. In conclusion, at least three resistant genes were detected in 28 isolates (70%), but the majority of isolates were sensitive against most of the tested antibiotics. This fact might relate to the low expression of these genes within the isolates. The horizontal gene transfer of the present genes may confer resistance to other related bacterial species in humans or domestic animals.

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