Novel aadA5 and dfrA17 variants of class 1 integron in multidrug-resistant Escherichia coli causing bovine mastitis

Mobile genetic elements (MGEs) are associated with the emergence of multidrug resistance in extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae. This study explores the role of class 1 integrons and IS26 elements in breaching taxonomic barriers. A total of 110 E. coli bacteria were isolated from 300 clinical mastitis milk samples. The 98% E. coli isolates were extended-spectrum beta-lactamase- producers. About 83% of these isolates carried co-resistance for fluoroquinolones. The co-existence of (extended-spectrum beta-lactamase + quinolone resistance determining region mutations) and (extended-spectrum beta-lactamase + plasmid-mediated quinolone resistance genes) was found in 76% and 44% of isolates, respectively. The MGEs were detected in 88% of isolates with IS26 in 82% and class 1 integrase in 40% of isolates. The types of class 1 integron gene cassettes detected includes dfrA7, (dfrA17 + aadA5), and (dfrA1 + aadA1). We discovered 2 and 4 novel variants of the dfrA17 and aadA5 genes, respectively. We report a variant of aadA5 with mutation E235G in the Indian subcontinent earlier reported only in a human clinical isolate from Belgium. About 19 isolates carried IS26 linked to integrase gene intI1 with an internal deletion of 265 bp in the 5`CS of integrase gene intI1, earlier reported only in E. coli ST131 isolates from human clinical, wastewater samples. This study suggests intercontinental dissemination of antibiotic resistant genes (ARGs) across different microbiomes via mobile genetic elements. KEY POINTS: • The role of mobile genetic elements in the emergence of multidrug-resistant E. coli in bovine mastitis. • Novel variants of the aadA5 (aminoglycoside adenyl transferase) and dfrA17 (dihydrofolate reductase) genes were identified in pathogenic E. coli isolated from bovine mastitis in class 1 integron gene cassette. • Sequence analysis of mobile genetic components revealed the physical connection between IS26 and intI1 genes with an internal deletion in 5'CS of class 1 integrase.