Antimicrobial susceptibility and virulence phenotypes of Escherichia coli isolated from bovine milk and feces in Brazil.

Abstract

Escherichia coli is a versatile pathogen that causes bovine mastitis and acts as a reservoir of virulence and antimicrobial resistance genes, causing risks to both animal and public health. This study characterized 83 E. coli isolates from milk samples of cows with clinical (n = 19) or subclinical mastitis (n = 33) and fecal samples from lactating cows (n = 31) in Brazil. The isolates were evaluated for phylogenetic group, antimicrobial susceptibility, extended-spectrum β-lactamase (ESBL), plasmid-mediated colistin resistance (mcr), O157 genotype, and biofilm formation. Phylogenetic classification revealed predominance of phylogroups A and B1. Antimicrobial susceptibility testing showed resistance to phenicols (38.6%), β-lactams (27.7%), polymyxins (20.5%), and tetracyclines (20.5%), with multidrug resistance (MDR) observed in 20.5% of isolates, most frequently among clinical mastitis and fecal strains. ESBL production was detected in 47.4% of the isolates evaluated, often concomitant with MDR. All isolates tested negative for the mcr genes, despite 20.5% being colistin-resistant, and were also negative for the O157 antigen. Biofilm formation was widespread, observed in 79.5% of the isolates. These results demonstrated the circulation of diverse antimicrobial-resistant profiles of E. coli strains in Brazilian dairy farms. Taken together, the coexistence of multidrug resistance, ESBL production, and biofilm-forming ability among isolates from mastitic milk and bovine feces indicates that antimicrobial-resistant E. coli strains are present in both clinical and intestinal reservoirs within dairy herds. Furthermore, the detection of colistin-resistant isolates lacking the investigated mcr genes suggests the involvement of alternative resistance mechanisms that warrant further investigation.