Seasonal variation in Antimicrobial Resistance Profiles of Escherichia coli Isolates from Dairy Cattle in Puerto Rico

Abstract

Elevated temperatures in Puerto Rico can increase the presence of Escherichia coli in dairy cattle by inducing heat stress, which reduces milk production and compromises the animals’ immune systems. E. coli is a key contributor to mastitis and foodborne illness outbreaks. In this study, isolates were recovered from clinical mastitis cases, which lead to decreased milk quality and economic losses in the dairy industry. The widespread use of antibiotics to treat mastitis contributes to antimicrobial resistance (AMR). This study evaluated E. coli transmission and AMR gene profiles across seasons in Puerto Rico, with a focus on mitigating AMR. Fifty-one milk samples with high somatic cell counts were collected from dairy farms across different regions during the hot and cold seasons. Samples were pre-enriched and cultured on selective media, including MacConkey Agar and CHROMAgar (ESBL). Biochemical tests and morphological analyses confirmed E. coli identification, followed by sequencing and bioinformatic analysis to determine AMR gene profiles. E. coli was identified in 27/64 (42%) of the samples collected during the hot season, compared to 24/62 (39%) during the cold season. The phylogenetic tree showed a diversity of serogroups and sequence types among raw milk isolates. Bioinformatic analysis revealed that the blaEC-18 gene had the highest detection frequency across both seasons. Seasonal variation was observed, with more AMR genes identified in hot season samples, including tet(A), tet(B), sul1, sul2, aph(3')-Ia, aph(3'')-Ib, and aph(6)-Id, compared to the cold season. These findings underscore the influence of seasonal stressors on the prevalence and diversity of antimicrobial-resistant E. coli in dairy cattle and emphasize the need for targeted surveillance and intervention strategies to reduce AMR transmission within the dairy supply chain.