Effects of dairy processing on antibiotic resistance genes in milk and associated changes in the murine gut resistome.

This study evaluated the effects of dairy processing on antibiotic resistance genes (ARGs) in milk and examined whether pasteurized milk exposure is associated with changes in the murine gut resistome. Raw milk was subjected to pasteurization (63°C, 30 min), microwave treatment, high-pressure processing, spray drying, or lactic acid fermentation. Microbial enumeration, metagenomic sequencing, and quantitative PCR were used to assess bacterial communities and ARG abundance. Mice were orally administered an ARG-carrying Escherichia coli strain or pasteurized milk for 4 weeks to determine alterations in gut microbial composition and ARG profiles. Non-fermentation processing treatments reduced culturable bacterial counts, whereas most sequencing-detected ARGs showed limited changes in relative abundance across thermal, microwave, high-pressure, and spray-dried treatments. Lactic acid fermentation increased the relative abundance of several ARGs, including Erm(K), vanT, and tetA, concurrent with dominance of fermentative taxa. In mice, administration of ARG-carrying Escherichia coli increased multiple gut ARGs, including β-lactam and quinolone resistance genes. Pasteurized milk intake was associated with changes in gut microbial composition and relative abundance of selected ARGs. These findings indicate that dairy processing reduced viable bacteria but did not fully eliminate detectable ARG signals. Pasteurized milk exposure was associated with gut resistome shifts in mice, although causality was not established.