Characterization of Listeria monocytogenes isolates from lactating dairy cows in a Wisconsin farm: antibiotic resistance, mammalian cell infection, and impacts on the fecal microbiota

Abstract

Listeria monocytogenes is an invasive foodborne pathogen that is ubiquitously present in the dairy farm environment. Although cattle are a reservoir of L. monocytogenes, most adult animals do not exhibit clinical symptoms, suggesting a homeostasis between this pathogen and the bovine gastrointestinal ecosystem. Nevertheless, substantial prevalence of L. monocytogenes fecal shedding by dairy cattle has been reported in many studies, posing threats of transmission within the herd and contamination of the human food supply. Accordingly, understanding the L. monocytogenes ecology within the bovine gastrointestinal tract is important to prevent clinical illness in the animal host, reduce transmission, and guide intervention strategies. In this study, we conducted a longitudinal sampling of fecal samples from twenty lactating dairy cows in one Wisconsin farm over a 29-day period, and found a strikingly high incidence of L. monocytogenes shedding, in 90% of sampled animals. The L. monocytogenes isolates were genetically diverse, representing all common serotypes previously identified from cattle. Additionally, most tested isolates were resistant to ampicillin, and a few were also resistant to gentamicin or trimethoprim/sulfamethoxazole. Most isolates effectively infected human epithelial cells (Caco-2) and murine fibroblasts (L2), suggesting that they are all capable of causing systemic infection if the intestinal barrier is breached. Finally, we investigated the impact of L. monocytogenes colonization on the gastrointestinal tract microbiota, by analyzing the fecal bacterial communities of some shedding and non-shedding cows. Whereas L. monocytogenes did not impact the alpha and beta diversity of tested animals, a subset of shedding cows exhibited different abundances of certain operational taxonomic units within the Bacteroidetes and Firmicutes phyla compared to non-shedding cows. Overall, our findings highlight the threat of antibiotic resistance among some L. monocytogenes isolates, the need for a strain-specific approach in listeriosis treatment, and suggest the potential negative impacts of asymptomatic L. monocytogenes carriage on animal gut health.