Impact of Pediococcus acidilactici and tylvalosin on porcine Enterobacterales abundance, antimicrobial resistance and conjugative potential.

Antimicrobial resistance impacts the health of humans, animals, and plants, as well as the metabolic functions supported by environmental microbiomes. Mitigation approaches against this global crisis are being developed to minimise its spread, the associated deaths and economic burden. This study aims to assess the impact of a macrolide, and of a probiotic adopted by pig producers to support reductions in antimicrobial use. Ileal digesta and feces from live pigs with ileal cannulas were collected before and after the administration of Pediococcus acidilactici and/or tylvalosin. Viable antibiotic-resistant enterobacteria were counted on selective media. The detection of Enterobacterales, Lactobacillaceae, β-lactamase genes and plasmid replicons was performed using PCR methods. The propagation potential of resistance to cefotaxime and erythromycin was assessed by conjugation. Pigs receiving P. acidilactici exhibited a significant reduction in the abundances of ileal cefotaxime-resistant Enterobacterales (1.5 logs compared to the control), and a decline in excretion of the blaCMY (3.4 folds) and blaTEM (1.4 folds) genes compared to animals receiving both the probiotic and antibiotic. Still, Enterobacterales from pigs ingesting P. acidilactici exhibited higher transfer frequencies of cefotaxime resistance. These pigs also displayed a significant expansion of the erythromycin-resistant subpopulation in the ileum (1.5 logs), and its decline in feces (2.5 logs). Tylvalosin administration significantly increased the kanamycin-resistant Enterobacterales sub-population (2 logs compared to the control). This study reveals dynamics of bacterial populations, the flux of antimicrobial resistance genes and their interplay with conjugative plasmids, highlighting the complexity of evaluating mitigation approaches.