The Impact of Direct-Fed Microbials and Phytogenic Feed Additives on Prevalence and Transfer of Extended-Spectrum Beta-Lactamase Genes in Broiler Chicken

Poultry frequently account for the highest prevalence of extended-spectrum beta-lactamase (ESBL)-producing <i>Enterobacteriaceae</i> in livestock. To investigate the impact of direct-fed microbials (DFM) and phytobiotic feed additives on prevalence and conjugation of extended-spectrum beta-lactamase (ESBL)-producing <i>Enterobacteriaceae</i>, an animal trial was conducted. <i>Lactobacillus agilis</i> LA73 and <i>Lactobacillus salivarius</i> LS1 and two commercial phytogenic feed additives (consisting of carvacrol, cinnamaldehyde, and eugenol) were used as feed additives either alone or as a combination of DFM and phytogenic feed additive. An ESBL-producing <i>E.</i> <i>coli</i> donor and a potentially pathogenic <i>Salmonella</i> Typhimurium recipient were inoculated at 5 &times; 10⁹ cells/mL in cecal contents from 2-week-old broilers. Conjugation frequencies were determined after 4 h aerobic co-incubation at 37 &deg;C and corrected for the impact of the sample matrix on bacterial growth of donor and recipient. Surprisingly, indigenous <i>Enterobacteriaceae</i> acted as recipients instead of the anticipated <i>Salmonella</i> recipient. The observed increase in conjugation frequency was most obvious in the groups fed the combinations of DFM and phytogenic product, but merely up to 0.6 log units. Further, cecal samples were examined for ESBL-producing <i>Enterobacteriaceae</i> on five consecutive days in broilers aged 27&ndash;31 days. All samples derived from animals fed the experimental diet showed lower ESBL-prevalence than the control. It is concluded that <i>Lactobacillus</i> spp. and essential oils may help to reduce the prevalence of ESBL-harboring plasmids in broilers, while the effect on horizontal gene transfer is less obvious.