Antibiotic degradation by commensal microbes shields pathogens
The complex bacterial populations that constitute the gut microbiota can harbor antibiotic-resistance genes (ARGs), including those encoding for {beta}-lactamase enzymes (BLA), which degrade commonly prescribed antibiotics such as ampicillin. While it is known that ARGs can be transferred between bacterial species, with dramatic public health implications, whether expression of such genes by harmless commensal bacterial species shields antibiotic-sensitive pathogens in trans by destroying antibiotics in the intestinal lumen is unknown. To address this question, we colonized GF mice with a model intestinal commensal strain of E. coli that produces either functional or defective BLA. Mice were subsequently infected with Listeria monocytogenes or Clostridioides difficile followed by treatment with oral ampicillin. Production of functional BLA by commensal E. coli markedly reduced clearance of these pathogens and enhanced systemic dissemination during ampicillin treatment. Pathogen resistance was independent of ARG acquisition via horizontal gene transfer but instead relied on antibiotic degradation in the intestinal lumen by BLA. We conclude that commensal bacteria that have acquired ARGs can mediate shielding of pathogens from the bactericidal effects of antibiotics.