Amino acid competition shapes Acinetobacter baumannii gut carriage

Antimicrobial resistant (AMR) infections are a critical threat to human health. Asymptomatic colonization is often critical for persistence of AMR pathogens. Gut colonization by the AMR priority pathogen Acinetobacter baumannii is associated with increased risk of clinical infection. Ecological factors shaping A. baumannii colonization and mechanisms of competition with the resident microbiota remain unclear. Here we show that A. baumannii encodes a second ast operon to utilize ornithine, a non-preferred carbon source, to compete with the resident microbiota and persist in the gut. Supplemental dietary ornithine promotes long-term fecal shedding of A. baumannii. By contrast, supplementation of a preferred carbon source-monosodium glutamate (MSG)-abolishes the requirement for A. baumannii ornithine catabolism. We report evidence for diet promoting asymptomatic A. baumannii gut colonization in humans. Together, these results highlight that evolution of ornithine catabolism allows A. baumannii to compete with the microbiota in the gut, a reservoir for pathogen spread.