Members of the gut microbiome originate from external sources such as the environment, food and other individuals, but how individual microbial constituents regulate their transmission into new hosts is not well-explored. In a recent study, Robinson et al. uncover that sensing of host amino acids in Aeromonas veronii regulates bacterial motility, enhancing gut colonization of zebrafish. Using a tractable experimental evolution model in gnotobiotic zebrafish, the authors identified traits in A. veronii that promoted host colonization, including mutations in a gene that encodes an amino-acid-sensing diguanylate cyclase (spdE). Interestingly, loss of spdE in A. veronii led to rapid immigration into zebrafish and fish-to-fish transmission. The authors found that SpdE predominantly recognizes the amino acid proline as well as valine and isoleucine, leading to a reduction in the synthesis of cyclic-di-GMP, an intracellular second messenger that regulates bacterial motility. Notably, binding of amino acids to SpdE increased bacterial motility through chemotaxis and chemokinesis, and bolstered host colonization. The authors also observed that bacterial colonization and SpdE-dependent motility was regulated by the resident microbiota, as bacterial collagenolytic activity can liberate SpdE ligands from the host, thus increasing bacterial motility and enhancing host transmission.