Cross-feeding of amino acid pathway intermediates is common in co-cultures of auxotrophic Escherichia coli

Abstract

Amino acid auxotrophy refers to an organism's inability to synthesize one or more amino acids that are required for cell growth. In microbiome research, co-cultures of amino acid auxotrophs are often used to investigate metabolite cross-feeding interactions and model community dynamics. Thus far, it has been implicitly assumed that amino acids are mainly cross-fed between these auxotrophs. However, this assumption has not been fully verified. For example, it could be that intermediates of amino acid biosynthesis pathways are exchanged instead, or in addition to amino acids. If true, this would significantly increase the complexity of metabolic interactions that needs to be considered. Here, we show that metabolic pathway intermediates are indeed exchanged in many co-cultures of amino acid auxotrophs. To demonstrate this, we selected 25 E. coli single gene knockouts that are auxotrophic for five different amino acids: arginine, histidine, isoleucine, proline, and tryptophan. In co-culture experiments, we paired strains that shared the same amino acid auxotrophy and monitored cell growth. We observed growth in 23 out of 55 strain pairings, indicating that pathway intermediates were exchanged between the strains. To provide further support for cross-feeding of pathway intermediates, auxotrophic E. coli strains were cultured in media supplemented with commercially available metabolic pathway intermediates at different concentrations. Supplementing media with these metabolites recovered cell growth as was predicted from the co-culture experiments. Most of these metabolites supported high growth rates, even when present at low concentrations (10 μM), suggesting the presence of high affinity transporters for these metabolites. In total, we identified eight metabolic pathway intermediates that were likely exchanged between the auxotrophic E. coli strains and verified six of these, including histidinol, N-acetyl-L-ornithine, L-ornithine, L-citrulline, keto-isoleucine and anthranilate. Taken together, this work demonstrates that exchange of metabolic pathway intermediates is more common than has been assumed so far. In future, these exchanges must be explicitly considered when constructing models of metabolite cross-feeding interactions in microbial communities and when interpreting results from microbiome studies involving auxotrophic strains.