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Enhanced nutrient uptake is sufficient to drive emergent cross-feeding between bacteria in a synthetic community

By Ryan K Fritts, Jordan T Bird, Megan G Behringer, Anna Lipzen, Joel Martin, Michael Lynch, James B McKinlay

Posted 16 Sep 2019
bioRxiv DOI: 10.1101/770727 (published DOI: 10.1038/s41396-020-00737-5)

Interactive microbial communities are ubiquitous, influencing biogeochemical cycles and host health. One widespread interaction is nutrient exchange, or cross-feeding, wherein metabolites are transferred between microbes. Some cross-fed metabolites, such as vitamins, amino acids, and ammonium (NH4+), are communally valuable and impose a cost on the producer. The mechanisms that enforce cross-feeding of communally valuable metabolites are not fully understood. Previously we engineered mutualistic cross-feeding between N2-fixing Rhodopseudomonas palustris and fermentative Escherichia coli . Engineered R. palustris excreted essential nitrogen as NH4+ to E. coli while E. coli excreted essential carbon as fermentation products to R. palustris . Here, we enriched for nascent cross-feeding in cocultures with wild-type R. palustris , not known to excrete NH4+. Emergent NH4+ cross-feeding was driven by adaptation of E. coli alone. A missense mutation in E. coli NtrC, a regulator of nitrogen scavenging, resulted in constitutive activation of an NH4+ transporter. This activity likely allowed E. coli to subsist on the small amount of leaked NH4+ and better reciprocate through elevated excretion of organic acids from a larger E. coli population. Our results indicate that enhanced nutrient uptake by recipients, rather than increased excretion by producers, is an underappreciated yet possibly prevalent mechanism by which cross-feeding can emerge.

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