Nitrogen assimilation by E. coli in the mammalian intestine

Nitrogen is an essential element for all living organisms, including Escherichia coli. Potential nitrogen sources are abundant in the intestine, but knowledge of those used specifically by E. coli to colonize remains limited. Here, we sought to determine the specific nitrogen sources used by E. coli to colonize the streptomycin-treated mouse intestine. We began by investigating whether nitrogen is limiting in the intestine. The NtrBC two-component system upregulates approximately 100 genes in response to nitrogen limitation. We showed that NtrBC is crucial for E. coli colonization, although most genes of the NtrBC regulon are not induced, which indicates that nitrogen is not limiting in the intestine. RNA-seq identified upregulated genes in colonized E. coli involved in transport and catabolism of seven amino acids, dipeptides and tripeptides, purines, pyrimidines, urea, and ethanolamine. Competitive colonization experiments revealed that L-serine, N-acetylneuraminic acid, N-acetylglucosamine, and di- and tripeptides serve as nitrogen sources for E. coli in the intestine. Furthermore, the colonization defect of a L-serine deaminase mutant was rescued by excess nitrogen in the drinking water but not by an excess of carbon and energy, demonstrating that L-serine serves primarily as a nitrogen source. Similar rescue experiments showed that N-acetylneuraminic acid serves as both a carbon and nitrogen source. To a minor extent, aspartate and ammonia also serve as nitrogen sources. Overall, these findings demonstrate that E. coli utilizes multiple nitrogen sources for successful colonization of the mouse intestine, the most important of which is L-serine.

While much is known about the carbon and energy sources that are used by E. coli to colonize the mammalian intestine, very little is known about the sources of nitrogen. Interrogation of colonized E. coli by RNA-seq revealed that nitrogen is not limiting, indicating an abundance of nitrogen sources in the intestine. Pathways for assimilation of nitrogen from several amino acids, dipeptides and tripeptides, purines, pyrimidines, urea, and ethanolamine were induced in mice. Competitive colonization assays confirmed that mutants lacking catabolic pathways for L-serine, N-acetylneuraminic acid, N-acetylglucosamine, and di- and tripeptides had colonization defects. Rescue experiments in mice showed that L-serine serves primarily as a nitrogen source, whereas N-acetylneuraminic acid provides both carbon and nitrogen. Of the many nitrogen assimilation mutants tested, the largest colonization defect was for an L-serine deaminase mutant, which demonstrates L-serine is the most important nitrogen source for colonized E. coli.