Serine Deamination as a New Acid Resistance Mechanism in Escherichia coli

Abstract

Escherichia coli associates with humans early in life and can occupy several body niches either as a commensal in the gut and vagina, or as a pathogen in the urinary tract. As such, E. coli has an arsenal of acid response mechanisms that allow it to withstand the different levels of acid stress encountered within and outside the host. Here, we report the discovery of an additional acid response mechanism that involves the deamination of L-serine to pyruvate by the conserved L-serine deaminases SdaA and SdaB. L-serine is the first amino acid to be imported in E. coli during growth in laboratory media, as the culture senesces. However, there remains a lack in knowledge as to why L-serine is preferred and how it is utilized. We show that in acidified media, L-serine is brought into the cell via the SdaC transporter and deletion of both SdaA and SdaB renders E. coli susceptible to acid stress, with a phenotype similar to other acid stress deletion mutants. We also show that the pyruvate produced by L-serine de-amination activates the pyruvate sensor BtsS, which in concert with the non-cognate response regulator YpdB upregulates the putative transporter YhjX, similar to what has been reported for this system during transition of E. coli to stationary phase. Based on these observations, we propose that L-serine deamination constitutes another acid response mechanism in E. coli that may function to protect E. coli as it transitions to stationary phase of growth. IMPORTANCEThe observation that L-serine uptake occurs as an E. coli culture senesces is well-established, yet the benefit E. coli garners from this uptake remains unclear. Here, we report a novel acid resistance mechanism, where L-serine is deaminated to pyruvate and ammonia, promoting acid tolerance in E. coli. This study is important as it provides evidence of the use of L-serine as an acid response strategy, not previously reported for E. coli.