Stress-Induced Block in Dicarboxylate Uptake and Utilization in Salmonella enterica Serovar Typhimurium

ABSTRACT

Bacteria have evolved to sense and respond to their environment by altering gene expression and metabolism to promote growth and survival. In this work, we demonstrate that Salmonella displays an extensive (>30-h) lag in growth when subcultured into medium where dicarboxylates such as succinate are the sole carbon source. This growth lag is regulated in part by RpoS, the RssB antiadaptor IraP, translation elongation factor P, and to a lesser degree the stringent response. We also show that small amounts of proline or citrate can trigger early growth in succinate medium and that, at least for proline, this effect requires the multifunctional enzyme/regulator PutA. We demonstrate that activation of RpoS results in the repression of dctA, encoding the primary dicarboxylate importer, and that constitutive expression of dctA induces growth. This dicarboxylate growth lag phenotype is far more severe across multiple Salmonella isolates than it is in its close relative Escherichia coli. Replacing 200 nucleotides of the Salmonella dctA promoter region with that of E. coli was sufficient to eliminate the observed lag in growth. We hypothesized that this cis-regulatory divergence might be an adaptation to Salmonella’s virulent lifestyle, where levels of phagocyte-produced succinate increase in response to bacterial LPS; however, we found that impairing dctA repression had no effect on Salmonella’s survival in acidified succinate or in macrophages.

IMPORTANCE Bacteria have evolved to sense and respond to their environment to maximize their chance of survival. By studying differences in the responses of pathogenic bacteria and closely related nonpathogens, we can gain insight into what environments they encounter inside of an infected host. Here, we demonstrate that Salmonella diverges from its close relative E. coli in its response to dicarboxylates such as the metabolite succinate. We show that this is regulated by stress response proteins and ultimately can be attributed to Salmonella repressing its import of dicarboxylates. Understanding this phenomenon may reveal a novel aspect of the Salmonella virulence cycle, and our characterization of its regulation yields a number of mutant strains that can be used to further study it.