Biology subjects

Hersch, S. J.

Publications and source records attributed to Hersch, S. J..

2 recordsLinked to original sources

bioRxiv2019

Salmonella stress response mediates growth repression in succinate media

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 hour) lag in growth when subcultured into media where dicarboxylates such as succinate are the sole carbon source. This growth lag is regulated in part by RpoS, the RssB anti-adaptor 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 media 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 induced growth. This dicarboxylate growth lag phenotype is far more severe across multiple Salmonella isolates than in its close relative E. coli. Replacing 200 nt of the Salmonella dctA promoter region with that of E. coli was sufficient to eliminate the observed lag in growth. We hypothesize that this cis-regulatory divergence might be an adaptation to Salmonellas virulent lifestyle where levels of phagocyte-produced succinate increase in response to bacterial LPS. We found that impairing dctA repression had no effect on Salmonellas survival in acidified succinate or in macrophage but propose alternate hypotheses of fitness advantages acquired by repressing dicarboxylate uptake. ImportanceBacteria 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 non-pathogens, 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.

microbiology↗

bioRxiv2019

Salmonella ItaR responds to itaconate in macrophage

Itaconate is a dicarboxylic acid that inhibits the isocitrate lyase enzyme of the bacterial glyoxylate shunt. Activated macrophages have been shown to produce itaconate, suggesting that these immune cells may employ this metabolite as a weapon against invading bacteria. Here we demonstrate that, in vitro, itaconate can exhibit bactericidal effects under acidic conditions similar to the pH of a macrophage phagosome. In parallel, successful pathogens including Salmonella have acquired a genetic operon encoding itaconate degradation proteins, which are induced heavily in macrophages. We characterize the regulation of this operon by the neighbouring gene, ripR, in specific response to itaconate. Moreover, we develop an itaconate biosensor based on the operon promoter that can detect itaconate in a semi-quantitative manner and, when combined with the ripR gene, is sufficient for itaconate-regulated expression in E. coli. Using this biosensor with fluorescence microscopy, we observe bacteria responding to itaconate in the phagosomes of macrophage and provide additional evidence that interferon-{gamma} stimulates macrophage itaconate synthesis and that J774 mouse macrophages produce substantially more itaconate than the human THP-1 monocyte cell line. In summary, we examine the role of itaconate as an antibacterial metabolite in mouse and human macrophage, characterize the regulation of Salmonellas defense against it, and develop it as a convenient itaconate biosensor and inducible promoter system. ImportanceIn response to invading bacteria, immune cells can produce a molecule called itaconate, which can inhibit microbial metabolism. Here we show that itaconate can also directly kill Salmonella when combined with moderate acidity, further supporting itaconates role as an antibacterial weapon. We also discover how Salmonella recognizes itaconate and activates a defense to degrade it, and we harness this response to make a biosensor that detects the presence of itaconate. This biosensor is versatile, working in Salmonella enterica or lab strains of Escherichia coli, and can detect itaconate quantitatively in the environment and in immune cells. By understanding how immune cells kill bacteria and how the microbes defend themselves, we can better develop novel antibiotics to inhibit pathogens such as Salmonella.

microbiology↗