Listeria monocytogenes Relies on the Heme-Regulated Transporter hrtAB to Resist Heme Toxicity and Uses Heme as a Signal to Induce Transcription of lmo1634, Encoding Listeria Adhesion Protein

Abstract

For pathogenic bacteria, host-derived heme represents an important metabolic cofactor and a source for iron. However, high levels of heme are toxic to bacteria. We have previously shown that excess heme has a growth-inhibitory effect on the Gram-positive foodborne pathogen <i>Listeria monocytogenes</i>, and we have learned that the LhrC1-5 family of small RNAs, together with the two-component system (TCS) LisRK, play a role in the adaptation of <i>L. monocytogenes</i> to heme stress conditions. However, a broader knowledge on how this pathogen responds to heme toxicity is still lacking. Here, we analyzed the global transcriptomic response of <i>L. monocytogenes</i> to heme stress. We found that the response of <i>L. monocytogenes</i> to excess heme is multifaceted, involving various strategies acting to minimize the toxic effects of heme. For example, heme exposure triggers the SOS response that deals with DNA damage. In parallel, <i>L. monocytogenes</i> shuts down the transcription of genes involved in heme/iron uptake and utilization. Furthermore, heme stress resulted in a massive increase in the transcription of a putative heme detoxification system, <i>hrtAB</i>, which is highly conserved in Gram-positive bacteria. As expected, we found that the TCS HssRS is required for heme-mediated induction of <i>hrtAB</i> and that a functional heme efflux system is essential for <i>L. monocytogenes</i> to resist heme toxicity. Curiously, the most highly up-regulated gene upon heme stress was <i>lmo1634</i>, encoding the <i>Listeria</i> adhesion protein, LAP, which acts to promote the translocation of <i>L. monocytogenes</i> across the intestinal barrier. Additionally, LAP is predicted to act as a bifunctional acetaldehyde-CoA/alcohol dehydrogenase. Surprisingly, a mutant lacking <i>lmo1634</i> grows well under heme stress conditions, showing that LAP is not required for <i>L. monocytogenes</i> to resist heme toxicity. Likewise, a functional ResDE TCS, which contributes to heme-mediated expression of <i>lmo1634</i>, is not required for the adaptation of <i>L. monocytogenes</i> to heme stress conditions. Collectively, this study provides novel insights into the strategies employed by <i>L. monocytogenes</i> to resist heme toxicity. Our findings indicate that <i>L. monocytogenes</i> is using heme as a host-derived signaling molecule to control the expression of its virulence genes, as exemplified by <i>lmo1634</i>.