The PrfA regulon of Listeria monocytogenes is induced by growth in low-oxygen microaerophilic conditions

<i>Listeria monocytogenes</i> is a food-borne pathogen that must adapt to several environments both inside and outside the host. One such environment is the microaerophilic conditions encountered in the host intestine proximal to the mucosal surface. The aim of this study was to investigate the expression of the PrfA regulon in response to microaerophilic growth conditions in the presence of either glucose or glycerol as a carbon source using four transcriptional (P<i>hly</i>, P<i>actA</i>, P<i>/prfA</i> and P<i>/plcA</i>) gene fusions. Further, RNAseq analysis was used to identify global changes in gene expression during growth in microaerophilic conditions. Following microaerophilic growth, there was a PrfA-dependent increase in transcription from the P<i>hly</i>, P<i>actA</i> and P<i>/plcA</i> promoters, indicating that microaerophilic growth induces the PrfA regulon regardless of the carbon source with increased expression of the PrfA, LLO and ActA proteins. A <i>sigB</i> mutation had no effect on the induction of the PrfA regulon under microaerophilic conditions when glucose was used as a carbon source. In contrast, when glycerol was the carbon source, a <i>sigB</i> mutation increased expression from the P<i>hly</i> and P<i>actA</i> promoters regardless of the level of oxygen. The RNAseq analysis showed that 273 genes were specifically regulated by microaerophilic conditions either up or down including the PrfA regulon virulence factors. Overall, these data indicated that <i>L. monocytogenes</i> PrfA regulon is highly responsive to the low-oxygen conditions likely to be encountered in the small intestine and that SigB has an input into the regulation of the PrfA regulon when glycerol is the sole carbon source.