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Background  The saprophytic pathogen  Listeria monocytogenes  has to cope with a variety of acidic habitats during its life cycle. The impact of low-temperature coupled with pH decrease for global gene expression and subsequent virulence properties, however, has not been elucidated.    Results  qRT-PCR revealed for the first time a transient, acid triggered  prfA  induction of approximately 4-fold, 5.7-fold, 7-fold and 9.3-fold 60 to 90 min after acid shock of  L. monocytogenes  at 37°C, 25°C, 18°C, and 10°C, respectively. Comparable data were obtained for seven different  L. monocytogenes  strains, demonstrating that  prfA  induction under these conditions is a general response of  L. monocytogenes . Transcriptome analysis revealed that the  in vivo -relevant genes  bsh ,  clpP, glpD, hfq, inlA, inlB ,  inlE ,  lisR,  and  lplA1  as well as many other genes with a putative role during infection are transiently induced upon acid shock conducted at 25°C and 37°C. Twenty-five genes repressed upon acid shock are known to be down regulated during intracellular growth or by virulence regulators. These data were confirmed by qRT-PCR of twelve differentially regulated genes and by the identification of acid shock-induced genes influenced by σ B . To test if up regulation of virulence genes at temperatures below 37°C correlates with pathogenicity, the capacity of  L. monocytogenes  to invade epithelial cells after acid shock at 25°C was measured. A 12-fold increased number of intracellular bacteria was observed (acid shock, t = 60 min) that was reduced after adaptation to the level of the unshocked control. This increased invasiveness was shown to be in line with the induction of  inlAB . Using a nematode infection assay, we demonstrated that  Caenorhabditis elegans  fed with acid-shocked  L. monocytogenes  exhibits a shorter time to death of 50% (TD 50 ) of the worms (6.4 days) compared to infection with unshocked bacteria (TD 50  = 10.2 days).    Conclusions   PrfA  and other listerial virulence genes are induced by an inorganic acid in a temperature-dependent manner. The data presented here suggest that low pH serves as a trigger for listerial pathogenicity at environmental temperatures.

bmc genomics

Acid shock of Listeria monocytogenes at low environmental temperatures induces prfA, epithelial cell invasion, and lethality towards Caenorhabditis elegans