Contemp Clin Trials 2009,30(5):490–496.PubMedCrossRef Competing interests The authors declare that they have no competing interests. Authors’ contributions PT performed the experiments, HK performed molecular modeling, JW conceived the study; PT, FR and JW wrote the manuscript. Small molecule library clinical trial KEJ and HCF coordinate the work. All authors read and approved the final manuscript.”
“Background The foodborne pathogen Listeria monocytogenes uses complex regulatory mechanisms to adapt to a variety of environmental conditions and to cause listeriosis, a life-threatening infection, in humans and animals. A key mechanism used by L. monocytogenes

to regulate transcript and protein levels in order to adapt to changing environmental conditions is through alternative sigma (σ) factors. Alternative σ factors reprogram the RNA polymerase holoenzyme to recognize specific promoters and hence allow for rapid induction of transcription of potentially large groups of genes under specific

environmental conditions [1]. In L. monocytogenes, four alternative σ factors, σB, σC, σH, and σL , have been identified. However, σC has only been described in L. monocytogenes strains that group into lineage PR-171 purchase II, a well defined phylogenetic group that includes serotypes 1/2a and 1/2c [2–4]. A number of studies that have explored σB-mediated stress response as well as σB-mediated gene expression and protein production in L. monocytogenes[1, 5–16] have shown that this alternative σ factor controls a large regulon and contributes to both stress response and virulence. σH, σL, and σC have not been as extensively characterized as σB in L. monocytogenes, at least partially because studies to date have only identified limited phenotypic consequences of null mutations in these σ factors in L. monocytogenes. Among these three alternative σ factors, σH appears to control the largest regulon; Chaturongakul et al. (2011) identified

97 and 72 genes as positively and negatively regulated by σH, respectively, in L. monocytogenes strain 10403S [7]. While a L. monocytogenes EGD-e sigH mutant was reported to have significantly impaired growth in minimal medium PtdIns(3,4)P2 and under alkaline stress conditions as well as slightly reduced virulence potential in a mouse model [17], phenotypic studies in a L. monocytogenes 10403S ΔsigH strain did not find evidence for an effect of this mutation on virulence in a guinea pig model, cell invasion and intracellular growth, or resistance to heat stress [7]. With regard to σL, 31 and 20 genes were identified as positively and negatively regulated, respectively, by this σ factor, in L. monocytogenes 10403S [7]. A more recent study in L. monocytogenes EGD-e identified 237 and 203 genes as positively regulated by σL when the parent and ΔsigL mutant strains were grown at 3°C and 37°C, respectively; most of the 47 genes that showed positive regulation by σL under both temperatures were located within prophage A118 [18].