Characterisation of the starvation-survival response in Listeria monocytogenes
Listeria monocytogenes is a food-borne pathogen able to adapt and survive in a wide range of habitats in addition to being able to overcome host defences. The need to prevent L. monocytogenes entering the food chain and the role that stress plays during the course of an infection, means that the starvation-survival and stress resistance mechanism of this organism are thus of significant interest. The starvation-survival response of L. monocytogenes EGD in a chemically defined medium was induced under glucose- or multiple nutrient-, but not amino acid-limitation. This resulted in 90 to 99.9% loss of viability within 2 days, with viability maintained during prolonged starvation. Surviving cells were reduced in size and developed increased general stress resistance. L. monocytogenes EGD demonstrated densitydependent starvation-survival under multiple nutrient- but not under glucose-limitation. Protein synthesis was required for long-term survival only for the first 8 hours of starvation and survival became independent of cell wall biosynthesis during long-term starvation. Strains bearing mutations in the gene regulators sigB (DESOII) or prfA (DES012) showed a to-fold reduction in starvation-survival compared to EGD after 20 days of glucose limitation. DESOl1 had reduced exponential phase acid stress resistance, but increased H202 resistance. Resistance to H20 2 in exponential phase and long-term starved DES012 cells was over 290- fold and 380-fold greater than in EGD (after 20 minutes and 50 minutes exposure respectively), whilst exponential- and post-exponential-phase acid resistance in the DESOl2 was at least 10-fold greater than in EGD. Both DESOII and DESOl2 also exhibited altered catalase expression. Four transposon insertion mutants (two pairs of siblings) defective in starvation-survival were isolated from a glucose-limitation screen. Both sets of mutations resulted in decreased starvation-survival and altered stress resistance properties. Characterisation of the transposon insertion sites in DES028 and DES029 revealed disruption of a putative ORF encoding for a homologue of YuIB, a DeoR-family transcriptional regulator from Bacillus subtilis. In the isolates DES035 and DES045, the transposon insertion was found to disrupt a putative ORF encoding for a homologue of PhaQ, a protein associated with inclusion bodies of the storage polymer polyhydroxyalkanoic acid in Bacillus megaterium. The roles of these two loci in the starvation-survival response and in stress resistance are discussed.