The htrA (degP) gene of Listeria monocytogenes 10403S is essential for optimal growth under stress conditions

This report describes a mutant of Listeria monocytogenes strain 10403S (serotype 1/2a) with a defective response to conditions of high osmolarity, an environment that L. monocytogenes encounters in some ready-to-eat foods. A library of L. monocytogenes clones mutagenized with Tn917 was generated and scored for sensitivity to 4% NaCl in order to identify genes responsible for growth or survival in elevated-NaCl environments. One of the L. monocytogenes Tn917 mutants, designated strain OSM1, was selected, and the gene interrupted by the transposon was sequenced. A BLAST search with the putative translated amino acid sequence indicated that the interrupted gene product was a homolog of htrA (degP), a gene coding for a serine protease identified as a stress response protein in several gram-positive and gram-negative bacteria. An htrA deletion strain, strain LDW1, was constructed, and the salt-sensitive phenotype of this strain was complemented by introduction of a plasmid carrying the wild-type htrA gene, demonstrating that htrA is necessary for optimal growth under conditions of osmotic stress. Additionally, strain LDW1 was tested for its response to temperature and H(2)O(2) stresses. The results of these growth assays indicated that strain LDW1 grew at a lower rate than the wild-type strain at 44 degrees C but at a rate similar to that of the wild-type strain when incubated at 4 degrees C. In addition, strain LDW1 was significantly more sensitive to a 52 degrees C heat shock than the wild-type strain. Strain LDW1 was also defective in its response to H(2)O(2) challenge at 37 degrees C, since 100 or 150 micro g of H(2)O(2) was more inhibitory for the growth of strain LDW1 than for that of the parent strain. The stress response phenotype observed for strain LDW1 is similar to that observed for other HtrA(-) organisms, which suggests that L. monocytogenes HtrA may play a role in degrading misfolded proteins that accumulate under stress conditions.     

Identification and disruption of btlA,a locus involved in bile tolerance and general stress resistance in Listeria monocytogenes

A transposon Tn917 mutant of Listeria monocytogenes L028 was isolated on the basis of reduced growth on agar adjusted to pH 5.5. The disrupted gene, designated btlA (bile tolerance locus), encodes a putative secondary transporter of the major facilitator superfamily, which has significant homology to yxiO in Bacillus subtilis (lmo1417 in L. monocytogenes EGDe). The mutant demonstrated decreased growth rates relative to the wild-type when grown in sub-lethal levels of various stressors (acid, salt, ethanol, bile, SDS, ampicillin and phosphomycin). The mutant was also more sensitive to lethal levels of bile. A pORI19 insertion mutant demonstrated similar phenotypes. Murine virulence studies indicated that disruption of btlA does not influence virulence potential. BtlA therefore represents a membrane protein essential for the maintenance of homeostasis under stress conditions, but is not involved in pathogenicity.     

Identification of an essential gene of Listeria monocytogenes involved in teichoic acid biogenesis

Listeria monocytogenes is a facultative intracellular gram-positive bacterium responsible for severe opportunistic infections in humans and animals. We had previously identified a gene encoding a putative UDP-N-acetylglucosamine 2-epimerase, a precursor of the teichoic acid linkage unit, in the genome of L monocytogenes strain EGD-e. This gene, now designated lmo2537, encodes a protein that shares 62% identity with the cognate epimerase MnaA of Bacillus subtilis and 55% identity with Cap5P of Staphylococcus aureus. Here, we addressed the role of lmo2537 in L. monocytogenes pathogenesis by constructing a conditional knockout mutant. The data presented here demonstrate that lmo2537 is an essential gene of L. monocytogenes that is involved in teichoic acid biogenesis. In vivo, the conditional mutant is very rapidly eliminated from the target organs of infected mice and thus is totally avirulent.     

Listeria monocytogenes sigma B regulates stress response and virulence functions

While the stress-responsive alternative sigma factor sigma(B) has been identified in different species of Bacillus, Listeria, and Staphylococcus, the sigma(B) regulon has been extensively characterized only in B. subtilis. We combined biocomputing and microarray-based strategies to identify sigma(B)-dependent genes in the facultative intracellular pathogen Listeria monocytogenes. Hidden Markov model (HMM)-based searches identified 170 candidate sigma(B)-dependent promoter sequences in the strain EGD-e genome sequence. These data were used to develop a specialized, 208-gene microarray, which included 166 genes downstream of HMM-predicted sigma(B)-dependent promoters as well as selected virulence and stress response genes. RNA for the microarray experiments was isolated from both wild-type and Delta sigB null mutant L. monocytogenes cells grown to stationary phase or exposed to osmotic stress (0.5 M KCl). Microarray analyses identified a total of 55 genes with statistically significant sigma(B)-dependent expression under the conditions used in these experiments, with at least 1.5-fold-higher expression in the wild type over the sigB mutant under either stress condition (51 genes showed at least 2.0-fold-higher expression in the wild type). Of the 55 genes exhibiting sigma(B)-dependent expression, 54 were preceded by a sequence resembling the sigma(B) promoter consensus sequence. Rapid amplification of cDNA ends-PCR was used to confirm the sigma(B)-dependent nature of a subset of eight selected promoter regions. Notably, the sigma(B)-dependent L. monocytogenes genes identified through this HMM/microarray strategy included both stress response genes (e.g., gadB, ctc, and the glutathione reductase gene lmo1433) and virulence genes (e.g., inlA, inlB, and bsh). Our data demonstrate that, in addition to regulating expression of genes important for survival under environmental stress conditions, sigma(B) also contributes to regulation of virulence gene expression in L. monocytogenes. These findings strongly suggest that sigma(B) contributes to L. monocytogenes gene expression during infection.     

Identification and characterization of Di- and tripeptide transporter DtpT of Listeria monocytogenes EGD-e

Listeria monocytogenes is a gram-positive intracellular pathogen responsible for opportunistic infections in humans and animals. Here we identified and characterized the dtpT gene (lmo0555) of L. monocytogenes EGD-e, encoding the di- and tripeptide transporter, and assessed its role in growth under various environmental conditions as well as in the virulence of L. monocytogenes. Uptake of the dipeptide Pro-[14C]Ala was mediated by the DtpT transporter and was abrogated in a DeltadtpT isogenic deletion mutant. The DtpT transporter was shown to be required for growth when the essential amino acids leucine and valine were supplied as peptides. The protective effect of glycine- and proline-containing peptides during growth in defined medium containing 3% NaCl was noted only in L. monocytogenes EGD-e, not in the DeltadtpT mutant strain, indicating that the DtpT transporter is involved in salt stress protection. Infection studies showed that DtpT contributes to pathogenesis in a mouse infection model but has no role in bacterial growth following infection of J774 macrophages. These studies reveal that DptT may contribute to the virulence of L. monocytogenes.     

Investigation of the role of ZurR in the physiology and pathogenesis of Listeria monocytogenes

Listeria monocytogenes is a Gram positive pathogen that is ubiquitous in the environment. It is a facultative anaerobic rod that causes listeriosis, a disease with potentially lethal consequences for susceptible individuals. During infection, the pathogen is capable of sequestering metal ions to act as vital biocatalysts in cellular processes. The zinc uptake regulator (ZurR) is predicted to coordinate uptake of zinc from the external environment. An in-frame deletion of the zurR gene resulted in a mutant exhibiting a small colony phenotype and a smaller cell size. The zurR mutant was unaffected under conditions of zinc limitation but demonstrated increased sensitivity to toxic levels of zinc. The mutant also demonstrated a significant (1-log) reduction in virulence potential in the murine model of infection. Using a bioinformatic approach, we identified a number of potentially Zur-regulated genes in the genome of L. monocytogenes. Quantitative RT-PCR demonstrated significant de-repression of zurA, lmo0153, and lmo1671 in the zurR mutant background indicating that these putative transporters are ZurR regulated.     

Regulation of growth inhibition at high temperature, autolysis, transformation and adherence in Streptococcus pneumoniae by clpC

The ClpC ATPase is a subfamily of HSP100/Clp molecular chaperones-regulators of proteolysis. By screening a library of loss of function mutants for the ability to survive treatment with penicillin, we identified the gene clpC. The corresponding protein was identified as a ClpC ATPase, sharing strong peptide sequence identity with ClpC of Bacillus subtilis, Listeria monocytogenes and Lactococcus lactis. Northern blot experiments showed that expression of clpC was induced in response to high temperature (40-42 degrees C) versus 37 degrees C, suggesting that ClpC is a heat shock protein. Insertional duplication mutagenesis of clpC resulted in increased tolerance to high temperature; a result in contrast to other bacterial Clp proteases. The clpC-deficient mutant formed long chains and failed to undergo lysis after treatment with penicillin or vancomycin. The effect of the clpC mutation extended to deficiency of adherence to the human type II alveolar cells. Finally, the clpC disruption resulted in decreased genetic transformation. Western blot analysis demonstrated that the mutant failed to express pneumolysin and the choline-binding proteins LytA, CbpA, CbpE, CbpF, CbpJ. These results suggest that the heat shock protein ClpC plays an essential complex pleiotropic role in pneumococcal physiology, including cell growth under heat stress, cell division, autolysis, adherence and transformation.