Hemolysin from Listeria--biochemistry, genetics and function in pathogenesis

Switching Lifestyles Is an in vivo Adaptive Strategy of Bacterial Pathogens

Gram-positive and Gram-negative pathogens exist as planktonic cells only at limited times during their life cycle. In response to environmental signals such as temperature, pH, osmolality, and nutrient availability, pathogenic bacteria can adopt varied cellular fates, which involves the activation of virulence gene programs and/or the induction of a sessile lifestyle to form multicellular surface-attached communities. In Salmonella, SsrB is the response regulator which governs the lifestyle switch from an intracellular virulent state to form dormant biofilms in chronically infected hosts. Using the Salmonella lifestyle switch as a paradigm, we herein compare how other pathogens alter their lifestyles to enable survival, colonization and persistence in response to different environmental cues. It is evident that lifestyle switching often involves transcriptional regulators and their modification as highlighted here. Phenotypic heterogeneity resulting from stochastic cellular processes can also drive lifestyle variation among members of a population, although this subject is not considered in the present review.

The metabolic regulator CodY links Listeria monocytogenes metabolism to virulence by directly activating the virulence regulatory gene prfA

Metabolic adaptations are critical to the ability of bacterial pathogens to grow within host cells and are normally preceded by sensing of host-specific metabolic signals, which in turn can influence the pathogen's virulence state. Previously, we reported that the intracellular bacterial pathogen Listeria monocytogenes responds to low availability of branched-chain amino acids (BCAAs) within mammalian cells by up-regulating both BCAA biosynthesis and virulence genes. The induction of virulence genes required the BCAA-responsive transcription regulator, CodY, but the molecular mechanism governing this mode of regulation was unclear. In this report, we demonstrate that CodY directly binds the coding sequence of the L. monocytogenes master virulence activator gene, prfA, 15 nt downstream of its start codon, and that this binding results in up-regulation of prfA transcription specifically under low concentrations of BCAA. Mutating this site abolished CodY binding and reduced prfA transcription in macrophages, and attenuated bacterial virulence in mice. Notably, the mutated binding site did not alter prfA transcription or PrfA activity under other conditions that are known to activate PrfA, such as during growth in the presence of glucose-1-phosphate. This study highlights the tight crosstalk between L. monocytogenes metabolism and virulence, while revealing novel features of CodY-mediated regulation.

ActA promotes Listeria monocytogenes aggregation, intestinal colonization and carriage

Listeria monocytogenes (Lm) is a ubiquitous bacterium able to survive and thrive within the environment and readily colonizes a wide range of substrates, often as a biofilm. It is also a facultative intracellular pathogen, which actively invades diverse hosts and induces listeriosis. So far, these two complementary facets of Lm biology have been studied independently. Here we demonstrate that the major Lm virulence determinant ActA, a PrfA-regulated gene product enabling actin polymerization and thereby promoting its intracellular motility and cell-to-cell spread, is critical for bacterial aggregation and biofilm formation. We show that ActA mediates Lm aggregation via direct ActA-ActA interactions and that the ActA C-terminal region, which is not involved in actin polymerization, is essential for aggregation in vitro. In mice permissive to orally-acquired listeriosis, ActA-mediated Lm aggregation is not observed in infected tissues but occurs in the gut lumen. Strikingly, ActA-dependent aggregating bacteria exhibit an increased ability to persist within the cecum and colon lumen of mice, and are shed in the feces three order of magnitude more efficiently and for twice as long than bacteria unable to aggregate. In conclusion, this study identifies a novel function for ActA and illustrates that in addition to contributing to its dissemination within the host, ActA plays a key role in Lm persistence within the host and in transmission from the host back to the environment.

Listeria monocytogenes (Lm) is a ubiquitous bacterium that survives and thrives within the environment, and a facultative intracellular pathogen that induces listeriosis. So far, these two complementary facets of Lm biology have been studied independently. Here we identify ActA, which is a major Lm virulence determinant mediating actin-based motility, as critical for bacterial aggregation and biofilm formation. ActA promotes Lm aggregation via direct ActA-ActA interaction and ActA C-terminal region, which is not involved in actin polymerization, is essential for aggregation. Whereas ActA-mediated Lm aggregation is not observed in infected tissues, it occurs in the gut lumen. Strikingly, ActA-dependent aggregating bacteria exhibit an increased ability to persist within the gut lumen, and are shed in the feces three order of magnitude more and for twice as long than bacteria unable to aggregate. This study identifies a novel function for ActA, which plays a key role in Lm persistence within the host and transmission.

A naturally occurring mutation K220T in the pleiotropic activator PrfA of Listeria monocytogenes results in a loss of virulence due to decreasing DNA-binding affinity

The sequencing of prfA, encoding the transcriptional regulator of virulence genes, in 26 low-virulence field Listeria monocytogenes strains showed that eight strains exhibited the same single amino-acid substitution: PrfAK220T. These strains exhibited no expression of PrfA-regulated proteins and thus no virulence. This substitution inactivated PrfA, since expression of the PrfAK220T mutant gene in an EGDDeltaprfA strain did not restore the haemolytic and phosphatidylcholine phospholipase C activities, in contrast to the wild-type prfA gene. The substitution of the lysine at position 220 occurred in the helix alphaH. However, the data showed that the PrfAK220T protein is dimerized just as well as its wild-type counterpart, but does not bind to PrfA-boxes. PrfAK220T did not form a PrfA-DNA complex in electrophoretic mobility shift assays, but low concentrations of CI complexes (PrfAK220T-RNA polymerase-DNA complex) were formed by adding RNA polymerase, suggesting that PrfA interacted with RNA polymerase in solution in the absence of DNA. Formation of some transcriptionally active complexes was confirmed by in vitro runoff transcription assays and quantitative RT-PCR. Crystallographic analyses described the structure of native PrfA and highlighted the key role of allosteric changes in the activity of PrfA and especially the role of the Lys220 in the conformation of the helix-turn-helix (HTH) motif.

Rapid purification of recombinant listeriolysin O (LLO) from Escherichia coli

Listeria monocytogenes is an emerging foodborne pathogen that is responsible for about 28% of the food-related deaths in the United States. It causes meningitis, septicaemia and in pregnant women, abortions and stillbirths. It secretes the toxin listeriolysin O (LLO) that allows the bacteria to enter the cytoplasm of host cells, where they can replicate and cause further infection. The rapid and sensitive detection of LLO in food samples is a key to monitoring and prevention of listeriosis. To facilitate the development of an assay for the specific detection of LLO, a source of LLO is essential. We outline a method of producing a large amount of functional LLO by expressing the hlyA gene (encoding LLO) in Escherichia coli and purifying the recombinant LLO using a one-step purification method. Purification of the protein takes only about 4 h. We compared three different expression constructs for the production of the toxin, which tends to interact strongly with a number of column surfaces. The first construct, using an intein fusion system, could not be purified from the column. The second LLO construct contained an N-terminus His tag; it gave a yield of 3.5-8 mg l(-1). The third contained a C-terminus His tag; it gave a yield of 2.5 mg l(-1) LLO. The purified LLO from the latter two constructs retained its activity at 4 degrees C for over a year as determined by bovine red blood cell hemolysis assay. This paper provides a much-needed, high-yield, one-step purification method of recombinant LLO, and is the first to provide evidence of long-term stability of the toxin for further applications.

A gene-expression program reflecting the innate immune response of cultured intestinal epithelial cells to infection by Listeria monocytogenes

BACKGROUND

Listeria monocytogenes is a Gram-positive, facultative, intracellular bacterial pathogen found in soil, which occasionally causes serious food-borne disease in humans. The outcome of an infection is dependent on the state of the infected individual's immune system, neutrophils being key players in clearing the microorganism from the body. The first line of host defense, however, is the intestinal epithelium.

RESULTS

We have examined the transcriptional response of cultured human intestinal epithelial cells to infection by L. monocytogenes, which replicates in the host cell cytoplasm and spreads from cell to cell using a form of actin-based motility. We found that the predominant host response to infection was mediated by NFkappaB. To determine whether any host responses were due to recognition of specific virulence factors during infection, we also examined the transcriptional response to two bacterial mutants; actA which is defective in actin-based motility, and prfA, which is defective in the expression of all L. monocytogenes virulence genes. Remarkably, we found no detectable difference in the host transcriptional response to the wild-type and mutant bacteria.

CONCLUSIONS

These results suggest that cultured intestinal epithelial cells are capable of mounting and recruiting a powerful innate immune response to L. monocytogenes infection. Our results imply that L. monocytogenes is not specifically detected in the host cytoplasm of Caco-2 cells by intracellular signals. This suggests that entry of bacteria is mediated in the host cell post-translationally, and that these bacteria seek the cytosol not only for the nutrient-rich environment, but also for protection from detection by the immune system.

The Listeria monocytogenes-secreted p60 protein is an N-end rule substrate in the cytosol of infected cells. Implications for major histocompatibility complex class I antigen processing of bacterial proteins

Cytosolic antigen degradation is an initial step in the generation of major histocompatibility complex (MHC) class I-associated cytolytic T lymphocyte epitopes. Intracellular Listeria monocytogenes secretes p60, a murein hydrolase, into the host cell cytosol, where it is degraded by proteasomes. Roughly 3% of degraded p60 gives rise to p60 217-225, a nonamer peptide that is bound by H-2Kd MHC class I molecules. Herein, we introduce targeted deletions throughout the p60 gene to identify potential proteolytic signals within p60. Degradation of mutant forms of p60 was investigated in macrophages infected with recombinant L. monocytogenes. We found that deletions within the amino-terminal two-thirds of p60 enhanced cytosolic degradation. In contrast, truncation of the C terminus resulted in modest stabilization of p60 in the host cell cytosol. Because a protein's N-terminal amino acid can determine its rate of degradation, we mutagenized this residue in p60 into known stabilizing and destabilizing residues. Valine substitution dramatically stabilized cytosolic p60 molecules, while substitution with aspartic acid resulted in rapid degradation. The number of p60 217-225 epitopes isolated from infected cells directly correlated with the rates of p60 degradation. Our data, therefore, indicate that the N-terminal amino acid and multiple internal regions of p60 influence its stability in the cytosol of infected cells. Antigen degradation and epitope generation are linked, and different degradation signals can channel bacterial proteins into the MHC class I antigen processing pathway.

Development and characterization of monoclonal antibodies specific for the genus Listeria

Monoclonal antibodies were obtained by the classic hybridoma technique with lymphocytes of BALB/c mice immunized with formalin killed Listeria monocytogenes cells. Among 1000 hybridomas issued from the fusion, four monoclonal antibodies (mAbs A6 A E4, C10 A F7, G4 A D6, G7 A D5) gave interesting results. By Western-blot analysis with various soluble extracts of different Listeria species, the four mAbs reacted with two major antigens of 38 and 41 kDa, with all Listeria species tested. The mAb A6 A E4 is an IgG2b with kappa light chains and reacted only with Listeria antigens without any cross reaction with other organisms tested by ELISA, dot-blotting and Western-blotting. With the same conditions, the three other mAbs reacted with Listeria and with other genus extracts, particularly with Streptococcus and Enterococcus. mAb A6 A E4-reactive antigens are proteins, and glycoprotein immunoassay indicated that the epitope is devoid of carbohydrate moiety. This mAb A6 A E4-reactive protein was neither expressed on cell surface nor released outside the bacteria; immunogold electron microscopy showed that these antigens were localized in the cytoplasma area.

Listeria ivanovii is capable of cell-to-cell spread involving actin polymerization

Listeria ivanovii has been considered to be pathogenic to animals but has rarely been found associated with human infections. It has been claimed that L. ivanovii lacks the actA gene, which in L. monocytogenes encodes a protein required for interaction with host cell actin. Using fluorescence microscopy and electron microscopy, we demonstrate that L. ivanovii can invade mammalian cells, lyse the phagosomal membrane, polymerize host cell actin, reorganize actin to form tails, and spread from cell to cell. However, no DNA homologous to the actA gene could be detected by polymerase chain reaction. Further, L. ivanovii lacks the 90-kDa surface protein which in L. monocytogenes is encoded by actA. Despite the ability to spread from cell to cell, L. ivanovii differed significantly from L. monocytogenes in being unable to form plaques on monolayers of 3T3 fibroblast cells.

Is any strain of Listeria monocytogenes detected in food a health risk?

Listeria spp. have been isolated from various food items. This fact does not mean in any case a true health risk. A balanced appraisal should be based on quantitative as well as qualitative aspects. Actually, there is still an open debate whether a limited number of Listeria has to be tolerated at least in certain food items. In addition, the pathogenic potency of an isolate may be put into account. Pathogenicity of various Listeria spp. definitely varies. Most Listeria spp., except Listeria monocytogenes, can be regarded as harmless to man. Also, not all strains of L. monocytogenes are pathogenic: rough variants possess only reduced virulence; non-hemolytic mutants have completely lost their pathogenic potency. Furthermore, several other virulence factors may be lost under natural conditions, so that among the majority of hemolytic, pathogenic isolates there may be others which are non-pathogenic or of low virulence only. Unfortunately, these strains actually cannot be recognized and characterized by common laboratory tests, so that animal pathogenicity seems to be the only way to get a final conclusion on the health risk of an isolate of L. monocytogenes from any food. The problem raised by this is which animal test is able to predict a true health risk either for normal hosts or for immunocompromised patients?

Listeria monocytogenes, a food-borne pathogen

The gram-positive bacterium Listeria monocytogenes is an ubiquitous, intracellular pathogen which has been implicated within the past decade as the causative organism in several outbreaks of foodborne disease. Listeriosis, with a mortality rate of about 24%, is found mainly among pregnant women, their fetuses, and immunocompromised persons, with symptoms of abortion, neonatal death, septicemia, and meningitis. Epidemiological investigations can make use of strain-typing procedures such as DNA restriction enzyme analysis or electrophoretic enzyme typing. The organism has a multifactorial virulence system, with the thiol-activated hemolysin, listeriolysin O, being identified as playing a crucial role in the organism's ability to multiply within host phagocytic cells and to spread from cell to cell. The organism occurs widely in food, with the highest incidences being found in meat, poultry, and seafood products. Improved methods for detecting and enumerating the organism in foodstuffs are now available, including those based on the use of monoclonal antibodies, DNA probes, or the polymerase chain reaction. As knowledge of the molecular and applied biology of L. monocytogenes increases, progress can be made in the prevention and control of human infection.

Expression of the iap gene coding for protein p60 of Listeria monocytogenes is controlled on the posttranscriptional level

Expression of the iap gene of Listeria monocytogenes encoding a major extracellular protein (p60) was analyzed. Different start sites for transcription of the iap gene were identified by primer extension analysis in L. monocytogenes and in a recombinant Escherichia coli clone. The mutant RIII of L. monocytogenes represents a member of the frequently occurring L. monocytogenes R mutants, which form cell chains and produce greatly reduced amounts of p60. However, the concentrations of iap-specific mRNA were similar in mutant RIII and the wild-type strain. The introduction of additional copies of the iap gene from wild-type L. monocytogenes led to an equal increase of iap mRNA in both strains, but overexpression of protein p60 was only observed in the wild-type strain. The nucleotide sequences of both iap genes and their 5' noncoding regions were identical in all parts that are essential for efficient transcription of the iap gene, translation of the iap-specific mRNA, and transport of the p60 protein. These data suggest that the expression of the iap gene in L. monocytogenes is controlled on the posttranscriptional level by a specific factor that is defective in mutant RIII.

Maternal listeriosis in pregnancy without fetal or neonatal infection

Maternal infection with Listeria monocytogenes without fetal or neonatal involvement is relatively rare. Eleven cases arising in England and Wales between 1967 and 1988 are presented.

The gene coding for protein p60 of Listeria monocytogenes and its use as a specific probe for Listeria monocytogenes

The gene of Listeria monocytogenes that encodes a major extracellular protein (p60) was cloned in Escherichia coli. The gene was designated iap, as p60 was previously shown to represent an invasion-associated protein (M. Kuhn and W. Goebel, Infect. Immun. 57:55-61, 1989). The recombinant E. coli clone expressed p60, as shown by immunoblotting. The complete nucleotide sequence of iap was determined. The deduced amino acid sequence of p60 (484 amino acids) contains a putative N-terminal signal sequence of 27 amino acids and an extended repeat region consisting of 19 threonine-asparagine units. Hybridization with the entire iap gene revealed the presence of homologous sequences in most other Listeria species. In contrast, a 400-base-pair internal iap probe which contained the whole repeat region hybridized only with genomic DNA from L. monocytogenes. Four oligonucleotides previously described as specific probes for the detection of L. monocytogenes (A. R. Datta, B. A. Wentz, D. Shook, and M. W. Trucksess, Appl. Environ. Microbiol. 54:2933-2937, 1988) were shown to be part of the iap gene.