A role for ActA in epithelial cell invasion by Listeria monocytogenes

We assessed the role of the actin-polymerizing protein, ActA, in host cell invasion by Listeria monocytogenes. An in frame DeltaactA mutant was constructed in a hyperinvasive strain of prfA* genotype, in which all genes of the PrfA-dependent virulence regulon, including actA, are highly expressed in vitro. Loss of ActA production in prfA* bacteria reduced entry into Caco-2, HeLa, MDCK and Vero epithelial cells to basal levels. Reintroduction of actA into the DeltaactA prfA* mutant fully restored invasiveness, demonstrating that ActA is involved in epithelial cell invasion. ActA did not contribute to internalization by COS-1 fibroblasts and Hepa 1-6 hepatocytes. Expression of actA in Listeria innocua was sufficient to promote entry of this non-invasive species into epithelial cell lines, but not into COS-1 and Hepa 1-6 cells, indicating that ActA directs an internalization pathway specific for epithelial cells. Scanning electron microscopy of infected Caco-2 human enterocytes suggested that this pathway involves microvilli. prfA* bacteria, but not wild-type bacteria (which express PrfA-dependent genes very weakly in vitro) or prfA* DeltaactA bacteria, efficiently invaded differentiated Caco-2 cells via their apical surface. Microvilli played an active role in the phagocytosis of the prfA* strain, and actA was required for their remodelling into pseudopods mediating bacterial uptake. Thus, ActA appears to be a multifunctional virulence factor involved in two important aspects of Listeria pathogenesis: actin-based motility and host cell tropism and invasion.

Comparative genomics of Listeria species

Listeria monocytogenes is a food-borne pathogen with a high mortality rate that has also emerged as a paradigm for intracellular parasitism. We present and compare the genome sequences of L. monocytogenes (2,944,528 base pairs) and a nonpathogenic species, L. innocua (3,011,209 base pairs). We found a large number of predicted genes encoding surface and secreted proteins, transporters, and transcriptional regulators, consistent with the ability of both species to adapt to diverse environments. The presence of 270 L. monocytogenes and 149 L. innocua strain-specific genes (clustered in 100 and 63 islets, respectively) suggests that virulence in Listeria results from multiple gene acquisition and deletion events.

Identification and mutagenesis by allelic exchange of choE, encoding a cholesterol oxidase from the intracellular pathogen Rhodococcus equi

The virulence mechanisms of the facultative intracellular parasite Rhodococcus equi remain largely unknown. Among the candidate virulence factors of this pathogenic actinomycete is a secreted cholesterol oxidase, a putative membrane-damaging toxin. We identified and characterized the gene encoding this enzyme, the choE monocistron. Its protein product, ChoE, is homologous to other secreted cholesterol oxidases identified in Brevibacterium sterolicum and Streptomyces spp. ChoE also exhibits significant similarities to putative cholesterol oxidases encoded by Mycobacterium tuberculosis and Mycobacterium leprae. Genetic tools for use with R. equi are poorly developed. Here we describe the first targeted mutagenesis system available for this bacterium. It is based on a suicide plasmid, a selectable marker (the aacC4 apramycin resistance gene from Salmonella), and homologous recombination. The choE allele was disrupted by insertion of the aacC4 gene, cloned in pUC19 and introduced by electroporation in R. equi. choE recombinants were isolated at frequencies between 10(-2) and 10(-3). Twelve percent of the recombinants were double-crossover choE mutants. The choE mutation was associated with loss of cooperative (CAMP-like) hemolysis with sphingomyelinase-producing bacteria (Listeria ivanovii). Functional complementation was achieved by expression of choE from pVK173-T, a pAL5000 derivative conferring hygromycin resistance. Our data demonstrate that ChoE is an important cytolytic factor for R. equi. The highly efficient targeted mutagenesis procedure that we used to generate choE isogenic mutants will be a valuable tool for the molecular analysis of R. equi virulence.

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal individuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research.

Pathogenicity islands and virulence evolution in Listeria

As in other bacterial pathogens, the virulence determinants of Listeria species are clustered in genomic islands scattered along the chromosome. This review summarizes current knowledge about the structure, distribution and role in pathogenesis of Listeria virulence loci. Hypotheses about the mode of acquisition and evolution of these loci in this group of Gram-positive bacteria are presented and discussed.

Regulation of virulence genes in Listeria

As in all pathogenic bacteria, virulence of the facultative intracellular Listeria species is a multifactorial trait. The expression of the bacterial genes involved in the different steps of the infectious process--invasion, intracellular multiplication and spreading--is temporally and spatially controlled, thus ensuring the presence of the respective gene products at the right moment and place. So far, one network which is involved in the regulation of listerial virulence, the PrfA regulon, has been characterized rather well. The key element of this regulon, PrfA, belongs to the Crp/Fnr family of transcriptional regulators. Its synthesis and activity are influenced by a variety of physico-chemical signals outside and inside of eukaryotic host cells. The analysis of virulence gene expression in vivo, i.e. in infected host cells, indicates that yet uncharacterized bacterial factors other than PrfA, and possibly also host factors, modulate the expression of the PrfA regulon. Essentially nothing is known about the signal transduction pathways involved in the observed differential expression of virulence genes. Fermentable carbon sources seem to have a particular role in virulence gene regulation. In addition to the PrfA regulon, the Clp stress proteins have an impact on Listeria virulence. These two regulons interact with each other by an unknown mechanism.

SmcL, a novel membrane-damaging virulence factor in Listeria

We describe here the fourth listerial membrane-damaging virulence factor, a sphingomyelinase C (SMase) that is produced specifically by the ruminant pathogen Listeria ivanovii. Its coding gene, smcL, is a monocistron expressed independently of PrfA. The smcL product, SmcL, is highly similar to the staphylococcal beta-toxin and is responsible for the differential hemolytic properties of L. ivanovii (bizonal hemolysis and CAMP-like reaction with R. equi). The role of SmcL in virulence was assessed by gene disruption and complementation. Our data show that SmcL mediates disruption of the membrane of primary phagosomes, thereby promoting bacterial intracellular proliferation. They also suggest that SmcL may play a role in host tropism. smcL is located in LIPI-2, a novel 18-kb pathogenicity island which also contains a cluster of internalin genes. LIPI-2 is unstable, L. ivanovii-specific and required for full virulence in mice and lambs.

The smcL gene of Listeria ivanovii encodes a sphingomyelinase C that mediates bacterial escape from the phagocytic vacuole

The ruminant pathogen Listeria ivanovii differs from Listeria monocytogenes in that it causes strong, bizonal haemolysis and a characteristic shovel-shaped co-operative haemolytic ('CAMP-like') reaction with Rhodococcus equi. We cloned the gene responsible for the differential haemolytic properties of L. ivanovii, smcL. It encodes a sphingomyelinase C (SMase) highly similar (> 50% identity) to the SMases from Staphylococcus aureus (beta-toxin), Bacillus cereus and Leptospira interrogans. smcL was transcribed monocistronically and was expressed independently of PrfA. Low-stringency Southern blots demonstrated that, within the genus Listeria, smcL was present only in L. ivanovii. We constructed an smcL knock-out mutant. Its phenotype on blood agar was identical to that of L. monocytogenes (i.e. weak haemolysis and no shovel-shaped CAMP-like reaction with R. equi ). This mutant was less virulent for mice, and its intracellular proliferation was impaired in the bovine epithelial-like cell line MDBK. The role of SmcL in intracellular survival was investigated using an L. monocytogenes mutant lacking the membrane-damaging determinants hly, plcA and plcB, being thus unable to grow intracellularly. Complementation of this mutant with smcL on a plasmid was sufficient to promote bacterial intracellular proliferation in MDBK cells. Transmission electron microscopy showed that SmcL mediates the disruption of the phagocytic vacuole and the release of bacteria into the cytosol. Therefore, L. ivanovii possesses a third phospholipase with membrane-damaging activity that, together with PlcA and PlcB, may act in concert with the pore-forming toxin Hly to mediate efficient escape from the vacuolar compartment. The 5' end of smcL is contiguous with the internalin locus i-inlFE, which is also specific to L. ivanovii and is required for full virulence in mice. Thus, smcL forms part of a novel virulence gene cluster in Listeria that is species specific.

The bvr locus of Listeria monocytogenes mediates virulence gene repression by beta-glucosides

The beta-glucoside cellobiose has been reported to specifically repress the PrfA-dependent virulence genes hly and plcA in Listeria monocytogenes NCTC 7973. This led to the hypothesis that beta-glucosides, sugars of plant origin, may act as signal molecules, preventing the expression of virulence genes if L. monocytogenes is living in its natural habitat (soil). In three other laboratory strains (EGD, L028, and 10403S), however, the effect of cellobiose was not unique, and all fermentable carbohydrates repressed hly. This suggested that the downregulation of virulence genes by beta-glucosides is not a specific phenomenon but, rather, an aspect of a global regulatory mechanism of catabolite repression (CR). We assessed the effect of carbohydrates on virulence gene expression in a panel of wild-type isolates of L. monocytogenes by using the PrfA-dependent phospholipase C gene plcB as a reporter. Utilization of any fermentable sugar caused plcB repression in wild-type L. monocytogenes. However, an EGD variant was identified in which, as in NCTC 7973, plcB was only repressed by beta-glucosides. Thus, the regulation of L. monocytogenes virulence genes by sugars appears to be mediated by two separate mechanisms, one presumably involving a CR pathway and another specifically responding to beta-glucosides. We have identified in L. monocytogenes a 4-kb operon, bvrABC, encoding an antiterminator of the BglG family (bvrA), a beta-glucoside-specific enzyme II permease component of the phosphoenolpyruvate-sugar phosphotransferase system (bvrB), and a putative ADP-ribosylglycohydrolase (bvrC). Low-stringency Southern blots showed that this locus is absent from other Listeria spp. Transcription of bvrB was induced by cellobiose and salicin but not by arbutin. Disruption of the bvr operon by replacing part of bvrAB with an interposon abolished the repression by cellobiose and salicin but not that by arbutin. Our data indicate that the bvr locus encodes a beta-glucoside-specific sensor that mediates virulence gene repression upon detection of cellobiose and salicin. Bvr is the first sensory system found in L. monocytogenes that is involved in environmental regulation of virulence genes.

Associations of veterinary services and farmer characteristics with the prevalences of brucellosis and border disease in small ruminants in Spain

We investigated the farm factors associated with the prevalences of brucellosis and border disease (BD) in small-ruminant herds in the Madrid region of Spain. These infections were used as models of diseases of well-known and totally unknown distribution, respectively, to assess the association between the perception of the importance of a given disease on the relative contributions of veterinary services and the farmer's attitudes to its prevention. Sera, farming-management information and data concerning veterinary assistance and farmer characteristics were collected from 60 sheep or goat herds. The overall sero-prevalence of brucellosis was 5.7% (complement fixation) and for BD was 17.9% (ELISA test). The relationship between sero-positivity and the variables in the questionnaire was assessed by multivariable analysis using random-effects logistic-normal regression. 'Availability of veterinary services' was a major protective factor for brucellosis. In contrast, no association with veterinary services was observed for BD, whereas 'membership in a farmers' organization' (a variable associated with good farming practice and animal care) was a protective factor. 'Membership of a farmers' organisation' and two other farmer variables indicative of good husbandry ('youth' and 'schooling') were associated with a lower sero-prevalence of brucellosis in univariable analysis but they did not remain significant in the multivariable model. Our observations suggest that veterinary-activity variables predominate over non-specific protective farm factors related to good husbandry in the case the disease is subject to disease surveillance. This underscores the importance of organized control programs for veterinary services to be effective in terms of animal disease prevention.

Functional similarities between the Listeria monocytogenes virulence regulator PrfA and cyclic AMP receptor protein: the PrfA* (Gly145Ser) mutation increases binding affinity for target DNA

Most Listeria monocytogenes virulence genes are positively regulated by the PrfA protein, a transcription factor sharing sequence similarities with cyclic AMP (cAMP) receptor protein (CRP). Its coding gene, prfA, is regulated by PrfA itself via an autoregulatory loop mediated by the upstream PrfA-dependent plcA promoter. We have recently characterized prfA* mutants from L. monocytogenes which, as a result of a single amino acid substitution in PrfA, Gly145Ser, constitutively overexpress prfA and the genes of the PrfA virulence regulon. Here, we show that about 10 times more PrfA protein is produced in a prfA* strain than in the wild type. Thus, the phenotype of prfA* mutants is presumably due to the synthesis of a PrfA protein with higher promoter-activating activity (PrfA*), which keeps its intracellular levels constantly elevated by positive feedback. We investigated the interaction of PrfA and PrfA* (Gly145Ser) with target DNA. Gel retardation assays performed with a DNA fragment carrying the PrfA binding site of the plcA promoter demonstrated that the PrfA* mutant form is much more efficient than wild-type PrfA at forming specific DNA-protein complexes. In footprinting experiments, the two purified PrfA forms interacted with the same nucleotides at the target site, although the minimum amount required for protection was 6 to 7 times lower with PrfA*. These results show that the primary functional consequence of the Gly145Ser mutation is an increase in the affinity of PrfA for its target sequence. Interestingly, similar mutations at the equivalent position in CRP result in a transcriptionally active, CRP* mutant form which binds with high affinity to target DNA in the absence of the activating cofactor, cAMP. Our observations suggest that the structural similarities between PrfA and CRP are also functionally relevant and support a model in which the PrfA protein, like CRP, shifts from transcriptionally inactive to active conformations by interaction with a cofactor.

A novel PrfA-regulated chromosomal locus, which is specific for Listeria ivanovii, encodes two small, secreted internalins and contributes to virulence in mice

Several large, cell wall-associated internalins and one small, secreted internalin (InlC) have been described previously in Listeria monocytogenes. Using degenerate primers derived from sequenced peptides of an L. ivanovii major secreted protein, we identified a new 4.25 kb internalin locus of L. ivanovii, termed i-inlFE. The two proteins encoded by this locus, i-InlE and i-InlF, belong to the group of small, secreted internalins. Southern blot analyses show that the i-inlFE locus does not occur in L. monocytogenes. These data also indicate that six genes encoding small, secreted internalins are present in L. ivanovii, in contrast to L. monocytogenes, in which inlC encodes the only small internalin. The mature i-InlE protein (198 amino acids) is secreted in large amounts into the brain-heart infusion (BHI) culture medium in the stationary growth phase. In minimum essential medium (MEM), which has been used previously to induce PrfA-dependent gene transcription, i-inlE mRNA and i-InlE protein are expressed at high levels. As shown by Northern blot analysis and primer extension, transcription of the tandemly arranged i-inlF and i-inlE genes is dependent on the virulence regulator PrfA, and characteristic palindromic sequences ('PrfA-boxes') were identified in the promoter regions of i-inlF and i-inlE. Non-polar i-inlE and i-inlF deletion mutants and an i-inlFE double deletion mutant were constructed and tested in the mouse infection model. After intravenous infection, all three mutants entirely failed to kill C57BL/6 mice even at high infectious doses of 109 bacteria per mouse, whereas the LD50 for the parental strain was determined as 4 x 107 bacteria per mouse. These data suggest an important role for i-InlE and i-InlF in L. ivanovii virulence.

Factors associated with seroprevalence to Mycobacterium paratuberculosis in small-ruminant farms in the Madrid region (Spain)

A cross-sectional study was conducted in a population of small ruminants in the Madrid region (Spain) to determine the Mycobacterium paratuberculosis seroprevalence and to identify farm factors possibly associated with paratuberculosis (PTB). Farming-management information and sera were collected from 60 sheep or goat flocks. The relationship between seropositivity and the variables in the questionnaire was assessed by unconditional logistic regression, followed by random-effects logistic regression analysis to adjust for overdispersion between herds. The seroprevalence to M. paratuberculosis was 11.7% (64 out of 546) using agar-gel immunodiffusion assay (AGID). According to the sensitivity and specificity of the AGID test the true prevalence could be as high as 44%. A herd size of between 200-400 head and the presence of foreign breeds and their crosses were significantly associated with seropositivity (OR = 4.05 and OR = 4.32, respectively). A higher replacement rate was also associated with seroprevalence to M. paratuberculosis (24.2% in positive herds vs. 18.1% in negative). All these three factors were related to more intensive management in the surveyed area. In contrast, membership of a professional livestock association appeared to be a protective factor against PTB (OR = 0.28). No variables related to veterinary assistance were associated with seroprevalence, probably reflecting the current lack of interest in PTB on the part of the animal-health administration and veterinary services in Spain.

Evidence for expressional crosstalk between the central virulence regulator PrfA and the stress response mediator ClpC in Listeria monocytogenes

Virulence is a multifactorial trait which depends on the coordinated expression of many bacterial products, hence it is to be expected that the regulatory circuits that control the relevant genetic determinants are somehow interconnected. Two pleiotropic regulatory elements acting at different levels, the transcription factor PrfA which controls virulence gene expression and the potential chaperone ClpC which is involved in tolerance to environmental stress, are required for Listeria monocytogenes survival within the host. We analyzed the influence of PrfA on clpC expression in L. monocytogenes. clpC transcription is maximal under heat-shock conditions, i.e. at 42 degrees C, and is very weak or undetectable at 37 degrees C. In a prfA* mutant which constitutively overexpresses PrfA and PrfA-dependent virulence genes, clpC transcription dropped to basal levels during exponential growth at 42 degrees C. This repression was not observed during stationary phase, indicating growth phase-dependent regulation of clpC. Culture in charcoal-treated medium, which triggers in wild-type strains the transcriptional activation of the PrfA regulon, also caused a strong downregulation of clpC. Moreover, in a prfA deletion mutant, clpC transcription during exponential growth at 37 degrees C was clearly enhanced, reaching the same high levels of the wild-type at 42 degrees C. Overall, our results indicate that clpC expression is negatively controlled at the transcriptional level, directly or indirectly, by the central virulence regulator PrfA.

Glucose-1-phosphate utilization by Listeria monocytogenes is PrfA dependent and coordinately expressed with virulence factors

Virulence genes of the facultative intracellular pathogen Listeria monocytogenes are coordinately regulated by the activator protein PrfA, encoded by prfA, a member of the cyclic AMP receptor protein family of bacterial transcription factors. We found that prfA* mutants that constitutively overexpress the virulence regulon due to a Gly145Ser substitution in PrfA (M.-T. Ripio, G. Domínguez-Bernal, M. Lara, M. Suárez, and J.-A. Vázquez-Boland, J. Bacteriol. 179:1533-1540, 1997) rapidly utilized glucose-1-phosphate (G-1-P) as a carbon source for growth, in contrast to wild-type strains, which characteristically do not. Wild-type strains acquired the capacity for readily metabolizing G-1-P upon exposure to environmental conditions that activate the expression of prfA and PrfA-dependent virulence genes (i.e., culture at 37 degrees C in charcoal-treated medium). In these strains, G-1-P utilization followed an expressional pattern identical to that of virulence genes controlled by PrfA, with repression at 20 degrees C. Tn917 insertions in L. monocytogenes mutants selected for G-1-P utilization deficiency mapped to the plcA-prfA operon, a deltaprfA strain was totally unable to utilize G-1-P, and trans complementation with prfA constructs restored the ability to efficiently metabolize and grow on G-1-P to these mutants. Thus, G-1-P utilization by L. monocytogenes is under the tight positive control of the central virulence regulator, PrfA, and is coexpressed with PrfA-dependent pathogenicity determinants. It was recently reported that readily utilized carbohydrates, such as glucose or cellobiose, repress virulence genes in L. monocytogenes. We confirmed this but, interestingly, found that G-1-P does not inhibit expression of the PrfA regulon, indicating that this sugar follows a catabolic pathway that bypasses the repressor mechanism triggered by other readily metabolized carbon sources. PrfA dependence and coexpression with virulence genes suggest that utilization of exogenous G-1-P may be relevant to Listeria pathogenesis. G-1-P is the precursor metabolite and primary degradation product of glycogen and is therefore available within the mammalian cell. Based on our results, we hypothesize that G-1-P could play an important role as a growth substrate for intracellular Listeria.

Actinobacillus pleuropneumoniae does not require urease activity to produce acute swine pleuropneumonia

The role in virulence of Actinobacillus pleuropneumoniae urease activity was investigated. A urease-negative mutant was isolated following transposon mutagenesis with a mini-Tn10 derivative. Both the parent strain and the urease-negative mutant exhibited identical LD50 values in a murine infection model. Pig challenge confirmed that the urease-negative mutant was fully virulent, since experimental inoculation with 5 x 10(7) colony forming units resulted in an acute disease indistinguishable from that produced by the wild-type strain at the same dose. Our results demonstrate that urease activity is not required for the development of acute pleuropneumonia.

Host cell heparan sulfate proteoglycans mediate attachment and entry of Listeria monocytogenes, and the listerial surface protein ActA is involved in heparan sulfate receptor recognition

The mechanisms by which the intracellular pathogen Listeria monocytogenes interacts with the host cell surface remain largely unknown. In this study, we investigated the role of heparan sulfate proteoglycans (HSPG) in listerial infection. Pretreatment of bacteria with heparin or heparan sulfate (HS), but not with other glycosaminoglycans, inhibited attachment and subsequent uptake by IC-21 murine macrophages and CHO epithelial-like cells. Specific removal of HS from target cells with heparinase III significantly impaired listerial adhesion and invasion. Mutant CHO cells deficient in HS synthesis bound and internalized significantly fewer bacteria than wild-type cells did. Pretreatment of target cells with the HS-binding proteins fibronectin and platelet factor 4, or with heparinase III, impaired listerial infectivity only in those cells expressing HS. Moreover, a synthetic peptide corresponding to the HS-binding ligand in Plasmodium falciparum circumsporozoite protein (pepPf1) inhibited listerial attachment to IC-21 and CHO cells. A motif very similar to the HS-binding site of pepPf1 was found in the N-terminal region of ActA, the L. monocytogenes surface protein responsible for actin-based bacterial motility and cell-to-cell spread. In the same region of ActA, several clusters of positively charged amino acids which could function as HS-binding domains were identified. An ActA-deficient mutant was significantly impaired in attachment and entry due to altered HS recognition functions. This work shows that specific interaction with an HSPG receptor present on the surface of both professional and nonprofessional phagocytes is involved in L. monocytogenes cytoadhesion and invasion and strongly suggests that the bacterial surface protein ActA may be a ligand mediating HSPG receptor recognition.

Identification of a ClpC ATPase required for stress tolerance and in vivo survival of Listeria monocytogenes

We identified a new chromosomal locus involved in the virulence of the facultative intracellular pathogen Listeria monocytogenes. This locus displays the same genetic organization as that of the clpC/mecB locus of Bacillus subtilis. It contains a thermoregulated operon of four genes, whose transcription is upregulated at 42 degrees C. The last gene of this operon is clpC, which encodes a protein of 826 amino acid residues, identified as a ClpC ATPase, sharing a strong peptide sequence identity (78%) with ClpC/MecB of B. subtilis. Tn917 insertions inactivating the entire operon, or only clpC, gave mutants highly susceptible to stress, including iron limitation, elevated temperatures and high osmolarity. The virulence of these mutants was severely impaired in the mouse. A clpC insertional mutant was also restricted in its capacity to grow in bone-marrow-derived macrophages. These results demonstrate that the ClpC ATPase of L. monocytogenes is a general stress protein involved in intracellular growth and in vivo survival of this pathogen in host tissues.

Virulence factors of the swine pathogen Actinobacillus pleuropneumoniae

The Gram-negative bacterium Actinobacillus pleuropneumoniae is the etiologic agent of swine pleuropneumonia, a highly contagious respiratory infection with great economic implications. In recent years, considerable efforts have been invested in the study of its virulence mechanisms. Here we review the current knowledge on the determinants of A. pleuropneumoniae pathogenicity, paying particular attention to the capsule, the lypopolysaccharide, the outer membrane proteins, and the RTX exotoxins. The contribution of other factors is also discussed.

Transcriptional activation of virulence genes in wild-type strains of Listeria monocytogenes in response to a change in the extracellular medium composition

A panel of 103 Listeria monocytogenes strains of different origins was examined for haemolysin and lecithinase production in brain-heart infusion (BHI). Three distinct phenotypes were observed. Phenotype 1 was characterized by low to undetectable levels of expression and was exhibited by almost all strains tested. Phenotype 2 expressed high levels of haemolysin and lecithinase and was displayed by five strains: one (P14-A) was a spontaneous mutant derived from a type 1 isolate (P14); the four others (EGD-A, NCTC 7973, SLCC 2373 and CLIP 545) were all laboratory strains kept under in vitro conditions for a long period. Phenotype 3 was intermediate and was exhibited by another laboratory strain (L028). We therefore concluded that phenotype 1 corresponded to the wild type, whereas phenotypes 2 and 3 represented mutant or variant phenotypes. Interestingly, wild-type strains were able to dramatically increase the expression of virulence factors when cultured in BHI treated with activated charcoal (BHIC), up to levels similar to those constitutively expressed by the hyperhaemolytic/lecithinase variants in BHI. Experiments with P14 and P14-A demonstrated that both charcoal and the hyperhaemolytic/lecithinase mutation exerted their effect by inducing (or derepressing) transcription of prfA, the pleiotropic transcriptional activator of the L. monocytogenes virulence regulon. Moreover, P14 and P14-A were equally virulent for mice despite the different levels of virulence factor expression in BHI. Taken together, these observations indicate that L. monocytogenes turns off virulence gene expression when growing in vitro in a rich medium, and suggest that the increased levels of virulence factors in the hyperhaemolytic/lecithinase mutants and in wild-type strains grown in BHIC might represent the levels of expression needed in vivo by L. monocytogenes for infecting host tissues.

Regulation of virulence gene expression in pathogenic Listeria

Dynamic interactions between host and pathogen are characteristic of infections caused by intracellular bacteria. This has favoured the evolution of highly effective control systems by which these pathogens regulate the expression of different virulence factors during sequential steps of the infection process. In the case of the facultative intracellular bacterium Listeria monocytogenes, these steps involve internalization by eukaryotic cells, lysis of the resulting phagosome, replication as well as movement within the host cytoplasm, direct cell-to-cell spread, and subsequent lysis of a double-membrane vacuole when entering neighbouring cells. Virulence factors which are involved in each of these steps have been identified and the expression of these factors is subject to a co-ordinate and differential control exerted by the major listerial virulence regulator PrfA. This protein belongs to the Crp/Fnr-family of transcriptional activators and recognizes specific target sequences in promoter regions of several listerial virulence genes. Differential expression of these genes during sequential steps of the infection seems to be at least partially mediated by different binding affinities of PrfA to its target sequences. Activity of PrfA-dependent genes and of prfA itself is under the control of several environmental variables which are used by the pathogen to recognize its transition from the free environment into a eukaryotic host.

Prevalence of agglutinating antibodies to Toxoplasma gondii in small ruminants of the Madrid region, Spain, and identification of factors influencing seropositivity by multivariate analysis

A seroepidemiological survey of Toxoplasma gondii infection in sheep and goats was conducted in the Madrid region of Spain. Sera were collected from 60 herds, for which farming management information and other relevant data for their characterization were also obtained through a questionnaire. The seroprevalence was 11.8% (64 out of 541), using the modified (2-mercaptoethanol) direct agglutination technique with a 1:64 cut-off titre. The relationship between seropositivity and the variables in the questionnaire was assessed by multivariate analysis. Four variables were found to be significantly associated with seroprevalence. Two of them, the presence of cats and a previous history of abortion outbreaks in the farm, were factors known to be linked with toxoplasmosis, indicating the validity of the serological data. Seropositivity was also related to a lack of replacements in the preceding year. Proximity to other farms appeared to be a protective factor negatively associated with seropositivity, probably because it was an indicator of proximity to an urban area and the availability of local sanitary facilities.

The sulphydryl-activated cytolysin and a sphingomyelinase C are the major membrane-damaging factors involved in cooperative (CAMP-like) haemolysis of Listeria spp

The negative mutant approach was used in this study to identify listerial cytolytic factors involved in cooperative haemolysis (CAMP-like phenomenon) with Staphylococcus aureus and Rhodococcus equi. A Listeria monocytogenes non-haemolytic mutant specifically impaired in listeriolysin O (LLO) production gave no CAMP reaction with S. aureus, and was virtually CAMP-negative with R. equi, indicating that the listerial sulphydryl-activated toxin played a major role in cooperative haemolysis. This was confirmed by direct evidence using purified LLO and alveolysin (from Bacillus alvei) in diffusion CAMP assays. To our knowledge, this is the first evidence of involvement of a sulphydryl-activated toxin in cooperative lytic processes. Phosphatidylcholine- and phosphatidylinositol-specific phospholipases C from L. monocytogenes did not seem to significantly contribute to cooperative haemolysis, as the corresponding mutants displayed wild-type CAMP reactions. In contrast, the sphingomyelinase C from Listeria iva-novii was the cytolytic factor responsible for the characteristic shovel-shaped CAMP reaction shown by this listerial species with R. equi. Possible mechanisms of lytic cooperation are discussed.

The RTX haemolysins ApxI and ApxII are major virulence factors of the swine pathogen Actinobacillus pleuropneumoniae: evidence from mutational analysis

The involvement of the RTX haemolysins (ApxI and ApxII) of the swine pathogen Actinobacillus pleuropneumoniae in virulence was investigated using haemolysin-deficient mutants constructed by a mini-Tn10 mutagenesis procedure. Two types of haemolysin mutant with single insertions of the transposon were obtained from a serotype 1 strain producing both ApxI and ApxII. One presented a complete loss of haemolytic activity because of the absence of ApxI and ApxII production. The other displayed weaker haemolysis than the wild type and produced only ApxII. The chromosomal regions flanking mini-Tn10 were cloned and sequenced. In the non-haemolytic mutant, the transposon had inserted in apxIB, a gene involved in the exportation of ApxI and ApxII toxins. The weakly haemolytic mutant resulted from the disruption of the structural gene for ApxI. Both mutations in the apxI operon were associated with a significant loss of virulence for mice and pigs, demonstrating that haemolysins are involved in A. pleuropneumoniae pathogenicity. The non-haemolytic mutant was apathogenic and the weakly haemolytic mutant retained some virulence for pigs, suggesting that both ApxI and ApxII are needed for full virulence.

Serological response in rabbits to Listeria monocytogenes after oral or intragastric inoculation

The serological response in rabbits against Listeria monocytogenes after oral or intragastric inoculation was investigated. Both the number of sero-positive animals and the average serum titres were higher in animals inoculated by the oral route. This difference was especially marked in rabbits inoculated with the lower dose (1 x 10(3) colony-forming units (cfu)), which developed a strong serological response (average serum titre of 1280 after 4 inoculations) in most of the inoculated animals (80%), without any clinical signs. The implication of these results in the epidemiology of listeriosis is discussed.

Epidemiologic investigation of a silage-associated epizootic of ovine listeric encephalitis, using a new Listeria-selective enumeration medium and phage typing

The role of silage feeding in the origin of an epizootic of encephalitic listeriosis in a sheep flock was investigated by use of a new direct Listeria-selective isolation and enumeration medium, in combination with serotyping and phage typing. The silage contained high numbers (about 10(6) cells/g) of a L monocytogenes strain indistinguishable with respect to serovar and phagovar from that isolated from the brains of sick sheep. These results provided unambiguous bacteriologic evidence of the epidemiologic link between silage consumption and listeriosis in ruminants.

Overlay technique for direct detection and identification of haemolytic Listeria on selective plating medium. Comparison of five media

An overlay technique is proposed for the identification and counting of haemolytic Listeria colonies directly on selective plating media. The technique was applied to different Listeria-selective plating media. In pure culture studies with collection strains, the overlay technique was more efficient and reliable for detection haemolytic Listeria species compared with the incorporation of blood into the agar. The efficacy of the overlay technique for the direct detection of haemolytic colonies of Listeria from raw milk samples was related to agar selectivity. The best results were obtained with Listeria-selective agar medium modified (LSAMM). Catalase assay, together with reactions for aesculin and tellurite, were useful and reliable criteria for the identification of Listeria. All colonies on LSAMM which were positive for catalase, tellurite and aesculin while those displaying typical haemolysis corresponded in most cases to L. monocytogenes.