A broad-host-range mobilizable shuttle vector for the construction of transcriptional fusions to beta-galactosidase in gram-positive bacteria

Bicarbonate enhances expression of the endocarditis and biofilm associated pilus locus, ebpR-ebpABC, in Enterococcus faecalis

Background

We previously identified ebpR, encoding a potential member of the AtxA/Mga transcriptional regulator family, and showed that it is important for transcriptional activation of the Enterococcus faecalis endocarditis and biofilm associated pilus operon, ebpABC. Although ebpR is not absolutely essential for ebpABC expression (100-fold reduction), its deletion led to phenotypes similar to those of an ebpABC mutant such as absence of pili at the cell surface and, consequently, reduced biofilm formation. A non-piliated ebpABC mutant has been shown to be attenuated in a rat model of endocarditis and in a murine urinary tract infection model, indicating an important participation of the ebpR-ebpABC locus in virulence. However, there is no report relating to the environmental conditions that affect expression of the ebpR-ebpABC locus.

Results

In this study, we examined the effect of CO₂/HCO₃^-, pH, and the Fsr system on the ebpR-ebpABC locus expression. The presence of 5% CO₂/0.1 M HCO₃^- increased ebpR-ebpABC expression, while the Fsr system was confirmed to be a weak repressor of this locus. The mechanism by which the Fsr system repressed the ebpR-ebpABC locus expression appears independent of the effects of CO₂^- bicarbonate. Furthermore, by using an ebpA::lacZ fusion as a reporter, we showed that addition of 0.1 M sodium bicarbonate to TSBG (buffered at pH 7.5), but not the presence of 5% CO₂, induced ebpA expression in TSBG broth. In addition, using microarray analysis, we found 73 genes affected by the presence of sodium bicarbonate (abs(fold) > 2, P < 0.05), the majority of which belong to the PTS system and ABC transporter families. Finally, pilus production correlated with ebpA mRNA levels under the conditions tested.

Conclusions

This study reports that the ebp locus expression is enhanced by the presence of bicarbonate with a consequential increase in the number of cells producing pili. Although the molecular basis of the bicarbonate effect remains unclear, the pathway is independent of the Fsr system. In conclusion, E. faecalis joins the growing family of pathogens that regulates virulence gene expression in response to bicarbonate and/or CO₂.

Defining a role for Hfq in Gram-positive bacteria: evidence for Hfq-dependent antisense regulation in Listeria monocytogenes

Small trans-encoded RNAs (sRNAs) modulate the translation and decay of mRNAs in bacteria. In Gram-negative species, antisense regulation by trans-encoded sRNAs relies on the Sm-like protein Hfq. In contrast to this, Hfq is dispensable for sRNA-mediated riboregulation in the Gram-positive species studied thus far. Here, we provide evidence for Hfq-dependent translational repression in the Gram-positive human pathogen Listeria monocytogenes, which is known to encode at least 50 sRNAs. We show that the Hfq-binding sRNA LhrA controls the translation and degradation of its target mRNA by an antisense mechanism, and that Hfq facilitates the binding of LhrA to its target. The work presented here provides the first experimental evidence for Hfq-dependent riboregulation in a Gram-positive bacterium. Our findings indicate that modulation of translation by trans-encoded sRNAs may occur by both Hfq-dependent and -independent mechanisms, thus adding another layer of complexity to sRNA-mediated riboregulation in Gram-positive species.

Capsular polysaccharide production in Enterococcus faecalis and contribution of CpsF to capsule serospecificity

Many bacterial species produce capsular polysaccharides that contribute to pathogenesis through evasion of the host innate immune system. The gram-positive pathogen Enterococcus faecalis was previously reported to produce one of four capsule serotypes (A, B, C, or D). Previous studies describing the four capsule serotypes of E. faecalis were based on immunodetection methods; however, the underlying genetics of capsule production did not fully support these findings. Previously, it was shown that capsule production for serotype C (Maekawa type 2) was dependent on the presence of nine open reading frames (cpsC to cpsK). Using a novel genetic system, we demonstrated that seven of the nine genes in the cps operon are essential for capsule production, indicating that serotypes A and B do not make a capsular polysaccharide. In support of this observation, we showed that serotype C and D capsule polysaccharides mask lipoteichoic acid from detection by agglutinating antibodies. Furthermore, we determined that the genetic basis for the difference in antigenicity between serotypes C and D is the presence of cpsF in serotype C strains. High-pH anion-exchange chromatography with pulsed amperometric detection analysis of serotype C and D capsules indicated that cpsF is responsible for glucosylation of serotype C capsular polysaccharide in E. faecalis.

Direct stimulus perception and transcription activation by a membrane-bound DNA binding protein

Few membrane proteins with a role in transcriptional regulation have been studied, and none are able to perceive their respective stimuli and activate transcription of their regulons without the aid of auxiliary proteins. The bacitracin resistance regulator, BcrR, of Enterococcus faecalis is a membrane-bound DNA binding protein and is required for bacitracin-dependent expression of the bacitracin resistance genes, bcrABD. Here, we show that BcrR interacts directly with Zn2+ bacitracin (Kd = 2-5 micropM), but not metal-free bacitracin. A solution-based DNA binding assay demonstrated that the affinity of BcrR for its target DNA is much higher (Kd = 40 nM) than previously found for transmembrane regulators and is comparable to that of soluble DNA binding proteins. A construct of BcrR that lacked the transmembrane domain was unable to bind to DNA, indicating that membrane localization was important for DNA binding. Bacitracin did not cause a change in the DNaseI footprint of BcrR on the bcrA promoter, but in vitro transcription assays with BcrR proteoliposomes showed bacitracin-dependent activation of transcription. These findings demonstrate that BcrR is a bona fide one-component transmembrane signal transduction system, which perceives an extracellular stimulus (presence of bacitracin) and relays it to an intracellular transcriptional response independent of any auxiliary proteins.

CsrRS regulates group B Streptococcus virulence gene expression in response to environmental pH: a new perspective on vaccine development

To identify factors involved in the response of group B streptococci (GBS) to environmental pH, we performed a comparative global gene expression analysis of GBS at acidic and neutral pHs. We found that the transcription of 317 genes was increased at pH 5.5 relative to that at pH 7.0, while 61 genes were downregulated. The global response to acid stress included the differential expression of genes involved in transport, metabolism, stress response, and virulence. Known vaccine candidates, such as BibA and pilus components, were also regulated by pH. We observed that many of the genes involved in the GBS response to pH are known to be controlled by the CsrRS two-component system. Comparison of the regulon of wild-type strain 2603 V/R with that of a csrRS deletion mutant strain revealed that the pH-dependent regulation of 90% of the downregulated genes and 59.3% of the up-regulated genes in strain 2603 V/R was CsrRS dependent and that many virulence factors were overexpressed at high pH. Beta-hemolysin regulation was abrogated by selective inactivation of csrS, suggesting the implication of the CsrS protein in pH sensing. These results imply that the translocation of GBS from the acidic milieu of the vagina to the neutral pH of the neonatal lung signals the up-regulation of GBS virulence factors and conversion from a colonizing to an invasive phenotype. In addition, the fact that increased exposure of BibA on the bacterial surface at pH 7.0 induced opsonophagocytic killing of GBS in immune serum highlights the importance of pH regulation in the protective efficacy of specific antibodies to surface-exposed GBS proteins.

Role of class A penicillin-binding proteins in the expression of beta-lactam resistance in Enterococcus faecium

Peptidoglycan is polymerized by monofunctional d,d-transpeptidases belonging to class B penicillin-binding proteins (PBPs) and monofunctional glycosyltransferases and by bifunctional enzymes that combine both activities (class A PBPs). Three genes encoding putative class A PBPs (pbpF, pbpZ, and ponA) were deleted from the chromosome of Enterococcus faecium D344R in all possible combinations in order to identify the glycosyltransferases that cooperate with low-affinity class B Pbp5 for synthesis of peptidoglycan in the presence of beta-lactam antibiotics. The viability of the triple mutant indicated that glycan strands can be polymerized independently from class A PBPs by an unknown glycosyltranferase. The susceptibility of the DeltapbpF DeltaponA mutant and triple mutants to extended spectrum cephalosporins (ceftriaxone and cefepime) identified either PbpF or PonA as essential partners of Pbp5 for peptidoglycan polymerization in the presence of the drugs. Mass spectrometry analysis of peptidoglycan structure showed that loss of PonA and PbpF activity led to a minor decrease in the extent of peptidoglycan cross-linking by the remaining PBPs without any detectable compensatory increase in the participation of the L,D-transpeptidase in peptidoglycan synthesis. Optical density measurements and electron microscopy analyses showed that the DeltapbpF DeltaponA mutant underwent increased stationary-phase autolysis compared to the parental strain. Unexpectedly, deletion of the class A pbp genes revealed dissociation between the expression of resistance to cephalosporins and penicillins, although the production of Pbp5 was required for resistance to both classes of drugs. Thus, susceptibility of Pbp5-mediated peptidoglycan cross-linking to different beta-lactam antibiotics differed as a function of its partner glycosyltransferase.

Two small c-type cytochromes affect virulence gene expression in Bacillus anthracis

Regulated expression of the genes for anthrax toxin proteins is essential for the virulence of the pathogenic bacterium Bacillus anthracis. Induction of toxin gene expression depends on several factors, including temperature, bicarbonate levels, and metabolic state of the cell. To identify factors that regulate toxin expression, transposon mutagenesis was performed under non-inducing conditions and mutants were isolated that untimely expressed high levels of toxin. A number of these mutations clustered in the haem biosynthetic and cytochrome c maturation pathways. Genetic analysis revealed that two haem-dependent, small c-type cytochromes, CccA and CccB, located on the extracellular surface of the cytoplasmic membrane, regulate toxin gene expression by affecting the expression of the master virulence regulator AtxA. Deregulated AtxA expression in early exponential phase resulted in increased expression of toxin genes in response to loss of the CccA-CccB signalling pathway. This is the first function identified for these two small c-type cytochromes of Bacillus species. Extension of the transposon screen identified a previously uncharacterized protein, BAS3568, highly conserved across many bacterial and archeal species, as involved in cytochrome c activity and virulence regulation. These findings are significant not only to virulence regulation in B. anthracis, but also to analysis of virulence regulation in many pathogenic bacteria and to the study of cytochrome c activity in Gram-positive bacteria.

The bicarbonate transporter is essential for Bacillus anthracis lethality

In the pathogenic bacterium Bacillus anthracis, virulence requires induced expression of the anthrax toxin and capsule genes. Elevated CO2/bicarbonate levels, an indicator of the host environment, provide a signal ex vivo to increase expression of virulence factors, but the mechanism underlying induction and its relevance in vivo are unknown. We identified a previously uncharacterized ABC transporter (BAS2714-12) similar to bicarbonate transporters in photosynthetic cyanobacteria, which is essential to the bicarbonate induction of virulence gene expression. Deletion of the genes for the transporter abolished induction of toxin gene expression and strongly decreased the rate of bicarbonate uptake ex vivo, demonstrating that the BAS2714-12 locus encodes a bicarbonate ABC transporter. The bicarbonate transporter deletion strain was avirulent in the A/J mouse model of infection. Carbonic anhydrase inhibitors, which prevent the interconversion of CO2 and bicarbonate, significantly affected toxin expression only in the absence of bicarbonate or the bicarbonate transporter, suggesting that carbonic anhydrase activity is not essential to virulence factor induction and that bicarbonate, and not CO2, is the signal essential for virulence induction. The identification of this novel bicarbonate transporter essential to virulence of B. anthracis may be of relevance to other pathogens, such as Streptococcus pyogenes, Escherichia coli, Borrelia burgdorferi, and Vibrio cholera that regulate virulence factor expression in response to CO2/bicarbonate, and suggests it may be a target for antibacterial intervention.

Transcriptional regulation of the citrate gene cluster of Enterococcus faecalis Involves the GntR family transcriptional activator CitO

The genome of the gram-positive bacterium Enterococcus faecalis contains the genes that encode the citrate lyase complex. This complex splits citrate into oxaloacetate and acetate and is involved in all the known anaerobic bacterial citrate fermentation pathways. Although citrate fermentation in E. faecalis has been investigated before, the regulation and transcriptional pattern of the cit locus has still not been fully explored. To fill this gap, in this paper we demonstrate that the GntR transcriptional regulator CitO is a novel positive regulator involved in the expression of the cit operons. The transcriptional analysis of the cit clusters revealed two divergent operons: citHO, which codes for the transporter (citH) and the regulatory protein (citO), and upstream from it and in the opposite direction the oadHDB-citCDEFX-oadA-citMG operon, which includes the citrate lyase subunits (citD, citE, and citF), the soluble oxaloacetate decarboxylase (citM), and also the genes encoding a putative oxaloacetate decarboxylase complex (oadB, oadA, oadD and oadH). This analysis also showed that both operons are specifically activated by the addition of citrate to the medium. In order to study the functional role of CitO, a mutant strain with an interrupted citO gene was constructed, causing a total loss of the ability to degrade citrate. Reintroduction of a functional copy of citO to the citO-deficient strain restored the response to citrate and the Cit(+) phenotype. Furthermore, we present evidence that CitO binds to the cis-acting sequences O(1) and O(2), located in the cit intergenic region, increasing its affinity for these binding sites when citrate is present and allowing the induction of both cit promoters.

Ethanolamine activates a sensor histidine kinase regulating its utilization in Enterococcus faecalis

Enterococcus faecalis is a gram-positive commensal bacterium of the human intestinal tract. Its opportunistic pathogenicity has been enhanced by the acquisition of multiple antibiotic resistances, making the treatment of enterococcal infections an increasingly difficult problem. The extraordinary capacity of this organism to colonize and survive in a wide variety of ecological niches is attributable, at least in part, to signal transduction pathways mediated by two-component systems (TCS). Here, the ability of E. faecalis to utilize ethanolamine as the sole carbon source is shown to be dependent upon the RR-HK17 (EF1633-EF1632) TCS. Ethanolamine is an abundant compound in the human intestine, and thus, the ability of bacteria to utilize it as a source of carbon and nitrogen may provide an advantage for survival and colonization. Growth of E. faecalis in a synthetic medium with ethanolamine was abolished in the response regulator RR17 mutant strain. Transcription of the response regulator gene was induced by the presence of ethanolamine. Ethanolamine induced a 15-fold increase in the rate of autophosphorylation in vitro of the HK17 sensor histidine kinase, indicating that this is the ligand recognized by the sensor domain of the kinase. These results assign a role to the RR-HK17 TCS as coordinator of the enterococcal response to specific nutritional conditions existing at the site of bacterial invasion, the intestinal tract of an animal host.

Dual promoters control expression of the Bacillus anthracis virulence factor AtxA

The AtxA virulence regulator of Bacillus anthracis is required for toxin and capsule gene expression. AtxA is a phosphotransferase system regulatory domain-containing protein whose activity is regulated by phosphorylation/dephosphorylation of conserved histidine residues. Here we report that transcription of the atxA gene occurs from two independent promoters, P1 (previously described by Dai et al. [Z. Dai, J. C. Sirard, M. Mock, and T. M. Koehler, Mol. Microbiol. 16:1171-1181, 1995]) and P2, whose transcription start sites are separated by 650 bp. Both promoters have -10 and -35 consensus sequences compatible with recognition by sigma(A)-containing RNA polymerase, and neither promoter depends on the sporulation sigma factor SigH. The dual promoter activity and the extended untranslated mRNA suggest that as-yet-unknown regulatory mechanisms may act on this region to influence the level of AtxA in the cell.

Identification of a sigma B-dependent small noncoding RNA in Listeria monocytogenes

In Listeria monocytogenes, the alternative sigma factor sigma(B) plays important roles in stress tolerance and virulence. Here, we present the identification of SbrA, a novel small noncoding RNA that is produced in a sigma(B)-dependent manner. This finding adds the sigma(B) regulon to the growing list of stress-induced regulatory circuits that include small noncoding RNAs.

Virulence gene expression is independent of ResDE-regulated respiration control in Bacillus anthracis

The ResDE two-component system regulates the synthesis of several components of the aerobic and anaerobic respiratory pathways in bacilli. The ResD response regulator transcription factor has been implicated in the regulation of virulence factors in a number of gram-positive species, including Bacillus anthracis. The precise deletions of resD and resE in B. anthracis that retained the classical respiratory phenotypes did not affect the expression of the gene for the protective antigen of the anthrax toxin, pagA, or that of the toxin regulator, atxA. The results indicate that the loss of ResDE-controlled respiratory capacity does not affect the synthesis of anthrax toxin.

The two-component system CesRK controls the transcriptional induction of cell envelope-related genes in Listeria monocytogenes in response to cell wall-acting antibiotics

The two-component system CesRK of Listeria monocytogenes responds to cell wall-acting antibiotics. We show here that CesRK controls the transcription of several cell envelope-related genes. The CesRK-dependent induction of these genes may be viewed as an attempt by L. monocytogenes to protect itself against the damaging effects of cell wall-acting antibiotics.

Evaluation of the Staphylococcus aureus class C nonspecific acid phosphatase (SapS) as a reporter for gene expression and protein secretion in gram-negative and gram-positive bacteria

A phosphatase secreted by Staphylococcus aureus strain 154 has previously been characterized and classified as a new member of the bacterial class C family of nonspecific acid phosphatases. As the acid phosphatase activity can be easily detected with a cost-effective plate screen assay, quantitatively measured by a simple enzyme assay, and detected by zymography, its potential use as a reporter system was investigated. The S. aureus acid phosphatase (sapS) gene has been cloned and expressed from its own regulatory sequences in Escherichia coli, Bacillus subtilis, and Bacillus halodurans. Transcriptional and translational fusions of the sapS gene with selected heterologous promoters and signal sequences were constructed and expressed in all three of the host strains. From the range of promoters evaluated, the strongest promoter for heterologous protein production in each of the host strains was identified, i.e., the E. coli lacZ promoter in E. coli, the B. halodurans alkaline protease promoter in B. subtilis, and the B. halodurans sigma(D) promoter in B. halodurans. This is the first report on the development of a class C acid phosphatase gene as a reporter gene with the advantage of being able to function in both gram-positive and gram-negative host strains.

Regions of Diversity 8, 9 and 13 contribute to Streptococcus pneumoniae virulence

BACKGROUND

Streptococcus pneumoniae is the leading cause of community-acquired pneumonia. Previously, using comparative genomic analyses, 13 regions of genomic plasticity have been identified in the S. pneumoniae genome. These "Regions of Diversity" (RDs) accounted for half the genomic variation observed amongst all pneumococci tested, moreover, were determined to encode a variety of putative virulence factors. To date, genes within 5 RDs have been unequivocally demonstrated to contribute to S. pneumoniae virulence. It is unknown if the remaining RDs also contribute to virulence.

RESULTS

Using allelic exchange, we created S. pneumoniae mutants that were deficient in RD2, 5, 7, 8, 9, 12 and 13. Mutants deficient in RD8, 9 and 13 were attenuated in a mouse model of disease. RD8 is 40,358 nucleotides in length and encodes 37 genes. Using a panel of isogenic mutants, we determined that RD8b3 is the operon within RD8 that is responsible for virulence. Mice infected with mutants deficient in RD8, RD8b3, RD9 and RD13 had significantly less bacteria in the blood two days after intranasal challenge and improved survival over time versus mice infected with wild type. In all instances mutants colonized the nasopharynx at levels equivalent to wild type.

CONCLUSION

Genes within RD1, 3, 4, 6, and 10 have previously been shown to contribute to virulence. This study demonstrates that genes within RD8, 9 and 13 also contribute to virulence. The ability of mutants deficient in RD2, 5, 7, 8, 9, 12, and 13 to colonize the nasopharynx indicates that genes within these RDs are not required for asymptomatic carriage. Nonetheless, the observation that mutants deficient in RD8b3, 9 and 13 are attenuated indicates that genes within these loci are necessary for spread of the bacteria beyond the nasopharynx to normally sterile sites.

Staphylococcus aureus RNAIII coordinately represses the synthesis of virulence factors and the transcription regulator Rot by an antisense mechanism

RNAIII is the intracellular effector of the quorum-sensing system in Staphylococcus aureus. It is one of the largest regulatory RNAs (514 nucleotides long) that are known to control the expression of a large number of virulence genes. Here, we show that the 3' domain of RNAIII coordinately represses at the post-transcriptional level, the expression of mRNAs that encode a class of virulence factors that act early in the infection process. We demonstrate that the 3' domain acts primarily as an antisense RNA and rapidly anneals to these mRNAs, forming long RNA duplexes. The interaction between RNAIII and the mRNAs results in repression of translation initiation and triggers endoribonuclease III hydrolysis. These processes are followed by rapid depletion of the mRNA pool. In addition, we show that RNAIII and its 3' domain mediate translational repression of rot mRNA through a limited number of base pairings involving two loop-loop interactions. Since Rot is a transcriptional regulatory protein, we proposed that RNAIII indirectly acts on many downstream genes, resulting in the activation of the synthesis of several exoproteins. These data emphasize the multitude of regulatory steps affected by RNAIII and its 3' domain in establishing a network of S. aureus virulence factors.

EbpR is important for biofilm formation by activating expression of the endocarditis and biofilm-associated pilus operon (ebpABC) of Enterococcus faecalis OG1RF

We identify ef1090 (renamed ebpR) and show its importance for the transcriptional regulation of expression of the Enterococcus faecalis pilus operon, ebpABC. An ebpR deletion (DeltaebpR) mutant was found to have reduced ebpABC expression with loss of pilus production and a defect in primary adherence with, as a consequence, reduced biofilm formation.

The putative glycosyltransferase-encoding gene cylJ and the group B Streptococcus (GBS)-specific gene cylK modulate hemolysin production and virulence of GBS

Group B streptococcus (GBS) expresses a hemolysin/cytolysin that plays an important role in pathogenesis. Using the Himar1 transposon mutagenesis system, a hypohemolytic mutant carrying an interrupted cylJ gene was characterized. cylJ, encoding a putative glycosyltransferase, and cylK, whose product is unknown, are both required for the full hemolytic/cytolytic activity, pigment formation, and virulence of GBS.

Negative regulation of Bacillus anthracis sporulation by the Spo0E family of phosphatases

The initiation of sporulation in Bacillus species is controlled by the phosphorelay signal transduction system. Multiple regulatory elements act on the phosphorelay to modulate the level of protein phosphorylation in response to cellular, environmental, and metabolic signals. In Bacillus anthracis nine possible histidine sensor kinases can positively activate the system, while two response regulator aspartyl phosphate phosphatases of the Rap family negatively impact the pathway by dephosphorylating the Spo0F intermediate response regulator. In this study, we have characterized the B. anthracis members of the Spo0E family of phosphatases that specifically dephosphorylate the Spo0A response regulator of the phosphorelay and master regulator of sporulation. The products of four genes were able to promote the dephosphorylation of Spo0A approximately P in vitro. The overexpression of two of these B. anthracis Spo0E-like proteins from a multicopy vector consistently resulted in a sporulation-deficient phenotype. A third gene was found to be not transcribed in vivo. A fourth gene encoded a prematurely truncated protein due to a base pair deletion that nevertheless was subject to translational frameshift repair in an Escherichia coli protein expression system. A fifth Spo0E-like protein has been structurally and functionally characterized as a phosphatase of Spo0A approximately P by R. N. Grenha et al. (J. Biol. Chem. 281:37993-38003, 2006). We propose that these proteins may contribute to maintain B. anthracis in the transition phase of growth during an active infection and therefore contribute to the virulence of this organism.

The transcriptional regulator RovS controls the attachment of Streptococcus agalactiae to human epithelial cells and the expression of virulence genes

Streptococcus agalactiae is part of the normal flora of the human gastrointestinal tract and also the leading cause of bacterial infections in human newborns and immunocompromised adults. The colonization and infection of different regions within the human host require a regulatory network in S. agalactiae that senses environmental stimuli and controls the formation of specific virulence factors. In the present study, we characterized an Rgg-like transcriptional regulator, designated RovS (regulator of virulence in Streptococcus agalactiae). Deletion of the rovS gene in the genome of S. agalactiae resulted in strain 6313 DeltarovS, which exhibited an increased attachment to immobilized fibrinogen and a significant increase in adherence to the eukaryotic lung epithelial cell line A549. Quantification of expression levels of known and putative S. agalactiae virulence genes by real-time PCR revealed that RovS influences the expression of fbsA, gbs0230, sodA, rogB, and the cyl operon. The altered gene expression in mutant 6313 DeltarovS was restored by plasmid-mediated expression of rovS, confirming the RovS deficiency as the cause for the observed changes in virulence gene expression in S. agalactiae. DNA electrophoretic mobility shift assays showed that RovS specifically binds to the promoter regions of fbsA, gbs0230, sodA, and the cyl operon, indicating that RovS directly regulates their expression. Deletion and mutation studies in the promoter region of fbsA, encoding the main fibrinogen receptor in S. agalactiae, identified a RovS DNA motif. Similar motifs were also found in the promoter regions of gbs0230, sodA, and the cyl operon, and alignments allowed us to propose a consensus sequence for the DNA-binding site of RovS.

The response regulator ResD modulates virulence gene expression in response to carbohydrates in Listeria monocytogenes

Listeria monocytogenes is a versatile bacterial pathogen that is able to accommodate to diverse environmental and host conditions. Presently, we have identified a L. monocytogenes two-component response regulator, ResD that is required for the repression of virulence gene expression known to occur in the presence of easily fermentable carbohydrates not found inside host organisms. Structurally and functionally, ResD resembles the respiration regulator ResD in Bacillus subtilis as deletion of the L. monocytogenes resD reduces respiration and expression of cydA, encoding a subunit of cytochrome bd. The resD mutation also reduces expression of mptA encoding the EIIABman component of a mannose/glucose-specific PTS system, indicating that ResD controls sugar uptake. This notion was supported by the poor growth of resD mutant cells that was alleviated by excess of selected carbohydrates. Despite the growth deficient phenotype of the mutant in vitro the mutation did not affect intracellular multiplication in epithelial or macrophage cell lines. When examining virulence gene expression we observed traditional induction by charcoal in both mutant and wild-type cells whereas the repression observed in wild-type cells by fermentable carbohydrates did not occur in resD mutant cells. Thus, ResD is a central regulator of L. monocytogenes when present in the external environment.

Cerebral microcirculation shear stress levels determine Neisseria meningitidis attachment sites along the blood-brain barrier

Neisseria meningitidis is a commensal bacterium of the human nasopharynx. Occasionally, this bacterium reaches the bloodstream and causes meningitis after crossing the blood–brain barrier by an unknown mechanism. An immunohistological study of a meningococcal sepsis case revealed that neisserial adhesion was restricted to capillaries located in low blood flow regions in the infected organs. This study led to the hypothesis that drag forces encountered by the meningococcus in the bloodstream determine its attachment site in vessels. We therefore investigated the ability of N. meningitidis to bind to endothelial cells in the presence of liquid flow mimicking the bloodstream with a laminar flow chamber. Strikingly, average blood flows reported for various organs strongly inhibited initial adhesion. As cerebral microcirculation is known to be highly heterogeneous, cerebral blood velocity was investigated at the level of individual vessels using intravital imaging of rat brain. In agreement with the histological study, shear stress levels compatible with meningococcal adhesion were only observed in capillaries, which exhibited transient reductions in flow. The flow chamber assay revealed that, after initial attachment, bacteria resisted high blood velocities and even multiplied, forming microcolonies resembling those observed in the septicemia case. These results argue that the combined mechanical properties of neisserial adhesion and blood microcirculation target meningococci to transiently underperfused cerebral capillaries and thus determine disease development.

Putative surface proteins encoded within a novel transferable locus confer a high-biofilm phenotype to Enterococcus faecalis

Enterococci are opportunistic pathogens and among the leading causes of nosocomial infections. Enterococcus faecalis, the dominant species among infection-derived isolates, has recently been recognized as capable of forming biofilms on abiotic surfaces in vitro as well as on indwelling medical devices. A few bacterial factors known to contribute to biofilm formation in E. faecalis have been characterized. To identify additional factors which may be important to this process, we utilized a Tn917-based insertional mutagenesis strategy to generate a mutant bank in a high-biofilm-forming E. faecalis strain, E99. The resulting mutant bank was screened for mutants exhibiting a significantly reduced ability to form biofilms. One mutant, P101D12, which showed greater than 70% reduction in its ability to form biofilms compared to the wild-type parent, was further characterized. The single Tn917 insertion in P101D12 was mapped to a gene, bee-2, encoding a probable cell wall-anchored protein. Sequence information for the region flanking bee-2 revealed that this gene was a member of a locus (termed the bee locus for biofilm enhancer in enterococcus) comprised of five genes encoding three putative cell wall-anchored proteins and two probable sortases. Contour-clamped homogeneous electric field gel and Southern hybridization analyses suggested that the bee locus is likely harbored on a large conjugative plasmid. Filter mating assays using wild-type E99 or mutant P101D12 as a donor confirmed that the bee locus could transfer conjugally at high frequency to recipient E. faecalis strains. This represents the first instance of the identification of a mobile genetic element conferring biofilm-forming property in E. faecalis.

Rap phosphatase of virulence plasmid pXO1 inhibits Bacillus anthracis sporulation

This study shows that the Bacillus anthracis pXO1 virulence plasmid carries a Rap-Phr system, BXA0205, which regulates sporulation initiation in this organism. The BXA0205Rap protein was shown to dephosphorylate the Spo0F response regulator intermediate of the phosphorelay signal transduction system that regulates the initiation of the developmental pathway in response to environmental, metabolic, and cell cycle signals. The activity of the Rap protein was shown to be inhibited by the carboxy-terminal pentapeptide generated through an export-import processing pathway from the associated BXA0205Phr protein. Deregulation of the Rap activity by either overexpression or lack of the Phr pentapeptide resulted in severe inhibition of sporulation. Five additional Rap-Phr encoding systems were identified on the chromosome of B. anthracis, one of which, BA3790-3791, also affected sporulation initiation. The results suggest that the plasmid-borne Rap-Phr system may provide a selective advantage to the virulence of B. anthracis.

Iron acquisition systems for ferric hydroxamates, haemin and haemoglobin in Listeria monocytogenes

Listeria monocytogenes is a Gram-positive bacterium that causes severe opportunistic infections in humans and animals. We biochemically characterized, for the first time, the iron uptake processes of this facultative intracellular pathogen, and identified the genetic loci encoding two of its membrane iron transporters. Strain EGD-e used iron complexes of hydroxamates (ferrichrome and ferrichrome A, ferrioxamine B), catecholates (ferric enterobactin, ferric corynebactin) and eukaryotic binding proteins (transferrin, lactoferrin, ferritin, haemoglobin). Quantitative determinations showed 10-100-fold lower affinity for ferric siderophores (Km approximately 1-10 nM) than Gram-negative bacteria, and generally lower uptake rates. Vmax for [59Fe]-enterobactin (0.15 pMol per 10(9) cells per minute) was 400-fold lower than that of Escherichia coli. For [59Fe]-corynebactin, Vmax was also low (1.2 pMol per 10(9) cells per minute), but EGD-e transported [59Fe]-apoferrichrome similarly to E. coli (Vmax=24 pMol per 10(9) cells per minute). L. monocytogenes encodes potential Fur-regulated iron transporters at 2.031 Mb (the fur-fhu region), 2.184 Mb (the feo region), 2.27 Mb (the srtB region) and 2.499 Mb (designated hupDGC region). Chromosomal deletions in the fur-fhu and hupDGC regions diminished iron uptake from ferric hydroxamates and haemin/haemoglobin respectively. In the former locus, deletion of fhuD (lmo1959) or fhuC (lmo1960) strongly reduced [59Fe]-apoferrichrome uptake. Deletion of hupC (lmo2429) eliminated the uptake of haemin and haemoglobin, and decreased the virulence of L. monocytogenes 50-fold in mice. Elimination of srtB region genes (Deltalmo2185, Deltalmo2186, Deltalmo2183), both sortase structural genes (DeltasrtB, DeltasrtA, DeltasrtAB), fur and feoB did not impair iron transport. However, deletion of bacterioferritin (Deltafri, lmo943; 0.97 Mb) decreased growth and altered iron uptake: Vmax of [59Fe]-corynebactin transport tripled in this strain, whereas that of [59Fe]-apoferrichrome decreased 20-fold.

Characterization of sporulation histidine kinases of Bacillus anthracis

The initiation of sporulation in Bacillus species is regulated by the phosphorelay signal transduction pathway, which is activated by several histidine sensor kinases in response to cellular and metabolic signals. Comparison of the protein components of the phosphorelay between Bacillus subtilis and Bacillus anthracis revealed high homology in the phosphorelay orthologs of Spo0F, Spo0B, and Spo0A. The sensor domains of sensor histidine kinases are poorly conserved between species, making ortholog recognition tenuous. Putative sporulation sensor histidine kinases of B. anthracis were identified by homology to the HisKA domain of B. subtilis sporulation sensor histidine kinases, which interacts with Spo0F. Nine possible kinases were uncovered, and their genes were assayed for complementation of kinase mutants of B. subtilis, for ability to drive lacZ expression in B. subtilis and B. anthracis, and for the effect of deletion of each on the sporulation of B. anthracis. Five of the nine sensor histidine kinases were inferred to be capable of inducing sporulation in B. anthracis. Four of the sensor kinases could not be shown to induce sporulation; however, the genes for two of these were frameshifted in all B. anthracis strains and one of these was also frameshifted in the pathogenic pXO1-bearing Bacillus cereus strain G9241. It is proposed that acquisition of plasmid pXO1 and pathogenicity may require a dampening of sporulation regulation by mutational selection of sporulation sensor histidine kinase defects. The sporulation of B. anthracis ex vivo appears to result from any one or a combination of the sporulation sensor histidine kinases remaining.

Fsr-independent production of protease(s) may explain the lack of attenuation of an Enterococcus faecalis fsr mutant versus a gelE-sprE mutant in induction of endocarditis

An Enterococcus faecalis gelE insertion disruption mutant (TX5128), which produces neither gelatinase (GelE) nor the cotranscribed (in the wild type) serine protease (SprE), was found to be attenuated in a rat endocarditis model with a significant decrease in the endocarditis induction rate versus wild-type E. faecalis OG1RF (GelE(+), SprE(+)). TX5266, which has a nonpolar deletion in fsrB and, like TX5128, is phenotypically GelE(-) under usual conditions, was also studied; fsrB is a homologue of agrB of staphylococci and participates in regulation of gelE-sprE expression. Unexpectedly, TX5266 approximated wild-type OG1RF in the endocarditis model and was significantly less attenuated than TX5128. This is in contrast to other models which have found fsr mutants to be as or more attenuated than TX5128. Further study found that the fsrB mutant produced very low levels of gelatinase activity after prolonged incubation in vitro versus no gelatinase activity with TX5128 and did not show the extensive chaining characteristic of TX5128. Reverse transcription-PCR confirmed that gelE was expressed in TX5266 at a very low level versus wild-type OG1RF and was not expressed at all in TX5128. Possible explanations for the increased induction of endocarditis by TX5266 versus TX5128 include the production of low levels of protease(s) or some other effect(s) of the inactivation of the E. faecalis fsr regulator. The equivalent ability of OG1RF and its fsr mutant to initiate endocarditis may explain why we did not find naturally occurring fsr mutants, which account for ca. 35% of E. faecalis isolates, unrepresented in endocarditis versus fecal isolates.

Staphylococcus aureus RNAIII and the endoribonuclease III coordinately regulate spa gene expression

Staphylococcus aureus RNAIII is one of the largest regulatory RNAs, which controls several virulence genes encoding exoproteins and cell-wall-associated proteins. One of the RNAIII effects is the repression of spa gene (coding for the surface protein A) expression. Here, we show that spa repression occurs not only at the transcriptional level but also by RNAIII-mediated inhibition of translation and degradation of the stable spa mRNA by the double-strand-specific endoribonuclease III (RNase III). The 3' end domain of RNAIII, partially complementary to the 5' part of spa mRNA, efficiently anneals to spa mRNA through an initial loop-loop interaction. Although this annealing is sufficient to inhibit in vitro the formation of the translation initiation complex, the coordinated action of RNase III is essential in vivo to degrade the mRNA and irreversibly arrest translation. Our results further suggest that RNase III is recruited for targeting the paired RNAs. These findings add further complexity to the expression of the S. aureus virulon.

Revising the role of the pneumococcal vex-vncRS locus in vancomycin tolerance

Vancomycin is used increasingly to treat invasive infections caused by multidrug-resistant Streptococcus pneumoniae. Although no vancomycin-resistant strains have been isolated to date, tolerant strains that fail to die rapidly and that cause relapsing disease have been described. The vex123-pep27-vncRS locus, consisting of an ABC transporter, a presumed signaling peptide, and a two-component system, respectively, has been implicated in vancomycin tolerance. Recent findings, however, challenged this model. The data presented here indicate that erythromycin in the growth medium induces a vancomycin-tolerant phenotype and that loss of function of Pep27 or VncRS does not alter autolysis. However, a role for the ABC transporter encoded by the vex123 genes in tolerance was confirmed. A vex3 mutant was considerably more tolerant to vancomycin treatment than the wild-type strain T4, and the strength of the phenotype depended on the orientation of the resistance cassette used to construct the mutant. Microarray results suggested a number of genes that might be involved in tolerance in the vex3 mutant. Although the exact function and regulation of the vex123-pep27-vncRS locus remains to be determined, several factors influence the autolysis behavior of S. pneumoniae, including the bacterial capsule, erythromycin, and the lytA and vex3 gene products.

Impact of specific pbp5 mutations on expression of beta-lactam resistance in Enterococcus faecium

We tested the impact of individual PBP 5 mutations on expression of ampicillin resistance in Enterococcus faecium using a shuttle plasmid designed to facilitate expression of cloned pbp5 in ampicillin-susceptible E. faecium D344SRF. Substitutions that had been implicated in contributing to the resistance of clinical strains conferred only modest levels of resistance when they were present as single point mutations. The levels of resistance were amplified when some mutations were present in combination. In particular, a methionine-to-alanine change at position 485 (in close proximity to the active site) combined with the insertion of a serine at position 466 (located in a loop that forms the outer edge of the active site) was associated with the highest levels of resistance to all beta-lactams. Affinity for penicillin generally correlated with beta-lactam MICs for the mutants, but these associations were not strictly proportional.

The Enterococcus faecalis fsr two-component system controls biofilm development through production of gelatinase

Bacterial growth as a biofilm on solid surfaces is strongly associated with the development of human infections. Biofilms on native heart valves (infective endocarditis) is a life-threatening disease as a consequence of bacterial resistance to antimicrobials in such a state. Enterococci have emerged as a cause of endocarditis and nosocomial infections despite being normal commensals of the gastrointestinal and female genital tracts. We examined the role of two-component signal transduction systems in biofilm formation by the Enterococcus faecalis V583 clinical isolate and identified the fsr regulatory locus as the sole two-component system affecting this unique mode of bacterial growth. Insertion mutations in the fsr operon affected biofilm formation on two distinct abiotic surfaces. Inactivation of the fsr-controlled gene gelE encoding the zinc-metalloprotease gelatinase was found to prevent biofilm formation, suggesting that this enzyme may present a unique target for therapeutic intervention in enterococcal endocarditis.

Structure and DNA-binding properties of the cytolysin regulator CylR2 from Enterococcus faecalis

Enterococcus faecalis is one of the major causes for hospital-acquired antibiotic-resistant infections. It produces an exotoxin, called cytolysin, which is lethal for a wide range of Gram-positive bacteria and is toxic to higher organisms. Recently, the regulation of the cytolysin operon was connected to autoinduction by a quorum-sensing mechanism involving the CylR1/CylR2 two-component regulatory system. We report here the crystal structure of CylR2 and its properties in solution as determined by heteronuclear NMR spectroscopy. The structure reveals a rigid dimer containing a helix-turn-helix DNA-binding motif as part of a five-helix bundle that is extended by an antiparallel beta-sheet. We show that CylR2 is a DNA-binding protein that binds specifically to a 22 bp fragment of the cytolysin promoter region. NMR chemical shift perturbation experiments identify surfaces involved in DNA binding and are in agreement with a model for the CylR2/DNA complex that attributes binding specificity to a complex network of CylR2/DNA interactions. Our results propose a mechanism where repression is achieved by CylR2 obstruction of the promoter preventing biosynthesis of the cytolysin operon transcript.

CesRK, a two-component signal transduction system in Listeria monocytogenes, responds to the presence of cell wall-acting antibiotics and affects beta-lactam resistance

Listeria monocytogenes is a food-borne pathogen that can cause a variety of illnesses ranging from gastroenteritis to life-threatening septicemia. The beta-lactam antibiotic ampicillin remains the drug of choice for the treatment of listeriosis. We have previously identified a response regulator of a putative two-component signal transduction system that plays a role in the virulence and ethanol tolerance of L. monocytogenes. Here we present evidence that the response regulator, CesR, and a histidine protein kinase, CesK, which is encoded by the gene downstream from cesR, are involved in the ability of L. monocytogenes to tolerate ethanol and cell wall-acting antibiotics of the beta-lactam family. Furthermore, CesRK controls the expression of a putative extracellular peptide encoded by the orf2420 gene, located immediately downstream from cesRK. Inactivation of orf2420 revealed that it contributes to ethanol tolerance and pathogenesis in mice. Interestingly, we found that transcription of orf2420 was strongly induced by subinhibitory concentrations of various cell wall-acting antibiotics, ethanol, and lysozyme. The induction of orf2420 expression was abolished in the absence of CesRK. Our data suggest that CesRK is involved in regulating aspects of the cell envelope architecture and that changes in cell wall integrity provide a potent stimulus for CesRK-mediated regulation. These results further our understanding of how L. monocytogenes senses and responds to antibiotics that are used therapeutically in the treatment of infectious diseases.

The RNA-binding protein Hfq of Listeria monocytogenes: role in stress tolerance and virulence

In gram-negative bacteria, the RNA-binding protein Hfq has emerged as an important regulatory factor in a variety of physiological processes, including stress resistance and virulence. In Escherichia coli, Hfq modulates the stability or the translation of mRNAs and interacts with numerous small regulatory RNAs. Here, we studied the role of Hfq in the stress tolerance and virulence of the gram-positive food-borne human pathogen Listeria monocytogenes. We present evidence that Hfq is involved in the ability of L. monocytogenes to tolerate osmotic and ethanol stress and contributes to long-term survival under amino acid-limiting conditions. However, Hfq is not required for resistance to acid and oxidative stress. Transcription of hfq is induced under various stress conditions, including osmotic and ethanol stress and at the entry into the stationary growth phase, thus supporting the view that Hfq is important for the growth and survival of L. monocytogenes in harsh environments. The stress-inducible transcription of hfq depends on the alternative sigma factor sigmaB, which controls the expression of numerous stress- and virulence-associated genes in L. monocytogenes. Infection studies showed that Hfq contributes to pathogenesis in mice, yet plays no role in the infection of cultured cell lines. This study provides, for the first time, information on the role of Hfq in the stress tolerance and virulence of a gram-positive pathogen.

Relevance of peptide uptake systems to the physiology and virulence of Streptococcus agalactiae

Streptococcus agalactiae is a major cause of invasive infections in human newborns. To satisfy its growth requirements, S. agalactiae takes up 9 of the 20 proteinogenic amino acids from the environment. Defined S. agalactiae mutants in one or several of four putative peptide permease systems were constructed and tested for peptide uptake, growth in various media, and expression of virulence traits. Oligopeptide uptake by S. agalactiae was shown to be mediated by the ABC transporter OppA1-F, which possesses two substrate-binding proteins (OppA1 and OppA2) with overlapping substrate specificities. Dipeptides were found to be taken up in parallel by the oligopeptide permease OppA1-F, by the dipeptide ABC transporter DppA-E, and by the dipeptide symporter DpsA. Reverse transcription-PCR analysis revealed a polycistronic organization of the genes oppA1-F and dppA-E and a monocistronic organization of dpsA in S. agalactiae. The results of quantitative real-time PCR revealed a medium-dependent expression of the operons dppA-E and oppA1-F in S. agalactiae. Growth of S. agalactiae in human amniotic fluid was shown to require an intact dpsA gene, indicating an important role of DpsA during the infection of the amniotic cavity by S. agalactiae. Deletion of the oppB gene reduced the adherence of S. agalactiae to epithelial cells by 26%, impaired its adherence to fibrinogen and fibronectin by 42 and 33%, respectively, and caused a 35% reduction in expression of the fbsA gene, which encodes a fibrinogen-binding protein in S. agalactiae. These data indicate that the oligopeptide permease is involved in modulating virulence traits and virulence gene expression in S. agalactiae.

pbp2229-mediated nisin resistance mechanism in Listeria monocytogenes confers cross-protection to class IIa bacteriocins and affects virulence gene expression

It was previously shown that enhanced nisin resistance in some mutants was associated with increased expression of three genes, pbp2229, hpk1021, and lmo2487, encoding a penicillin-binding protein, a histidine kinase, and a protein of unknown function, respectively. In the present work, we determined the direct role of the three genes in nisin resistance. Interruption of pbp2229 and hpk1021 eliminated the nisin resistance phenotype. Interruption of hpk1021 additionally abolished the increase in pbp2229 expression. The results indicate that this nisin resistance mechanism is caused directly by the increase in pbp2229 expression, which in turn is brought about by the increase in hpk1021 expression. We also found a degree of cross-protection between nisin and class IIa bacteriocins and investigated possible mechanisms. The expression of virulence genes in one nisin-resistant mutant and two class IIa bacteriocin-resistant mutants of the same wild-type strain was analyzed, and each mutant consistently showed either an increase or a decrease in the expression of virulence genes (prfA-regulated as well as prfA-independent genes). Although the changes mostly were moderate, the consistency indicates that a mutant-specific change in virulence may occur concomitantly with bacteriocin resistance development.

The CroRS two-component regulatory system is required for intrinsic beta-lactam resistance in Enterococcus faecalis

Enterococcus faecalis produces a specific penicillin-binding protein (PBP5) that mediates high-level resistance to the cephalosporin class of beta-lactam antibiotics. Deletion of a locus encoding a previously uncharacterized two-component regulatory system of E. faecalis (croRS) led to a 4,000-fold reduction in the MIC of the expanded-spectrum cephalosporin ceftriaxone. The cytoplasmic domain of the sensor kinase (CroS) was purified and shown to catalyze ATP-dependent autophosphorylation followed by transfer of the phosphate to the mated response regulator (CroR). The croR and croS genes were cotranscribed from a promoter (croRp) located in the rrnC-croR intergenic region. A putative seryl-tRNA synthetase gene (serS) located immediately downstream from croS did not appear to be a target of CroRS regulation or to play a role in ceftriaxone resistance. A plasmid-borne croRp-lacZ fusion was trans-activated by the CroRS system in response to the presence of ceftriaxone in the culture medium. The fusion was also induced by representatives of other classes of beta-lactam antibiotics and by inhibitors of early and late steps of peptidoglycan synthesis. The croRS null mutant produced PBP5, and expression of an additional copy of pbp5 under the control of a heterologous promoter did not restore ceftriaxone resistance. Deletion of croRS was not associated with any defect in the synthesis of the nucleotide precursor UDP-MurNAc-pentapeptide or of the D-Ala(4)-->L-Ala-L-Ala-Lys(3) peptidoglycan cross-bridge. Thus, the croRS mutant was susceptible to ceftriaxone despite the production of PBP5 and the synthesis of wild-type peptidoglycan precursors. These observations constitute the first description of regulatory genes essential for PBP5-mediated beta-lactam resistance in enterococci.

Molecular analysis of the Enterococcus faecalis serotype 2 polysaccharide determinant

We previously described a 15-kb genetic cluster consisting of 11 open reading frames (cps2A to cps2K) of Enterococcus faecalis FA2-2 that is responsible for the production of the serotype 2 capsular polysaccharide. By using transcriptional fusions to a promoterless lacZ gene, we identified two independent promoters related to the expression of the polysaccharide. Both transcription initiation sites were mapped by primer extension. Reverse transcription-PCR (RT-PCR) demonstrated the transcriptional linkage of genes present in both transcripts. Real-time RT-PCR quantification of transcripts revealed maximum transcription during log phase growth, an observation confirmed by promoter fusion studies. The heterologous expression of this pathway in Escherichia coli caused reactivity with E. faecalis type 2 antiserum, thus demonstrating the essential role of this pathway in the synthesis of the type-specific polysaccharide.

Development of a thermally regulated broad-spectrum promoter system for use in pathogenic gram-positive species

Selectively regulating gene expression is an essential molecular tool that is lacking for many pathogenic gram-positive bacteria. In this report, we describe the evaluation of a series of promoters regulated by the bacteriophage P1 temperature-sensitive C1 repressor in Enterococcus faecium, Enterococcus faecalis, and Staphylococcus aureus. Using the lacZ gene to monitor gene expression, we examined the strength, basal expression, and induced expression of synthetic promoters carrying C1 operator sites. The promoters exhibited extremely low basal expression and, under inducing conditions, gave high levels of expression (100- to 1,000-fold induction). We demonstrate that the promoter system could be modulated by temperature and showed rapid induction and that the mechanism of regulation occurred at the level of transcription. Controlled expression with the same constructs was also demonstrated in the gram-negative bacterium Escherichia coli. However, low basal expression and the ability to achieve derepression were dependent on both the number of mismatches in the C1 operator sites and the promoter driving c1 expression. Since the promoters were designed to contain conserved promoter elements from gram-positive species and were constructed in a broad-host-range plasmid, this system will provide a new opportunity for controlled gene expression in a variety of gram-positive bacteria.

Enterococcus faecalis heme-dependent catalase

Enterococcus faecalis cells cannot synthesize porphyrins and do not rely on heme for growth but can take up heme and use it to synthesize heme proteins. We recently described a cytochrome bd in E. faecalis strain V583 and here report the identification of a chromosomal gene, katA, encoding a heme-containing cytoplasmic catalase. The 54-kDa KatA polypeptide shows sequence similarity to members of the family of monofunctional catalases. A hexahistidyl-tagged version of the catalase was purified, and major characteristics of the enzyme were determined. It contains one protoheme IX group per KatA polypeptide. Catalase activity was detected only in E. faecalis cells grown in the presence of heme in the medium; about 2 and 10 micro M hemin was required for half-maximal and maximal production of catalase, respectively. Our finding of a catalase whose synthesis is dependent on the acquisition of heme in the opportunistic pathogen E. faecalis might be of clinical importance. Studies of cellular heme transport and heme protein assembly and in vivo synthesis of metalloprotein analogs for biotechnological applications are impeded by the lack of experimental systems. We conclude that the E. faecalis cell potentially provides such a desired system.

Regulation of D-alanyl-lipoteichoic acid biosynthesis in Streptococcus agalactiae involves a novel two-component regulatory system

The dlt operon of gram-positive bacteria comprises four genes (dltA, dltB, dltC, and dltD) that catalyze the incorporation of D-alanine residues into the lipoteichoic acids (LTAs). In this work, we characterized the dlt operon of Streptococcus agalactiae, which, in addition to the dltA to dltD genes, included two regulatory genes, designated dltR and dltS, located upstream of dltA. The dltR gene encodes a 224-amino-acid putative response regulator belonging to the OmpR family of regulatory proteins. The dltS gene codes for a 395-amino-acid putative histidine kinase thought to be involved in the sensing of environmental signals. The dlt operon of S. agalactiae is mainly transcribed from the P(dltR) promoter, which directs synthesis of a 6.5-kb transcript encompassing dltR, dltS, dltA, dltB, dltC, and dltD, and from a weaker promoter, P(dltA), which is located in the 3' extremity of dltS. We demonstrate that P(dltR), but not P(dlA), is activated by DltR in the presence of DltS in D-Ala-deficient LTA mutants resulting from insertional inactivation of the dltA gene, which encodes the cytoplasmic D-alanine-D-alanyl carrier ligase DltA. Expression of the dlt operon does not require DltR and DltS, since the basal activity of P(dltR) is high, being 20-fold that of the constitutive promoter P(aphA-3) which directs synthesis of the kanamycin resistance gene aphA-3 in various gram-positive bacteria. We hypothesize that the role of DltR and DltS in the control of expression of the dlt operon is to maintain the level of D-Ala esters in LTAs at a constant and appropriate value whatever the environmental conditions. The DltA(-) mutant displayed the ability to form clumps in standing culture and exhibited an increased susceptibility to the cationic antimicrobial polypeptide colistin.

Role of penicillin-binding protein 5 in expression of ampicillin resistance and peptidoglycan structure in Enterococcus faecium

The contribution of penicillin-binding protein 5 (PBP 5) to intrinsic and acquired beta-lactam resistance was investigated by constructing isogenic strains of Enterococcus faecium producing different PBP 5. The pbp5 genes from three E. faecium clinical isolates (BM4107, D344, and H80721) were cloned into the shuttle vector pAT392 and introduced into E. faecium D344S, a spontaneous derivative of E. faecium D344 highly susceptible to ampicillin due to deletion of pbp5 (MIC, 0.03 microg/ml). Immunodetection of PBP5 indicated that cloning of the pbp5 genes into pAT392 resulted in moderate overproduction of PBP 5 in comparison to wild-type strains. This difference may be attributed to a difference in gene copy number. Expression of the pbp5 genes from BM4107 (MIC, 2 microg/ml), D344 (MIC, 24 microg/ml), and H80721 (MIC, 512 microg/ml) in D344S conferred relatively low levels of resistance to ampicillin (MICs, 6, 12, and 20 microg/ml, respectively). A methionine-to-alanine substitution was introduced at position 485 of the BM4107 PBP 5 by site-directed mutagenesis. In contrast to previous hypotheses based on comparison of nonisogenic strains, this substitution resulted in only a 2.5-fold increase in the ampicillin MIC. The reversed-phase high-performance liquid chromatography muropeptide profiles of D344 and D344S were similar, indicating that deletion of pbp5 was not associated with a detectable defect in cell wall synthesis. These results indicate that pbp5 is a nonessential gene responsible for intrinsic resistance to moderate levels of ampicillin and by itself cannot confer high-level resistance.

Characterization of fsr, a regulator controlling expression of gelatinase and serine protease in Enterococcus faecalis OG1RF

We have previously identified a locus, fsr, a homologue of staphylococcal agr loci, which positively regulates the expression of gelatinase and serine protease (encoded by gelE and sprE, respectively) in Enterococcus faecalis OG1RF. The expression of the three genes in the fsr locus, fsrA, fsrB, and fsrC, appears to be autoregulated, and we have shown that mutants with insertion disruptions in each of these three genes were significantly attenuated in a mouse peritonitis model compared to the parent strain. In the present study, we showed that fsrB and fsrC are highly expressed in the postexponential growth phase and that their expression is cell density dependent. Reverse transcriptase PCR using primers covering the intergenic regions in the fsr/gelE loci confirmed that fsrB and fsrC, as well as gelE and sprE, are cotranscribed. We also showed, using a nonpolar fsrB deletion mutant, that fsrB, the homologue of agrB of staphylococci with unknown function, is required for the regulatory function of fsr. Primer extension and analysis of transcriptional fusions indicated the presence of promoters immediately upstream of fsrA, of fsrB, and of gelE and that the fsrB and gelE promoters are fsr dependent, while the fsrA promoter is an fsr-independent weak constitutive promoter. Two conserved 7-bp direct repeats were found immediately upstream of the fsrB and gelE promoters, similar to the repeats found upstream of P2 and P3 promoters of the agr locus; deletions and mutations in the repeated sequences completely abolished the fsrB and gelE promoter activities, suggesting that the repeats are important for the regulatory function in the fsrB and gelE promoter regions.

Penicillin-binding protein 5 and expression of ampicillin resistance in Enterococcus faecium

We report a structural and transcriptional analysis of the pbp5 region of Enterococcus faecium C68. pbp5 exists within a larger operon that includes upstream open reading frames (ORFs) corresponding to previously reported psr (penicillin-binding protein synthesis repressor) and ftsW (whose product is a transmembrane protein that interacts with PBP3 in Escherichia coli septum formation) genes. Hybridization of mRNA from C68, CV133, and four ampicillin-resistant CV133 mutants revealed four distinct transcripts from this region, consisting of (i) E. faecium ftsW (ftsW(Efm)) alone; (ii) psr and pbp5; (iii) pbp5 alone; and (iv) ftsW(Efm), psr, and pbp5. Quantities of the different transcripts varied between strains and did not always correlate with quantities of PBP5 or levels of ampicillin resistance. Since the psr of C68 is presumably nonfunctional due to an insertion of an extra nucleotide in the codon for the 44th amino acid, the region extending from the ftsW(Efm) promoter through the pbp5 gene of C68 was cloned in E. coli to facilitate mutagenesis. The psr ORF was regenerated using site-directed mutagenesis and introduced into E. faecium D344-SRF on conjugative shuttle vector pTCV-lac (pCWR558 [psr ORF interrupted]; pCWR583 [psr ORF intact]). Ampicillin MICs for both D344-SRF(pCWR558) and D344-SRF(pCWR583) were 64 microg/ml. Quantities of pbp5 transcript and protein were similar in strains containing either construct regardless of whether they were grown in the presence or absence of ampicillin, arguing against a role for PSR as a repressor of pbp5 transcription. However, quantities of psr transcript were increased in D344-SRF(pCWR583) compared to D344-SRF(pCWR558), especially after growth in ampicillin; suggesting that PSR acts in some manner to activate its own transcription.

Listeriolysin O as a reporter to identify constitutive and in vivo-inducible promoters in the pathogen Listeria monocytogenes

Listeria monocytogenes is a facultative intracellular gram-positive bacterium capable of growing in the cytoplasm of infected host cells. Bacterial escape from the phagosomal vacuole of infected cells is mainly mediated by the pore-forming hemolysin listeriolysin O (LLO) encoded by hly. LLO-negative mutants of L. monocytogenes are avirulent in the mouse model. We have developed a genetic system with hly as a reporter gene allowing the identification of both constitutive and in vivo-inducible promoters of this pathogen. Genomic libraries were created by randomly inserting L. monocytogenes chromosomal fragments upstream of the promoterless hly gene cloned into gram-positive and gram-negative shuttle vectors and expressed in an LLO-negative mutant strain. With this hly-based promoter trap system, combined with access to the L. monocytogenes genome database, we identified 20 in vitro-transcribed genes, including genes encoding (i) p60, a previously known virulence gene, (ii) a putative new hemolysin, and (iii) two proteins of the general protein secretion pathway. By using the hly-based system as an in vivo expression technology tool, nine in vivo-induced loci of L. monocytogenes were identified, including genes encoding (i) the previously known in vivo-inducible phosphatidylinositol phospholipase C and (ii) a putative N-acetylglucosamine epimerase, possibly involved in teichoic acid biosynthesis. The use of hly as a reporter is a simple and powerful alternative to classical methods for transcriptional analysis to monitor promoter activity in L. monocytogenes.