Characterization of genes regulated directly by the VirR/VirS system in Clostridium perfringens

Identification of a two-component VirR/VirS regulon in Clostridium perfringens

Clostridium perfringens, a Gram-positive anaerobic pathogen, is a causative agent of human gas gangrene that leads to severe rapid tissue destruction and can cause death within hours unless treated immediately. Production of several toxins is known to be controlled by the two-component VirR/VirS system involving a regulatory RNA (VR-RNA) in C. perfringens. To elucidate the precise regulatory network governed by VirR/VirS and VR-RNA, a series of microarray screening using VirR/VirS and VR-RNA-deficient mutants was performed. Finally, by qRT-PCR analysis, 147 genes (30 single genes and 21 putative operons) were confirmed to be under the control of the VirR/VirS-VR-RNA regulatory cascade. Several virulence-related genes for alpha-toxin, kappa-toxin, hyaluronidases, sialidase, and capsular polysaccharide synthesis were found. Furthermore, some genes for catalytic enzymes, various genes for transporters, and many genes for energy metabolism were also found to be controlled by the cascade. Our data indicate that the VirR/VirS-VR-RNA system is a global gene regulator that might control multiple cellular functions to survive and multiply in the host, which would turn out to be a lethal flesh-eating infection.

Use of an EZ-Tn5-based random mutagenesis system to identify a novel toxin regulatory locus in Clostridium perfringens strain 13

Background

Although useful for probing bacterial pathogenesis and physiology, current random mutagenesis systems suffer limitations for studying the toxin-producing bacterium Clostridium perfringens.

Methodology/Principal Findings

An EZ-Tn5-based random mutagenesis approach was developed for use in C. perfringens. This mutagenesis system identified a new regulatory locus controlling toxin production by strain 13, a C. perfringens type A strain. The novel locus, encoding proteins with homology to the AgrB and AgrD components of the Agr quorum sensing system of Staphylococcus aureus and two hypothetical proteins, was found to regulate early production of both alpha toxin and perfringolysin O (PFO) by strain 13. PFO production by the strain 13 ΔagrB mutant could be restored by genetic complementation or by physical complementation, i.e. by co-culture of the strain 13 ΔagrB mutant with a pfoA mutant of either strain 13 or C. perfringens type C CN3685. A similar AgrB- and AgrD-encoding locus is identifiable in all sequenced C. perfringens strains, including type B, C, D, and E isolates, suggesting this regulatory locus contributes to toxin regulation by most C. perfringens strains. In strain 13, the agrB and agrD genes were found to be co-transcribed in an operon with two upstream genes encoding hypothetical proteins.

Conclusions/Significance

The new Tn5-based random mutagenesis system developed in this study is more efficient and random than previously reported C. perfringens random mutagenesis approaches. It allowed identification of a novel C. perfringens toxin regulatory locus with homology to the Agr system of S. aureus and which functions as expected of an Agr-like quorum sensing system. Since previous studies have shown that alpha toxin and perfringolysin O are responsible for strain 13-induced clostridial myonecrosis in the mouse model, the new agr regulatory locus may have importance for strain 13 virulence.

Functional analysis of the VirSR phosphorelay from Clostridium perfringens

Toxin production in Clostridium perfringens is controlled by the VirSR two-component signal transduction system, which comprises the VirS sensor histidine kinase and the VirR response regulator. Other studies have concentrated on the elucidation of the genes controlled by this network; there is little information regarding the phosphorelay cascade that is the hallmark of such regulatory systems. In this study, we have examined each step in this cascade, beginning with autophosphorylation of VirS, followed by phosphotransfer from VirS to VirR. We also have studied the effects of gene dosage and phosphorylation in vivo. We have used random and site-directed mutagenesis to identify residues in VirS that are important for its function and have identified a region in the putative sensory domain of VirS that appeared to be essential for function. In vitro phosphorylation studies showed that VirSc, a truncated VirS protein that lacked the N-terminal sensory domain, was capable of autophosphorylation and could subsequently act as a phosphodonor for its cognate response regulator, VirR. Conserved residues of both VirS and VirR, including the D57 residue of VirR, were shown to be essential for this process. By use of Targetron technology, we were able to introduce a single copy of virR or virR(D57N) onto the chromosome of a virR mutant of C. perfringens. The results showed that in vivo, when virR was present in single copy, the production of wild-type levels of perfringolysin O was dependent on the presence of virS and an unaltered D57 residue in VirR. These results provide good evidence that phosphorylation is critical for VirR function.

Contact with enterocyte-like Caco-2 cells induces rapid upregulation of toxin production by Clostridium perfringens type C isolates

Clostridium perfringens type C isolates cause necrotizing enteritis in humans and domestic animals. In vitro, type C isolates often produce beta toxin (CPB), beta2 toxin (CPB2), alpha toxin (CPA), perfringolysin O (PFO) and TpeL during (or after) late log-phase growth. In contrast, the current study found that many type C isolates respond to close contact with enterocyte-like Caco-2 cells by producing all toxins, except TpeL, much more rapidly than occurs during in vitro growth. This in vivo effect involves rapid transcriptional upregulation of the cpb, cpb2, pfoA and plc toxin genes. Rapid Caco-2 cell-induced upregulation of CPB and PFO production involves the VirS/VirR two-component system, since upregulated in vivo transcription of the pfoA and cpb genes was blocked by inactivating the virR gene and was reversible by complementation to restore VirR expression. However, the luxS quorum-sensing system is not required for the rapid upregulation of type C toxin production induced by contact with Caco-2 cells. These results provide the first indication of host cell:pathogen cross-talk affecting toxin production kinetics by any pathogenic Clostridium spp., identify in vivo versus in vitro differences in C. perfringens toxin expression, and implicate VirS/VirR as a possible contributor to some C. perfringens enteric diseases.

Virulence gene regulation by the agr system in Clostridium perfringens

A gram-positive anaerobic pathogen, Clostridium perfringens, causes clostridial myonecrosis or gas gangrene in humans by producing numerous extracellular toxins and enzymes that act in concert to degrade host tissue. The agr system is known to be important for the regulation of virulence genes in a quorum-sensing manner in Staphylococcus aureus. A homologue for S. aureus agrBD (agrBD(Sa)) was identified in the C. perfringens strain 13 genome, and the role of C. perfringens agrBD (agrBD(Cp)) was examined. The agrBD(Cp) knockout mutant did not express the theta-toxin gene, and transcription of the alpha- and kappa-toxin genes was also significantly decreased in the mutant strain. The mutant strain showed a recovery of toxin production after the addition of the culture supernatant of the wild-type strain, indicating that the agrBD(Cp) mutant lacks a signal molecule in the culture supernatant. An agr-virR double-knockout mutant was constructed to examine the role of the VirR/VirS two-component regulatory system, a key virulence regulator, in agrBD(Cp)-mediated regulation of toxin production. The double-mutant strain could not be stimulated for toxin production with the wild-type culture supernatant. These results indicate that the agrBD(Cp) system plays an important role in virulence regulation and also suggest that VirR/VirS is required for sensing of the extracellular signal and activation of toxin gene transcription in C. perfringens.