*These authors contributed equally to this work.Clostridium difficile infection is the leading cause of healthcare associated diarrhoea in Europe and North America 1, 2 . During infection, C. difficile produces two key virulence determinants, toxin A and toxin B. Experiments with purified toxins have suggested that toxin A alone is able to evoke the symptoms of C. difficile infection, but toxin B is unable to do so unless it is mixed with toxin A, or there is prior damage to the gut mucosa 3 . However, a recent study suggested that toxin B is essential for C. difficile virulence and that a strain producing toxin A alone was avirulent 4 . This creates a paradox over the individual importance of toxin A and toxin B. Here we show that isogenic mutants of C. difficile producing either toxin A or toxin B alone can cause fulminant disease in the hamster model of infection. By using a gene knock-out system 5, 6 to permanently inactivate the toxin genes, we found that C. difficile producing either one or both toxins displayed cytotoxic activity in vitro, which translated directly into virulence in vivo.Furthermore, by constructing the first ever double mutant strain of C. difficile, in which both toxin genes were inactivated, we were able to completely attenuate virulence. Our findings re-establish the importance of both toxin A and toxin B and highlight the need to continue considering both toxins in the development of diagnostic tests and effective counter-measures against C. difficile.2 Toxin A and toxin B both catalyse the glucosylation, and hence inactivation, of Rho-GTPases; small regulatory proteins of the eukaryotic actin cell cytoskeleton. This leads to disorganisation of the cell cytoskeleton and cell death 7 . The toxin genes, tcdA and tcdB, are situated on the C. difficile chromosome in a 19.6 kilobase pathogenicity locus (PaLoc), along with the three accessory genes, tcdC, tcdR and tcdE (Fig. 1a). To address the individual importance of toxin A and toxin B, we used the ClosTron gene knock-out system 6 to inactivate the toxin genes of C. difficile. This system inactivates genes by inserting an intron into the protein-encoding DNA sequence of a gene, thus resulting in a truncated and non-functional protein. The intron sequence itself encompasses an erythromycin resistance determinant which permits selective isolation of mutants. Furthermore, it has been shown experimentally that the insertions are completely stable, meaning that inactivation of a gene is permanent 5 .Using the ClosTron system, we targeted insertions to tcdA and tcdB at nucleotide positions 1584 and 1511, respectively (Fig. 1a). In both cases, this placed the intron within DNA sequence encoding the toxin catalytic domain. Three separate isogenic The genotype of each toxin mutant was characterised by PCR and DNA sequence analysis to confirm the exact location of each intron insertion made (data not shown).Southern blot analysis of EcoRV-digested genomic DNA samples, using an intron-3 specific probe, confirmed that the A -B + and A + B -mutants ...

No abstract

bBacteria within biofilms are protected from multiple stresses, including immune responses and antimicrobial agents. The biofilm-forming ability of bacterial pathogens has been associated with increased antibiotic resistance and chronic recurrent infections. Although biofilms have been well studied for several gut pathogens, little is known about biofilm formation by anaerobic gut species. The obligate anaerobe Clostridium difficile causes C. difficile infection (CDI), a major health care-associated problem primarily due to the high incidence of recurring infections. C. difficile colonizes the gut when the normal intestinal microflora is disrupted by antimicrobial agents; however, the factors or processes involved in gut colonization during infection remain unclear. We demonstrate that clinical C. difficile strains, i.e., strain 630 and the hypervirulent strain R20291, form structured biofilms in vitro, with R20291 accumulating substantially more biofilm. Microscopic and biochemical analyses show multiple layers of bacteria encased in a biofilm matrix containing proteins, DNA, and polysaccharide. Employing isogenic mutants, we show that virulence-associated proteins, Cwp84, flagella, and a putative quorum-sensing regulator, LuxS, are all required for maximal biofilm formation by C. difficile. Interestingly, a mutant in Spo0A, a transcription factor that controls spore formation, was defective for biofilm formation, indicating a possible link between sporulation and biofilm formation. Furthermore, we demonstrate that bacteria in clostridial biofilms are more resistant to high concentrations of vancomycin, a drug commonly used for treatment of CDI. Our data suggest that biofilm formation by C. difficile is a complex multifactorial process and may be a crucial mechanism for clostridial persistence in the host.

Clostridium difficile is the most common hospital acquired pathogen in the United States, and infection is in many cases fatal. Toxins A and B are its major virulence factors, but increasingly a third toxin may be present, known as C. difficile transferase (CDT). An ADP-ribosyltransferase that causes actin cytoskeletal disruption, CDT is typically produced by the major, hypervirulent strains and has been associated with more severe disease. Here we show that CDT enhances the virulence of two PCR-ribotype 027 strains in mice. The toxin induces pathogenic host inflammation via a Toll-like Receptor 2 (TLR2) dependent pathway, resulting in the suppression of a protective host eosinophilic response. Finally, we show that restoration of TLR2 deficient eosinophils is sufficient for protection from a strain producing CDT. These findings offer an explanation for the enhanced virulence of CDT-expressing C. difficile and demonstrate a mechanism by which this binary toxin subverts the host immune response.