Clostridium difficile: an update on virulence mechanisms

The development of Clostridium difficile genetic systems

Clostridum difficile is a major cause of healthcare-associated disease in the western world, and is particularly prominent in the elderly. Its incidence is rising concomitant with increasing longevity. More effective countermeasures are required. However, the pathogenesis of C. difficile infection is poorly understood. The lack of effective genetic tools is a principal reason for this ignorance. For many years, the only tools available for the transfer of genes into C. difficile have been conjugative transposons, such as Tn916, delivered via filter mating from Bacillus subtilis donors. They insert into a preferred site within the genome. Therefore, they may not be employed for classical mutagenesis studies, but can be employed to modulate gene function through the delivery of antisense RNA. Attempts to develop transformation procedures have so far met with little success. However, in recent years the situation has been dramatically improved through the demonstration of efficient conjugative transfer of both replication-proficient and replication-deficient plasmids from Escherichia coli donors. This efficient transfer can only be achieved in certain strains through negation of the indigenous restriction barrier, and is generally most effective when the plasmid employed is based on the replicon of the C. difficile plasmid, pCD6.

Occurrence of Clostridium difficile in fecal samples of HIV-infected children in Poland

The prevalence of Clostridium difficile and its toxins (A and B) in HIV-positive children in Poland was investigated in a group of 18 children, aged 6 months to 8 1/2 years. Stool samples were tested using an antigen detection method for toxin A/B, cytotoxicity-neutralization and culture. In 3 cases (17%) C. difficile toxins were detected in both stool samples and strains recovered from culture. The three strains isolated were shown by PCR methods to contain toxins A and B genes. All children had been treated previously with antimicrobial and antiviral agents. All three C. difficile-positive children had mild diarrhea that resolved without specific therapy. Further studies involving a large number of children and molecular analyses of isolated C. difficile strains are necessary to determine the frequency and rate of carriage of C. difficile strains among HIV-positive children in Poland.

Transcription and analysis of polymorphism in a cluster of genes encoding surface-associated proteins of Clostridium difficile

Recent investigations of the Clostridium difficile genome have revealed the presence of a cluster of 17 genes, 11 of which encode proteins with similar two-domain structures, likely to be surface-anchored proteins. Two of these genes have been proven to encode proteins involved in cell adherence: slpA encodes the precursor of the two proteins of the S-layer, P36 and P47, whereas cwp66 encodes the Cwp66 adhesin. To gain further insight into the function of this cluster, we further focused on slpA, cwp66, and cwp84, the latter of which encodes a putative surface-associated protein with homology to numerous cysteine proteases. It displayed nonspecific proteolytic activity when expressed as a recombinant protein in Escherichia coli. Polymorphism of cwp66 and cwp84 genes was analyzed in 28 strains, and transcriptional organization of the three genes was explored by Northern blots. The slpA gene is strongly transcribed during the entire growth phase as a bicistronic transcript; cwp66 is transcribed only in the early exponential growth phase as a polycistronic transcript encompassing the two contiguous genes upstream. The putative proteins encoded by the cotranscribed genes have no significant homology with known proteins but may have a role in adherence. No correlation could be established between sequence patterns of Cwp66 and Cwp84 and virulence of the strains. The cwp84 gene is strongly transcribed as a monocistronic message. This feature, together with the highly conserved sequence pattern of cwp84, suggests a significant role in the physiopathology of C. difficile for the Cwp84 protease, potentially in the maturation of surface-associated adhesins encoded by the gene cluster.

Characterization of Clostridium difficile isolates from foals with diarrhea: 28 cases (1993-1997)

OBJECTIVE

To determine molecular characteristics of Clostridium difficile isolates from foals with diarrhea and identify clinical abnormalities in affected foals.

DESIGN

Retrospective study.

ANIMALS

28 foals with C difficile-associated diarrhea.

PROCEDURE

Toxigenicity, molecular fingerprinting, and antibiotic susceptibility patterns were determined. Information on signalment, clinical findings, results of clinicopathologic testing, whether antimicrobials had been administered prior to development of diarrhea, and outcome was obtained from the medical records.

RESULTS

Twenty-three (82%) foals survived. Toxin A and B gene sequences were detected in isolates from 24 of 27 foals, whereas the toxin B gene alone was detected in the isolate from 1 foal. Results of an ELISA for toxin A were positive for fecal samples from only 8 of 20 (40%) foals. Ten of 23 (43%) isolates were resistant to metronidazole. Molecular fingerprinting revealed marked heterogeneity among isolates, except for the metronidazole-resistant isolates. Sixteen foals had tachypnea. Hematologic abnormalities were indicative of inflammation. Common serum biochemical abnormalities included metabolic acidosis, hyponatremia, hypocalcemia, azotemia, hypoproteinemia, hyperglycemia, and high enzyme activities. Passive transfer of maternal antibodies was adequate in all 12 foals evaluated.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that a large percentage of C difficile isolates from foals with diarrhea will have the toxin A and B gene sequences. Because of the possibility that isolates will be resistant to metronidazole, susceptibility testing is warranted. Clostridium difficile isolates from foals may have a substantial amount of molecular heterogeneity. Clinical and hematologic findings in affected foals are similar to those for foals with diarrhea caused by other pathogens.

The induction of apoptosis by bacterial pathogens

Apoptosis is a highly regulated process of cell death that is required for the development and homeostasis of multicellular organisms. In contrast to necrosis, apoptosis eliminates individual cells without inducing an inflammatory response. Activation or prevention of cell death could be a critical factor in the outcome of an infection. Programmed cell death has been observed as a response to infection by a wide range of animal and plant pathogens and is mediated by an array of pathogen-encoded virulence determinants. Pathogen-induced modulation of the host cell-death pathway may serve to eliminate key immune cells or evade host defenses that can act to limit the infection. Alternatively, suppression of the death pathway may facilitate the proliferation of intracellular pathogens.

Antagonistic activity against Helicobacter infection in vitro and in vivo by the human Lactobacillus acidophilus strain LB

The purpose of the present study was to examine the activity of the human Lactobacillus acidophilus strain LB, which secretes an antibacterial substance(s) against Helicobacter pylori in vitro and in vivo. The spent culture supernatant (SCS) of the strain LB (LB-SCS) dramatically decreased the viability of H. pylori in vitro independent of pH and lactic acid levels. Adhesion of H. pylori to the cultured human mucosecreting HT29-MTX cells decreased in parallel with the viability of H. pylori. In conventional mice, oral treatment with the LB-SCS protected against infection with Helicobacter felis. Indeed, at both 8 and 49 days post-LB-SCS treatment (29 and 70 days postinfection), inhibition of stomach colonization by H. felis was observed, and no evidence of gastric histopathological lesions was found. LB-SCS treatment inhibits the H. pylori urease activity in vitro and in H. pylori that remained associated with the cultured human mucosecreting HT29-MTX cells. Moreover, a decrease in urease activity was detected in the stomach of the mice infected with H. felis and treated with LB-SCS.

Clostridium difficile toxin B induces apoptosis in intestinal cultured cells

Toxigenic strains of the anaerobic bacterium Clostridium difficile produce at least two large, single-chain protein exotoxins involved in the pathogenesis of antibiotic-associated diarrhea and colitis. Toxin A (CdA) is a cytotoxic enterotoxin, while toxin B (CdB) is a more potent cytotoxin lacking enterotoxic activity. This study dealt with CdB, providing the first evidence that intestinal cells exposed to this toxin exhibit typical features of apoptosis in that a significant proportion of the treated cells displayed nuclear fragmentation and chromatin condensation. In keeping with ultrastructural data, CdB-treated cells showed the typical flow cytometric hallmark of apoptosis consisting of a distinct sub-G1 peak. The CdB-induced apoptotic response was dose and time dependent and not simply due to the actin-disrupting effect of the toxin or to the subsequent impairment of cell anchorage. Rather, the inhibition of proteins belonging to the Rho family due to CdB seems to play a role in the induction of apoptosis in intestinal cells. The origin of cells and the growth rate may also be cofactors relevant to such a response.