Detection of type III secretion genes as a general indicator of bacterial virulence

Type III secretion systems of Gram-negative bacteria are specific export machineries for virulence factors which allow their translocation to eukaryotic cells. Since they correlate with bacterial pathogenicity, their presence is used as a general indicator of bacterial virulence. By comparing the genetic relationship of the major type III secretion systems we found the family of genes encoding the inner-membrane channel proteins represented by the Yersinia enterocolitica lcrD (synonym yscV) and its homologous genes from other species an ideal component for establishing a general detection approach for type III secretion systems. Based on the genes of the lcrD family we developed gene probes for Gram-negative human, animal and plant pathogens. The probes comprise lcrD from Y. enterocolitica, sepA from enteropathogenic Escherichia coli, invA from Salmonella typhimurium, mxiA from Shigella sonnei, as well as hrcV from Erwinia amylovora. In addition we included as a control probe the flhA gene from E. coli K-12 to validate our approach. FlhA is part of the flagellar export apparatus which shows a high degree of similarity with type III secretions systems, but is not involved in pathogenicity. The probes were evaluated by screening a series of pathogenic as well as non-pathogenic bacteria. The probes detected type III secretion in pathogens where such systems were either known or were expected to be present, whereas no positive hybridization signals could be found in non-pathogenic Gram-negative bacteria. Gram-positive bacteria were devoid of known type III secretion systems. No interference due to the genetic similarity between the type III secretion system and the flagellar export apparatus was observed. However, potential type III secretion systems could be detected in bacteria where no such systems have been described yet. The presented approach provides therefore a useful tool for the assessment of the virulence potential of bacterial isolates of human, animal and plant origin. Moreover, it is a powerful means for a first safety assessment of poorly characterized strains intended to be used in biotechnological applications.

BID-dependent release of mitochondrial SMAC dampens XIAP-mediated immunity against Shigella

The X‐linked inhibitor of apoptosis protein (XIAP) is a potent caspase inhibitor, best known for its anti‐apoptotic function in cancer. During apoptosis, XIAP is antagonized by SMAC, which is released from the mitochondria upon caspase‐mediated activation of BID. Recent studies suggest that XIAP is involved in immune signaling. Here, we explore XIAP as an important mediator of an immune response against the enteroinvasive bacterium Shigella flexneri, both in vitro and in vivo. Our data demonstrate for the first time that Shigella evades the XIAP‐mediated immune response by inducing the BID‐dependent release of SMAC from the mitochondria. Unlike apoptotic stimuli, Shigella activates the calpain‐dependent cleavage of BID to trigger the release of SMAC, which antagonizes the inflammatory action of XIAP without inducing apoptosis. Our results demonstrate how the cellular death machinery can be subverted by an invasive pathogen to ensure bacterial colonization.

Laribacter hongkongensis: a potential cause of infectious diarrhea

In this study, we describe the isolation of Laribacter hongkongensis, a recently described genus and species of bacterium, in pure culture on charcoal cefoperazone deoxycholate agar from the stool of six patients with diarrhea. Three patients were residents of Hong Kong, and three of Switzerland. In none of the stool samples obtained from these six patients was Salmonella, Shigella, enterohemorrhagic Escherichia coli, Vibrio, Aeromonas, Plesiomonas, or Campylobacter recovered. Rotavirus antigen detection, electron microscopic examination for viruses, and microscopic examinations for ova and cysts were all negative for the stool samples obtained from the three patients in Hong Kong. Enterotoxigenic E. coli was recovered from one of the patients in Hong Kong. Unlike L. hongkongensis type strain HKU1, all the six strains were motile with bipolar flagellae. Sequencing of the 16S ribosomal RNA genes of the six strains showed that they all had sequences with only 0-2 base differences to that of the type strain. Pulsed field gel electrophoresis of the SpeI digested genomic DNA of the six isolates and that of the type strain revealed that the seven isolates were genotypically unrelated strains. More extensive epidemiologic studies should be carried out to ascertain the causative association between L. hongkongensis and diarrhea and to define the reservoir and modes of transmission of L. hongkongensis.