Characterization of non-membrane-damaging cytotoxin of non-toxigenic Vibrio cholerae O1 and its relevance to disease

The neuraminidase gene is present in the non-toxigenic Vibrio cholerae Amazonia strain: a different allele in comparison to the pandemic strains

The neuraminidase gene, nanH, is present in the O1, non-toxigenic Vibrio cholerae Amazonia strain. Its location has been assigned to a 150 kb NotI DNA fragment, with the use of pulsed-field gel electrophoresis and DNA hybridization. This NotI fragment is positioned inside 630 kb SfiI and 1900 kb I-CeuI fragments of chromosome 1. Association of the pathogenicity island VPI-2, carrying nanH and other genes, with toxigenic strains has been described by other authors. The presence of nanH in a non-toxigenic strain is an exception to this rule. The Amazonia strain nanH was sequenced (Genbank accession No. AY825932) and compared to available V. cholerae sequences. The sequence is different from those of pandemic strains, with 72 nucleotide substitutions. This is the first description of an O1 strain with a different nanH allele. The most variable domain of the Amazonia NanH is the second lectin wing, comprising 13 out of 17 amino acid substitutions. Based on the presence of nanH in the same region of the genome, and similarity of the adjacent sequences to VPI-2 sequences, it is proposed that the pathogenicity island VPI-2 is present in this strain.

Cell vacuolation, a manifestation of the El tor hemolysin of Vibrio cholerae

Culture supernatants of nontoxigenic nonepidemic clinical strains of Vibrio cholerae belonging to diverse serogroups were found to induce vacuolation of nonconfluent HeLa cells. The vacuoles became prominent 18 h after introduction of culture supernatant, and vacuolated cells survived for 48 h and then died. Only a fraction of the vacuolated cells took up neutral red dye, implying that there were differences in the vacuolar microenvironment. Further tests showed that the factor responsible for vacuolation was heat labile and proteinaceous. Vacuolating activity was completely neutralized by antibody to hemolysin of V. cholerae but not by antibody to vacuolating cytotoxin of Helicobacter pylori. Partial purification of the vacuolating factor led to elution of fractions, which showed both hemolytic and vacuolating activity. PCR amplification and cloning of the hemolysin structural gene (hlyA) into Escherichia coli DH5alpha led to isolation of clones producing cell vacuolating factor in a cell-associated form. Further, a null insertion mutation in the hlyA gene of a high-vacuolating-factor-producing strain led to complete abolition of both cell vacuolating and hemolytic activities. These analyses establish vacuolation as a potentially important but previously unrecognized property of V. cholerae El Tor hemolysin.

Morphological and cytoskeletal changes caused by non-membrane damaging cytotoxin of Vibrio cholerae on int 407 and HeLa cells

Vibrio cholerae produces a non-membrane damaging cytotoxin (NMDCY), also known as cell rounding factor, which causes rapid rounding of cultured cells like HeLa, CHO and Vero and reportedly elicits enterotoxic activity in the rabbit ileal loop assay. Pursuing the concept that NMDCY might be an accessory factor contributing to the diarrhea caused by V. cholerae, we investigated the effect of NMDCY on Int 407 (intestinal cell line) and HeLa (non-intestinal cell line) cells using light, fluorescent and electron microscopy to gain insight into the cellular response evoked by NMDCY. Binding assays showed that NMDCY has affinity for both Int 407 and HeLa cells. Changes in the internal organelles and cytoskeletal structures of the cell lines were documented indicating changes in the secretory and metabolic function of the toxin-treated cells. Toxin-treated cells visualized under the electron microscope revealed retraction of cell body, formation of blebs on cell surface, changes in mitochondria having dilated and rarefied matrix and an extensively developed Golgi apparatus, endoplasmic reticulum and lysosomes compared to those in normal cells. Immunofluorescence study showed restructuring of microfilament network represented by actin, filamin and vinculin, as also of the microtubular component, tubulin and the intermediate filament, vimentin. Immunogold study further revealed that the toxin is internalized even within the nucleus. Moreover, a rise in the intracellular calcium level of the NMDCY-treated cells leads us to hypothesize that a cascade of events results in the final impairment of the cell machinery.