| Popis: |
The natural habitat of Vibrio cholerae is warm, nutrient-rich water, particularly estuarine water. Vibrio cholerae produces a chitinase that degrades the exoskeletons of insects and shellfish. The presence of chitin induces competence for genetic transformation that will permit horizontal gene transfer. Most strains of Vibrio cholerae do not cause disease but those that do acquired their virulence by horizontal gene transfer. Pathogenic strains mostly belong to the O1 serotype of which there are two biotypes: classical and El Tor. Disease-causing strains have at least two pathogenicity islands (VPI-1 and VPI-2) and carry the prophage CTXΦ. The prophage genome encodes cholera toxin. CTXΦ resemble coliphage M13 and the cholera toxin is similar to that found in enterotoxigenic Escherichia coli. Therefore, pathogenic strains of Vibrio cholerae may have acquired some of their virulence characteristics from Escherichia coli by horizontal gene transfer. The first six major epidemics of cholera were caused by the classical biotype of Vibrio cholerae. The seventh epidemic was caused by the El Tor biotype. The original El Tor isolates were of low virulence compared with the 7PET strain and the genetic changes that led to the creation of the 7PET strain have been identified. |
