Biology Reference
In-Depth Information
Khan et al . (2002) reported that O3 : K6 strains possessed a specifi c 850-
bp sequence that was absent in other Vibrio species and related organisms.
A set of primers VPF2/VPR2 (Table 5.4) was then developed that amplifi es
a 327-bp segment of this unique sequence.
Vibrio cholerae is a Gram-negative facultative anaerobic curved rod that
is polarly fl agellated, cytochrome oxidase positive and characteristically
ferments glucose to acid without gas production. The generally accepted
method for its isolation involves enrichment in alkaline peptone water
(APW) followed by culture on selective thiosulfate-citrate-bile salts-
sucrose (TCBS) agar. Sucrose positive, smooth yellow colonies on TCBS
agar are submitted to the oxidase test. Positive colonies are then purifi ed
on a nonselective agar and subjected to biochemical and serological tests.
Detection of cholera toxin (CTX) is usually by an ELISA (Enzyme Linked
Immuno Sorbent Assay) assay.
V. cholera is considered a heterogeneous species with 206 serotypes
presently recognized. However, only two serotypes are associated with
epidemic infections, 01 and 0139 (Islam et al., 2004). There are strains of
these two serogroups however that do not produce the cholera enterotoxin
(CTX) and are not infectious. In addition, CTX negative V. cholera 01 strains
have been implicated in occasional cases of diarrhea and extraintestinal
infections. Some non-01/non-0139 strains produce a heat stabile
enterotoxin designated NAG-ST (non-agglutinable Vibrio ST) encoded by
the sto gene. V. cholerae 01 is divided into two biotypes, classical and El Tor
(Keasler and Hall, 1993; Islam et al., 2004). The El Tor biotype is presently
considered the most signifi cant. In the USA crabs, shrimp, and oysters
have been the most frequently implicated vehicles.
Factors Associated With the Virulence of V. cholerae
Cholera toxin (CTX) consists of two polypeptides; the A subunit encoded
by ctxA is responsible for adenylate cyclase activation in enterocytes of the
small intestine, inducing extensive secretion resulting in tremendous loss
of water and electrolytes associated with cholera. The B subunit encoded
by ctxB consists of fi ve identical peptides responsible for binding to the
epithelial cell surface receptor GM 1 . In addition to CTX the pathogenicity
of V. cholera O1 and O139 strains also depends on the ability to adhere and
colonize the small intestine via production of pili encoded by tcpA .
There are a large number of genes associated with the virulence of
V. cholerae that have been used to detect and characterize isolates. The major
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