Amplification of the V. harveyi hemolysin gene (vhh)
PCR primers reported by Zhang and co-workers
(2001) were used to amplify the 1.3 kb open reading
frame of the hemolysin gene in the reference strain V.
harveyi IFO 15634 (Fig. 1). The identity of the PCRamplified
1.3 kb product was verified by direct sequencing
and BLAST homology search. The partial
nucleotide sequence displays 95% homology with the
reported Vibrio harveyi vhhA and vhhB genes deposited
in the GenBank database (data not shown).
Subsequently, other nested primers were designed in
this study in order to amplify internal hemolysin vhh
gene fragments in the V. harveyi reference strain.
Primer sequences used (Table 3) include a primer set
F2 and R2 designed in this study, generating a 216-bp
hemolysin gene fragment. PCR reactions using combinations
of the old and new primers and template DNA
from the reference V. harveyi strain, was observed to
yield amplified fragments with the following approximate
sizes: 308 bp and 1.2 kb (with primer sets, F1R2and F2R1, respectively; Fig. 1).
In order to test the universality of the primer sets in
producing the expected vhh gene amplified fragments,
genomic DNA extracts from ten (10) Vibrio harveyi isolates
from different hosts and geographical locations
were used as PCR templates (Fig. 2). The 1.3 kb PCR
product generated using the F1R1 primer set (designed
in an earlier study) was observed in all V. harveyi
strains tested except 2 strains, VIB 391 and STD
3-101, isolated from larval shrimp in Thailand and
Ecuador, respectively. Amplified DNA fragments were
also observed from all isolates tested after PCR using
the new set of primers F2R2 and the combined
primers F2R1 (216-bp and 1.2-kb, respectively), except
in two Vibrio harveyi strains that did not yield the
expected products with F1R1 either. Interestingly, the
308-bp amplified fragment produced by the F1R2
primer set with V. harveyi (IFO 15634) was consistently
obtained in all ten (10) V. harveyi strains. The expected 308-bp band and an additional 600-bp amplicon
were observed in profiles of one strain from
Ecuador (STD 3-101). The presence of this additional
band can not be explained at this time.
Test for the specificity of the PCR detection protocol
for V. harveyi vhh gene
The specificity of the primer set amplifying 308-bp
Vibrio harveyi vhh gene fragment was established by
using the target V. harveyi, non-target Vibrio species,
and several non-Vibrio DNA templates (data not
shown). The expected amplicon was only observed in
V. harveyi. Use of DNA templates from non-Vibrio
species and the black tiger shrimp (Penaeus
monodon) did not yield the 308-bp amplicon, indicating
that no false positive results will be obtained even in
the presence of these DNA in samples to be analyzed.