contain 20 ng μl–1 of nucleic acid. The nucleic acid was
10-fold serially diluted down to 2 fg μl–1 and 1 μl of
each dilution used as template in the m-PCR assay
described. To determine the sensitivity in mixed culture,
initial pure cultures of each bacterium (F.
columnare, E. ictaluri and A. hydrophila) at log-phase
growth were 10-fold diluted 4 times and then 2-fold
serially diluted in PBS pH 7.2. Corresponding dilutions
of each bacterial species were mixed. Total nucleic
acid was extracted from 200 μl of each mixture and 1μl
subjected to m-PCR as described above.
For additional analytical sensitivity assessment, tissues
(blood, gills and kidney) from naïve fish were
spiked with serial dilutions of log-phase cultures of the
3 bacteria Flavobacterium columnare, Edwardsiella
ictaluri and Aeromonas hydrophila. The concentration
of log-phase cultures was first adjusted to optical densities
(OD) at 540 nm of 0.7 for F. columnare and 1.0 for
E. ictaluri and A. hydrophila. These dilutions were
determined to contain 1.6 × 109 CFU ml–1 F. columnare,
3.5 × 109 CFU ml–1 E. ictaluri , and 2.5 × 109 CFU ml–1
A. hydrophila. One hundred microliters of each bacterial
dilution was mixed with 100 μl of tissue (blood) or
tissue homogenate (gills or kidney) and total nucleic
acid prepared from the mixture using guanidinium
thiocyanate acid as previously described (Casas et al.
1995); 1 μl of the 10 μl of isolated nucleic acid from
each sample was used in the m-PCR reaction mixture.
Diagnostic sensitivity of each primer set for its target
species was assessed by PCR amplification of nucleic
acid extracted from pure cultures of 10 isolates of each
species (Flavobacterium columnare, Edwardsiella
ictaluri and Aeromonas hydrophila). Template nucleic
acids from 1 isolate of each species of bacteria were
also amplified together in each possible combination.Additionally, nucleic acid extracted from 11 Gramnegative
bacteria listed in Table 1, which are either
taxonomically related or ubiquitous in the aquatic
environment and 2 Gram-positive bacteria (Streptococcus
iniae and S. agalactiae) previously isolated
from diseased fish (bass or mullet) were tested in the
m-PCR to confirm diagnostic specificity.