NRMRL
Table 2 shows the results from DNA hybridization assays
which were conducted on raw sludge samples obtained
from a clarification tank receiving spent wash water from
an egg processing facility. For this series of experiments,
all inoculated RV tubes were assayed using the DNA
hybridization technique and culture techniques. Therefore,
a direct comparison could be made between the responses
observed using each technique. Control samples responded
as expected with Salmonellae cultures yielding positive
results and E. coli cultures yielding negative results.
Results from this series of tests show that the sensitivity
of the DNA hybridization technique was quite good
yielding identical results to the cultural technique used in
all but two occasions. This equals a false negative response
of 3% for these samples. As noted above, all DNA
hybridization assays were performed on RV cultures
following 24 h of incubation. At the same time, aliquots
of the cultures were streaked to XLT4 agar plates, and the
original RV cultures were returned to the incubator. After
24 h of incubation the XLT4 plates were observed for
typical Salmonellae colonies. If none were observed, XLT4
plates were streaked with aliquots taken from the 48 h RV
cultures. As noted, in Table 2, one of the 24 h RV cultures
did not produce colonies on the selective media or a
positive result to DNA hybridization, but did yield
colonies confirmed as Salmonella spp. following 48 h of
RV enrichment.(164 h). The lime-treated biosolids were then neutralized
with dilute (1Æ0 N) hydrochloric acid and assayed for
Salmonellae using both the DNA hybridization and the
RV enrichment culture procedure. These results are shown
in Table 3.
Of the 90 assays conducted, all RV cultures which were
found to be positive for Salmonellae (33) using the DNA
hybridization technique were also confirmed by biochemical
and serologic assays. Likewise, no culturable Salmonellae
were found in the 57 assay tubes which were negative for
DNA hybridization. Therefore, no difference was observed
between the DNA hybridization assay and the RV culture
technique for these samples
Samples of raw sludge were then obtained from Cincinnati’s
Muddy Creek Wastewater Treatment Plant once a
week for 3 weeks. These samples were assayed as described
above for the egg wash water sludge. Additionally, the raw
sludge was treated with calcium hydroxide (hydrated lime),
to achieve a pH of