- A method was devised and tested for rapid and quantitative
identification of microbial flora in fresh seafoods. The rapid
identification of large numbers of isolates was made possible by
(1) a simplified identification scheme established by reference
culture studies and from the known reactions of microorganisms
reported in the literature, (2) the multiple transfer of large numbers
of isolates by means of replica-plating, and (3) the use of an
electronic computer to analyze data.
For the identification of microbial isolates, colonies
developing on initial isolation plates were picked by sterile toothpicks
and inoculated on a master-plate in prearranged spacing and
order. Growth on the master-plate was replicated on a series of
solid agar plates containing differential or selective agents. Identifying characteristics consisted of growth responses of the
isolates on media containing penicillin, tylosin, vancomycin,
streptomycin, chloramphenicol, neomycin and colistin; growth responses
on Bacto-SS, Bacto-S-110, Bacto-potato dextrose agar;
and culture pigmentation, cell morphology and the Gram-reaction.
Information was processed by an IBM 1410 digital computer which
sorted and grouped each isolate into one of ten microbial genera or
groups, according to a programmed identification key. The identification
system was tested by analyzing the microbial flora of dover
sole fillets (Microstomas pacificus) and ground beef.
This rapid identification method was employed in an investigation
designed to determine the nature of the microbial flora shifts
in dover sole resulting from irradiation and storage at 6°C. The
relationship between the microorganisms which initially survive irradiation,
and those making up the final spoilage flora, was determined.
A total of 2,723 isolates were examined in this study.
The spoilage of unirradiated control samples during storage
at 6°C was almost entirely due to the growth of Pseudomonas.
This group, which occupied 25 percent of the fresh flora, grew up to
nearly 100 percent in two days storage. In contrast, irradiation
doses of 0.1, 0.2, 0.3, and 0.4 megarad favored the growth of
Achromobacter and yeasts. Micrococcus species, which survived
radiation, did not grow at 6°C. At 0.5 megarad, spoilage of
fish samples at 6°C was due entirely to yeasts.