Evaluation of recovery methods of Shigella species from fresh marine fish and shellfishes

Recovery experiments of Shigella strains from fresh marine fish and shellfishes, including fresh sea urchin, which have been artificially contaminated with the strains, were performed using the improved Shigella broth-enrichment method and the culture method reported by Mehlman et al. All of the 43 Shigella stock cultures strains tested were recovered easily by the enrichment method from sea urchin individuals inoculated with a small number of viable cells of each strain. That is, a total of 24 strains (56%) were recovered from sea urchin individuals inoculated with less than 10 viable cells per one individual, and the other 19 strains were also recovered when 10 to 1,000 cells of each strain were inoculated. Recovery of Shigella strains from fish and shellfishes by the enrichment method was hardly affected by the number of contaminated bacteria (SPC, standard plate counts) in these materials. In order to confirm reliability of the enrichment method, similar experiments were performed using S. flexneri strain B as the inoculum and more fish and shellfishes as the samples (24 specimens of fresh sea urchins, 11 specimens of fresh oysters and 5 other specimens including prawns). Except for one oyster specimen which showed an especially high SPC value, the inoculum was able to be recovered from most of the materials inoculated with less than 10 viable cells, and all of the tested samples became Shigella positive when they were inoculated with up to 1,000 viable cells.(ABSTRACT TRUNCATED AT 250 WORDS)