Cytotoxin (CTX) production in 34 human and 22 animal strains of isolated in Japan and other countries was studied by three assay systems described previously. Furthermore, cholera-like enterotoxin production by these strains was tested by reversed passive latex agglutination (RPLA). CTX titres in the fetal calf serum (FCS) and newborn calf serum (NCS) assays were relatively lower, with a maximum of 4 and 8, respectively, than the maximum of 128 for the serum-free culture (SFC) assay. CTX detection rates were 62, 85 and 100% in human isolates and 64, 77 and 100% in animal isolates for the FCS, NCS and SFC assay systems, respectively. There was no significant difference in the detection rate of CTX between human and animal isolates, or between human isolates from Japan and other countries. With the three assay systems, the strains were divided into four groups from the pattern of CTX detection; 54% of strains gave positive results in all three assay systems, and 9 % of them were positive in the SFC assay only. Morphological changes on CHO cells showed distended instead of rounded cells with eight of 21 strains negative in the FCS assay. Cholera-like enterotoxin was not detected in the culture filtrate of any of the strains when tested by RPLA. These results indicate that cytotoxin production by is complex as compared with that of other enteric pathogens.


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