Saccharomyces cerevisiae, an ascomycetous yeast and a candidate for a host-vector system in recombinant DNA experiments, was examined for its patho genicity and colonization in mice and cynomolgus monkeys as the models of the biological containment level. Adult mice given perorally with 5.5 or 2.4 × 107cells of MC 16 strain S. cerevisiae excluded them rapidly and no colonization of the cells in the abdominal organs, lymph nodes or gastrointestinal wall was demonstrated. No change in the fecal flora was observed. Cynomolgus monkeys after peroral administration of 4.9 × 107 or 7.8 × 108 cells showed a similar tendency of rapid ex clusion and the lack of colonization. Cortisone acetate treatment had no significant effect. Intravenous administration of 3.9 × 107 yeast cells had no pathogenic effect and no viable yeast cell was detected in the blood. The biological containment level of S.cerevisiae was suggested to be not lower than that of E. coli K12 B1 level, and the possibility of achieving the B2 level was also suggested.
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