Survival of Salmonella typhi in oysters

Shell oysters were reared in experimental conditions to elucidate the survival rate of Salmonella typhi in the body of cultured oysters and the fate of S. typhi in the shucked oysters. Sixty to 80 cultured oysters were reared for 4 h at 15 degrees C in 40 l of sea water than contained 1 x 10(4) -- 2 x 10(4) cells/ml of S. typhi (V-form) added, then these contaminated oysters were transferred to a tank that contained 40 l of clean sea water circulated through a ultraviolet disinfectant apparatus with a filtration system, and oysters were reared therein for 7-9 d at 15 degrees C. During the time the oysters were in the clean sea water, 5 oysters were periodically sampled, shucked, and the number of viable cells of S. typhi in the oysters was determined. At the beginning of the rearing in the clean sea water, the number was 4 x 10(5) -- 8 x 10(5) cells/100 g (MPN) in the shucked oysters. The MPN gradually lowered thereafter during the first 4 h in the clean sea water, but 24 h later, the MPN was reduced as much as 4 log. Thereafter, 20-80 cells/100 g (MPN) or 2-12 cells/oyster of S. typhi were enumerated up to the end of the experiment and a complete elimination was not observed. When the contaminated oysters were preserved at 10 degrees C, 5 degrees C or -20 degrees C, as shucked oysters, he number of viable cells of S. typhi did not change beyond the range of 1 log which may be caused by individuality at all temperatures tested during the first 10 days, i.e. the value did not tend to be reduced during this period. When the oysters were stored by -20 degrees C, the viability of S. typhi was apparently unaffected up to 140 days when the experiment was terminated. All these experiments were highly reproducible.