Studies on growth of verotoxin-producing Escherichia coli (VTEC) O157 in enrichment broth and minimal broth

Abstract

Growth of 10 strains of VTEC O157, which were isolated from faecal specimens, was evaluated in conventionally recommended enrichment broth media Trypticase soy broth (TSB) and Davis's Minimal Medium (DMM) at different temperatures. Five strains of VTEC O157 used in the test proliferated well in TSB and reached to 10(9) CFU/ml at 36 degrees C in 24 hours incubation and to the same population densities at 25 degrees C in 48 hours. Whereas in DMM at 36 degrees C there was no proliferation in 6 hours, but grew to 10(6) CFU/ml in 24 hours and 10(8) CFU/ml in 48 hours. Three strains of VTEC O157 tested at 25 degrees C grew to 1.52 x 10(4) CFU/ml in 24 hours, 1.67 x 10(8) CFU/ml in 48 hours and 6.80 x 10(8) CFU/ml in 72 hours. No proliferation was observed at 4 degrees C in 72 hours in growth TSB and DMM. The growth in glucose free DMM was found in 7 out of 10 strains, and 4 out of these 7 strains grew profusely to 10(5) CFU/ml in 48 hours. One out of 3 strains showed no proliferation but survived 72 hours postinoculation, decreasing from 179 CFU to 29 CFU in 100 microliters of medium. The other 2 strains became completely extinct in 48 hours after inoculation with 263 CFU and 2,420 CFU in 100 microliters of medium respectively. However, these 3 strains which showed no growth in glucose free DMM could proliferate in DMM containing 1/10 of the usual glucose concentration, at a rate depending on the concentration. In conclusion, it is suggested that the finding of increase of VTEC O157 in glucose free DMM would be a useful solution for the food poisoning problem caused by VTEC O157.