Streptococcus thermophilus strain ATCC 19258 consistently showed low colony counts (<106 CFU/ml) when grown overnight (16–18 h) in M17 at 42°C. There was a rapid decrease in the total viable colony counts after 10 h incubation at 42°C with the maximum decrease after 16 to 18 h incubation. A similar decrease in colony counts was also noticed in M17G and M17S broths containing 0.5% glucose and sucrose, respectively. The decrease in colony counts were more pronounced at 42°C than at 37°C. Such decreases in total viable colony counts was not observed when cells were grown in M17 broth initially supplemented with 2% carbohydrate. The cell survival and turbidity of seven other cultures of S. thermophilus in M17 and M17-2 L (M17 containing 2% lactose) broths were also tested at 42°C. All the cultures showed significantly higher cell survival in M17-2 L broth than that observed in M17 medium. The results demonstrate that both growth and maintenance of S. thermophilus strains can be markedly improved by supplementing M17 medium with 2% carbohydrate as opposed to 0.5% used in regular M17 medium.
*Contribution No. 844 from the Food Research Centre.