Diversity of stress responses in dairy thermophilic streptococci

Exponential and stationary phase cells of 56 strains of the dairy Streptococcus species S. thermophilus, S. macedonicus and S. salivarius, were exposed to acid, osmotic, oxidative and heat stresses to investigate the diversity of their responses. Three strains of dairy related streptococci, Lactococcus lactis ATCC11454, Enterococcus faecium DSM20477 and Enterococcus faecalis DSM20478, were included for comparison purposes. Acid and heat adaptation and cross-protection to stress were studied in ten strains with different stress response patterns. Cell death and the changes in protein expression were evaluated by plate counts and Sodium Dodecyl Sulfate Polyacrilamide Gel Electrophoresis, respectively. All strains of all species were highly tolerant of osmotic stress. With a few exceptions, acid and oxidative treatments reduced the number of viable cells by >5 log units but responses to heat stress were more variable. For some, but not all strains, stationary phase cells were more resistant to some or all stresses. Matrix cluster analysis was used to group strains on the basis of their pattern of stress response in seven clusters. Significant associations between the sources of strains and stress resistance were found for acid and oxidative stresses. Adaptation to stress during the exponential phase enhanced the survival of acid and heat stressed cells from 1 to 60,000-folds, but a detrimental effect of adaptation on cell viability was evident for oxidative and osmotic stresses for three strains. Adaptation and entry into the stationary phase resulted in significant changes of protein bands whose estimated molecular masses corresponded with those of proteins (DnaK, GprE, GroEL, and GroES) involved in the general stress response but no statistically significant correlation between stress response and band intensity was evident.