Genetics of Streptococcus thermophilus: A Review

The genetics and molecular biology of several species of lactic acid bacteria have progressed impressively during the last years with a major emphasis on the mesophilic cheese lactococci. More recent efforts were also addressed to streptococci and lactobacilli. Among them, Streptococcus thermophilus (Streptococcus salivarius ssp. thermophilus) has received particular attention because of its use in yogurt and cheese manufacture. Several gene transfer techniques -conjugation, conjugative mobilization, spheroplast transformation, transduction, and electrotransformation-have been developed for this species. This enabled examination of the use of existing or newly constructed cloning vectors and elaboration of a first generation of expression vectors. Studies on both homologous and heterologous gene expression are being pursued in S. thermophilus. Ongoing research covers the lactose utilization determinants and reporter genes such as the one encoding for o~-amylase. Because of the potential problem of phage attack in dairy processes, collections of S. thermophilus bacteriophages have been analyzed and compared. In view of accumulating data collected for the thermophflic dairy streptococci, it may be concluded that their genetic modification is now feasible.