Streptococcus thermophilus LMD-9

Photo: Robert Hutkins, University of Nebraska
Streptococcus thermophilus was once described as a bacterium "marked more by the things which it cannot do than by it’s positive actions" (Sherman, 1937). Although it may be certainly be true that S. thermophilus is physiologically and biochemically less versatile than other lactic acid bacteria, the reality is that this organism can actually "do" quite a bit. In fact, research during the past two decades has revealed that S. thermophilus has properties that make it one of the most commercially important of all lactic acid bacteria.

Streptococcus thermophilus is used, along with Lactobacillus spp., as a starter culture for the manufacture of several important fermented dairy foods, including yogurt and Mozzarella cheese. Its use has increased significantly during the past two decades, as a result of the tremendous increase in consumption of these products. According to USDA statistics, in 1998, more than 2.24 billion pounds Mozzarella cheese and 1.37 billion pounds of yogurt were produced, respectively, with a combined economic value of nearly $5 billion.

The substantial increase in production of Mozzarella cheese and yogurt have led not only to increased use of S. thermophilus cultures, but also to new demands on their performance and production requirements. Industrial strains, for example, should be insensitive to bacteriophage, have stabile fermentation characteristics, and produce products having consistent flavor and texture properties. Although research on the physiology of S. thermophilus has revealed important information on some of these properties, including sugar and protein metabolism, polysaccharide production, and flavor generation, only recently has the genetic basis for many of these traits been determined. Clearly, future efforts aimed at improving this important industrial strain will require information that can only be obtained by genome analysis.

Currently, several traits in S. thermophilus have been targeted for strain improvement programs (Delcour et al., 2000). Since bacteriophage are responsible for considerable economic losses during cheese manufacture, efforts are underway to engineer restriction and other phage resistance systems into commercial strains. Enhancing stability and expression of exopolysaccharides that act as natural thickening agents has also attracted significant attention. Finally, S. thermophilus has an important role as a probiotic, alleviating symptoms of lactose intolerance and other gastrointestinal disorders.

The genome of S. thermophilus is 1.8 Mb, making it among the smallest genomes of all lactic acid bacteria. Although a moderate thermophile, it is phylogenetically related to the more mesophilic lactococci and has a comparable low G+C ratio (40%). Genes coding for metabolic pathways involved in sugar catabolism (Poolman et al., 1989; Vaughan et al., 2001), protein and peptide utilization (Fernandez-Espla et al., 2000; Garault et al., 2002), polysaccharide production (Almirón-Roig et al., 2000), the stress response system (Perrin et al., 1999), and phage resistance mechanisms (Burrus, 2001; Solow and Somkuti, 2000) have been sequenced and characterized. More than 100 DNA sequence entries are currently listed in GenBank. Although most strains do not harbor plasmids, other mobile elements have been reported (Guedon et. al., 1995), and techniques for gene transfer and mutagenesis have been developed (Baccigalupi et al., 2000; Coderre and Somkuti, 1999). A genome sequencing project using an industrially-relevant strain will undoubtedly reveal valuable information that could have substantial impact on agriculture, the food industry, and the consuming public.


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