Streptococcus thermophilus
Classification
Higher order taxa:
Kingdom: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Lactobacillales
Family: Streptococcaceae
Genus: Streptococcus
Species: Streptococcus salivarius
Description and Significance
Streptococcus thermophilus is an essential lactic acid bacterium used for commercial purposes, which includes the production of milk, cheese, and other diary products. This organism is a thermophilic Gram-positive bacterium with an optimal growth rate at 45 °C. It is also capable of generating energy, in the form of adenosine triphosphate (ATP), by aerobic respiration with the presence of oxygen; however, without the presence of oxygen, it still can produce ATP through fermentation. S. thermophilus lacks cytochrome, oxidase, and catalase enzymes. It does not have motility and it does not form spores. Although S. thermophilus is closely related to other pathogenic streptococci (such as S. pneumoniae and S. pyogenes), S. thermophilus is classified as a non-pathogenic, alpha-hemolytic species that is part of the viridians group. The increasing consumer need for diary products and booming manufacture of diary products ($40 billion industry) lead to the investigation and sequencing of S. thermophilus.
Genome Structure
Originally isolated for yogurt in 1974, the entire genome sequence of Streptococcus thermophilus LMG18311 has been sequenced. The sequencing of S. thermophilus LMG18311 was accomplished by a genome shotgun method, which was consummated with the joint efforts of Biochemistry Unit and Genetics Unit from Université Catholique de Louvain. Also, a different strain, CNRZ1066, of S. thermophilus was sequenced.
The genomes of S. thermophilus CNRZ1066 and LMG18311 are approximately 180,000 base pairs in length, which contain about 1900 coding regions. From these 1900 coding sequences, 1500 sequences are orthologus, genes from different species that are similar to each other, to various streptococcus genes, revealing that S. thermophilus share common physical and cellular properties with pathogenic streptococci. Both strains are single circular chromosomes; however, they differ by 3,000 single nucleotide base pairs. The difference in genome sequence may account for difference in extracellular polysaccharide biosythesis, bacteriocin synthesis, and immunities. S. thermophilus genes have low GC nucleotide base pairs (approximately 39%).
By sequencing the S. thermophilus genome, factors (which explain the non-pathogenic characteristics of S. thermophilus) have been observed. Nearly 10% of S. thermophilus genes are inactive or pseudogenes, which are caused by frame-shift, deletion, and or mutation of genes. Many virulent related genes (VRGs) that contribute to virulence of pathogenic streptococci are either not present or present as pseudogenes in S. thermophilus. [Virulence determinants such as pnemococcal surface protein A and C (PspA and C), pnemococcal manganese ABC transporter lipoprotein PsaA, IgA proteases, and choline binding proteins are inactivated in S. thermophilus.]
