Streptococcus equi subsp. zooepidemicus

Streptococci are non-motile, Gram-positive cocci with widely varying pathogenic potential that occur in pairs or chains. S.equi is the causative agent of strangles, the most common respiratory infectious disease of horses. It is characterized by prominent swelling of the throat lymph nodes and impaired breathing. The symptoms are severe, complications are common and, at present, there is no effective treatment or vaccine for the disease. The virulent strain 4047 (ST-179 by MLST) was isolated from a horse with strangles in the New Forest, England in 1990, and compared with another equine-isolated bacterium S.zooepidemicus H70.In comparison with S.zooepidemicus, the equine-restricted S.equi appears to have lost ancestral functions, including some involved in catabolic metabolism, transport and the cell envelope, and gained some new functions via mobile genetic elements (MGE) to be able to exploit a new environmental niche. 16.4% of the S.equi genome is composed of MGE while only 7.5% of the S.zooepidemicus is MGE. These MGEs encode virulence factors including 4 prophages that are more related to those found in the human pathogen S.pyogenes than they are to each other, suggesting a common phage pool among the Streptococci. One of the integrative conjugative elements in the S.equi genome could produce siderophores, which have been demonstrated to be required in S.equi for the production of a secreted molecule which enhances the ability of S.equi to acquire iron. This locus was present in a number of S.equi isolates, but in none of the diverse collection of S. zooepidemicus isolates examined. Given the importance of iron acquisition to other streptococcal pathogens, this acquisition may have contributed significantly to the increased pathogenesis of this Streptococcus (modified from PubMed 19325880). (HAMAP: STRE4)