Streptococcus

Streptococcus is a genus of gram-positive, mostly anaerobic cocci occurring in pairs or chains. The Greek word "streptos" means "twisted" or "chained", and the word "kokhos" means a "berry" or "seed", hence the name for this genus of bacteria. Over one hundred species of Streptococcus are recognized. Of these, about 50% may be found at some time in the human mouth. The classical name for the oral streptococci is the viridans group. Species of the viridans streptococci include S. anginosus, S. gordonii, S. intermedius, S. sanguinis, S. parasanguinis, and the mutans group.

Many species of streptococci may be assigned to Lancefield grouping system . The system depends on specific antibodies which react with various antigens in the bacterial cell wall. The groups comprise, A, B, C, D, E, F, G, H, K, L, and R/S. For example, S. pyogenes is a group A streptococcus (GAS). GAS strains are further distinguished according to their surface anti-phagocytic M proteins which enable these bacteria to evade the host defense mechanisms.¹

Animal Diseases Caused by Streptococci

Most streptococci of veterinary importance co-exist with their animal hosts without producing clinical form of diseases. They commonly inhabit upper respiratory, alimentary, and lower genital tract. Disease-causing streptococci trigger inflammatory processes that lead to formation of pus and abscesses. In horses, S. equi causes strangles and bacterial pneumonia. Other streptococci are associated with a variety conditions in horses, including osteomyelitis, arthritis, abscesses, and wounds.

In dogs and cats, secondary pneumonia, toxic shock syndrome, and necrotizing fasciitis, or gangrene, are sometimes caused by S. canis and some strains of S. pyogenes.^2,3 Bacterial infection in fish from freshwater farms and from salt-water environments has been associated with S. iniae. People handling affected fish are at risk of developing cellulitis, endocarditis or arthritis following a puncture wound.

Healthy individuals may carry all the streptococci discussed above and many infections arise from an animal or human own microbial flora. Strangles is a contagious disease affecting most often young animals. Animal streptococci present little harm to humans, although S. suis can cause serious infections in swine handlers.

Streptococcus pyogenes

S. pyogenes is an important human pathogen, but it can also be found in low numbers as an internal microflora of the upper respiratory tract. It is the primary cause of streptococcal pharyngitis (strep throat) and a common cause of skin infections such as impetigo, cellulitis, and wound infections. S. pyogenes can lead to toxic like shock syndrome (TSLS) produced by TSLS toxin called streptococcal pyrogenic toxin (SPE). S. pyogenes is also responsible for rheumatic fever (rheumatic heart disease), rheumatoid arthritis, and kidney disease. Some strains of S. pyogenes, such as so-called flesh-eating strains, also produce proteases and other enzymes that enable the streptococci to evade host defense mechanisms and cause extensive damage to body soft tissues.³ Most strains of S. pyogenes also produce the cytotoxins streptolysin O and streptolysin S that kill host white blood cells and red blood cells, thereby contributing to the pus formation characteristic of streptococcal disease. Some strains also secrete a toxin termed erythrogenic toxin that produces the characteristic rash of scarlet fever.

Immunity to S. pyogenes depends on antibodies that bind to a protein termed M protein. There are over 50 types of M protein and immunity to one type does not protect the host from infection by another.⁵

Role of Streptococci in Caries Formation

Streptococcus mutans and Streptococcus sobrinus initiate caries formation by producing acids. Lactic, acetic, propionic, and formic acids diffuse into the tooth enamel and break it down by demineralizatio, meaning the removal of minerals such as calcium. Enamel is extraordinary because it is utterly smooth and shiny. Nothing can really get a hold on the surface. To ordinary bacteria it is really like a glass mountain. Streptococcus mutans has a few superpowers not shared by other bacteria: it can cling onto flat surfaces, especially when there is a film of sugar around. Another S. mutans' properties is that it can produce various enzymes and other chemicals that begin to roughen up the previously glassy surface of the enamel. Which is how the bacterial biofilm starts. The layer of S. mutans provides an anchorage for other bacteria which will eventually evolve into dental plaque.⁴

Streptococci species

References:
1. Oral Microbiology at a Glance By Richard J. Lamont, Howard F. Jenkinson
2. Veterinary microbiology By Dwight C. Hirsh, Nigel James Maclachlan, Richard L. Walker
3. Laboratory diagnosis of infectious diseases: essentials of diagnostic microbiology by Paul G. Engelkirk, Janet L. Duben-Engelkirk
4. Human wildlife by Dr. Robert Buckman
5. The Microbial World by Roger Y. Stanier, John L. Ingraham, Mark L. Wheelis, Page R. Painter