Streptococci: Discoveries and Diversity
Earlier this month, a team of Moroccan scientists isolated two previously undiscovered strains of catalase- and oxidase-negative Streptococcus-like strains from raw camel milk. Published in the Journal of Systematic Evolutionary Microbiology, Streptococcus moroccensis and S. rifensis join over 50 others in the growing Streptococci genus, with Streptococcus rupicaprae as their nearest phylogenetic neighbour. 16S rRNA sequencing placed similarity of the two strains to to S. rupicaprae at 95.9 % and 95.7 %, respectively.
Although these novel strains are believed presently to be relatively benign, this is not the case for all members of this fascinating genus which although are commensal in many instances, are capable of producing an array of symptoms ranging from annoying to deadly in ways that are not always easy to predict.
Streptococci are spherical Gram-positive bacteria and are classified in accordance with their metabolism on blood agar; their ability to induce haemolysis (nonspecific killing of blood cells by metabolic by-products) of red blood cells on 5% sheep blood agar; inducing a visible change in the growth medium. This process is known as hemedigestion. They are also described according to the presence or absence of Lancefield carbohydrate group antigens on their cell walls.
Using these methods, Streptococci are organised thus:
Pyogenic (beta-haemolytic) including Groups A, B, C, E, F & G
Lactococci (generally not pathogenic)
Alpha-haemolytic Streptococci induce only partial haemolysis of red blood cells and thus turn blood agar green (see image below). An example of an alpha-haemolytic Streptococci is S. pneumoniae which forms part of the commensal flora of the nasopharynx in healthy individuals. Despite this, S. pneumoniae is associated with disease in some individuals particularly the elderly, neonates and the immunocompromised and is a leading cause of meningitis in adults and young adults. S. pneumoniae is not restricted to respiratory infection however and may induce septic arthritis, endocarditis, rhinitis and brain abscesses among other disease states.
The late microbiologist Rebecca Lancefield, established the system of Lancefield grouping over the course of her esteemed career, working extensively on beta-haemolytic Streptococci; placing them into one of 11 groups based on their immunochemical properties.
Group A (GAS) - S. pyogenes is of clinical importance as it is responsible for an array of infections which may be invasive or noninvasive. Non invasive infections are generally not severe and include impetigo, pharyngitis, the now uncommon scarlet fever and also rheumatic fever. More severe invasive Group A streptococcal infections include toxic shock syndrome, pneumonia as well as gruesome and disfiguring necrotising fasciitis.
Group B (GBS) - Such as S. agalactiae; a common cause of pneumonia and meningitis in neonates and the elderly. Group B Streptococci are also known to colonise the intestines and the birth canal wherein it may threaten the integrity of the amniotic membrane and infect the baby. To prevent this occurring, women in some countries are screened for Group B Streptococci and treated with prophylactic antibiotics during labour.
Group C (GCS) - GCS are generally animal pathogens. S. equi is not pathogenic in humans but certainly a bacteria of consequence in veterinary medicine and is responsible for the equine upper-respiratory tract infection known as strangles. Other GCS have been known, albeit rarely, to cause severe disease in humans such as meningitis.
Group D (GDS) - Many of the species that were previously thought to comprise the Group D Streptococci have largely been reclassified and allocated to the genus Enterococcus. An example of a GDS since reallocated is Enterococcus faecalis. Strep. bovis is an example of a true GDS and is a coliform common in the alimentary canals of ruminants although this bacterium has been linked to cancers and endocarditis in humans. Other members are associated with food borne disease.
Group F (GFS) - S. anginosus, also known as S. milleri is commonly associated with highly purulent abscesses. In previous years it was postulated that this microorganism is more common than previously thought as a pathogen due to misidentification in clinical laboratories, this seems to have been true to some degree but with the refinement of technology and understanding, misidentification is greatly reduced.
Group G (GGS) - Streptococcus canis is an example of a GGS and is a part of the natural flora of the oral mucosa in cats and dogs. Although not widely considered to be a common pathogen among humans, incidences of toxic shock syndrome and necrotising fasciitis attributable to S. canis are known. S. canis is also reported to complex albumin in human hosts as well as other animals and thus is highly likely to be vertically transferred.
Gamma-haemolytic (non-haemolytic) Streptococci do not induce haemolysis of red blood cells and thus do not alter the appearance of blood agar. Group D Streptococci such as Enterococcus faecalis are gamma haemolytic and as aforementioned are typically commensal coliforms also associated with food borne disease.
Other species discovered this month
More on Biochemical and Molecular methods in bacteriology
