Chapter 11 Streptococci, staphylococci and micrococci
Streptococci comprise a diverse group of Gram-positive cocci, which continuously undergo taxonomic revision. They are distributed widely in humans and animals, mostly forming part of their normal flora. A few species cause significant human morbidity. The oral streptococci, which include the cariogenic mutans group, are important members of the genus. Another common group of cocci, the staphylococci, live on the skin but are infrequently isolated from the oral cavity and are significant agents of many pyogenic (pus-producing) human infections.
Fig. 11.1 α- and β-haemolysis: β-haemolytic colonies (e.g. Streptococcus pyogenes) produce complete translucence of blood agar, whereas α-haemolytic colonies (e.g. Streptococcus pneumoniae) do not. Note also the sensitivity of S. pneumoniae to a disc impregnated with optochin.
The carbohydrate antigens found on the cell walls of the organisms are related to their virulence. Hence, serogrouping, termed Lancefield grouping, is useful in the identification of the more virulent β-haemolytic species. Currently, 20 Lancefield groups are recognized (A–H and K–V) but not all are equally important as human pathogens. The following are worthy of note:
It is found as a commensal in the nasopharynx of a minority of healthy adults, but more commonly (about 10%) in children. It grows well on blood agar, with a characteristic halo of β-haemolysis. Some strains produce mucoid colonies as a result of having a hyaluronic acid capsule. This may contribute to virulence by offering resistance to phagocytosis.
Culture on blood agar yields characteristic β-haemolytic colonies (lysis of blood due to streptolysins O and S). A Gram-stained smear may show characteristic cocci in chains (Fig. 11.2); these are more developed in liquid than in solid media. The isolate can be presumptively identified as Streptococcus pyogenes if it is sensitive to bacitracin.
Gram-stained smear and culture yielding β-haemolytic colonies on blood agar; colonies on blood agar are generally larger than Streptococcus pyogenes. Lancefield group is determined by antiserum against cell wall polysaccharide.
Oral streptococci, which live principally in the oropharynx, are a mixed group of organisms with variable characteristics. New typing techniques, particularly those based on molecular biology, have revealed the complex nature of the origin and the taxonomy of this group. Hence, the nomenclature of oral streptococci is in a constant state of flux. They typically show α-haemolysis on blood agar, but this is not a constant feature as some strains are non-haemolytic and others β-haemolytic. Oral streptococci can be divided into four main species groups as follows:
|mutans group||S. mutans, serotypes c, e, f|
|S. sobrinus, serotypes d, g|
|S. cricetus, serotype a|
|S. rattus, serotype b and others|
|salivarius group||S. salivarius|
|anginosus group||S. constellatus|
|mitis group||S. sanguinis|