Genus Streptococcus
Classification1. Group specific substance (Lancefield grouping) : according to the CHO content of cell wall. Lancefield groups A-H and K-U. Most important gps that cause diseases in humans are A, B, C, D,F, G . Group A .S pyogenes. Group B S. agalaciae Group D non enterococcal streptococci S. bovis ( Enterococcus are seperated genus)
2 . Hemolysis a. β hemolysis ( complete destruction of R.B.C with clearing of blood around the bacterial growth) e.g. Streptococcus pyogenes( gp A), Streptococcus agalactiae (gp B).
B. α hemolysis: incomplete destruction of the RBC with reduction of hemoglobin and the formation of green pigment e.g. viridans group streptococci .C. ᵞ hemolysis: non hemolytic streptococci e.g. Streptococcus bovis.
3. Capsular polysaccharides It is used to classify S pneumoniae into over 90 types and to group group B streptococci.
Microscopical appearance
Cultural characteristicsafter 18-24 hr. incubation on blood or choclate agar (fastidious m.o). aerobic., facultative anaerobic, strictly anaerobicColonies are small 0.5 mm , smooth, translucent. According to haemolysis: 1. β haemolytic gp 2. α haemolytic gp. 3.δ haemolytic .
Biochemical activities:
A) catalase negative (differentiate it from staphylococcus ). B) bile insoluble. C) bacitracin sensitivity test +ve for gp. A streptococcus .
Streptococcus pneumoniae
Microscopical appearance: Gram positive lancet shape cocci arranged in pairs (diplococci) or short chains, non motile non spore forming , capsulated m.o.Cultural characteristics
Colonies are several mm in diameter, mucoid , smooth (drop of oil on agar), surrounded with intense green α-hemolysis, some colonies may collapse from nail head appearance.It fastidious, requires 5- 10 % CO2 ,Differentiations between S. pneumoniae and α-hemolytic streptococcus Morphological characteristics.
Optochin sensitivity test. (S.pneumoniae sensitive). Genus Streptococci resistant
Bile solubility (2% sodium deoxycholate). pneumococcus = soluble (clear) α-hemolytic streptococcus= insoluble (turbid)