MICROCOCCI 223 



glucose render media more favourable for streptococci 

 cultivation, and alkalinity 0-5 per cent. 



Streptococci are easily stained with the usual aniline 

 dyes and are Gram-positive. They are non-sporing, non- 

 motile, and non-flagellar. They (at least the pyogenic 

 species) do not liquefy gelatin. Optimum temperature 

 37-5 C. Growth takes place at 15 to 20 C. (distinction 

 from pneumococci) . Aerobiosis is the suitable environ- 

 ment for most races, but strict anaerobiosis does not prevent 

 development in suitable media. Size 0-5 to i micron. 



Cultures. In broth (bouillon) minute granules, which 

 fall to the bottom and form a sparse powdery or sandy 

 deposit. Diffuse clouding is rare. The long chain forms 

 produce the coarser granules. 



In glucose broth, the rapid formation of lactic acid 

 arrests development (i per cent of sterile CaCO 3 obviates) . 



In gelatin stab, in 48 hours a thin line forms which 

 later is seen to be made up of minute rounded colonies 

 of whitish colour reaching the size of a pin's head in 5 

 to 6 days. No growth on surface nor liquefaction. 



On agar plates, the colonies are small, greyish, and 

 delicately opalescent, are round, and tend to remain 

 separate in stroke cultures. 



In milk, ready growth with acid formation but no clot 

 (in pyogenic varieties). 



On potato, no growth. 



On blood agar plates, most cause haemolysis and decolor- 

 ization (difference from pneumococci). Ferment lactose, 

 saccharose, and salicin ; not inulin in Hiss's medium. 



Thermal Death-point : 10 minutes at 54 C. 



Virulence varies ; greatest for white mice and rabbits. 



Habitat : skin and mucous membranes. 



Diseases : cellulitis, erysipelas, osteomyelitis, bronchitis, 

 pneumonia, empyema, pericarditis, otitis, pharyngitis, 

 tonsillitis, septic endocarditis, septicaemia, puerperal fever. 



Toxins: streptolysin (bsemolytic). 



Immunization : variable results. 



Differentiation : (Andrews and Horder). Streptococcus 

 pyogenes does not clot milk, nor give green fluorescence 

 with neutral red broth, nor produce acid in Lemco media 

 with ramnose, inulin, coniferin, and mannite. It produces 



