694 STREPTOCOCCUS 



Hsemolytic Streptococci of Group C are certainly pathogenic. They have been frequently 

 isolated from suppurative and acute inflammatory lesions in horses, cattle, guinea-pigs 

 and other animals. They have also been isolated from human infections ; but their patho- 

 genicity for man appears to be lower than that of Str. jyyogenes. The human pathogenic 

 strains that have been examined produce a fibrinolysin active against human fibrin ; the 

 strains derived from animals do not. 



Haemolytic Streptococci of Group F. — The strains that have been identified as belonging 

 to this group have been derived from minor infections of the respiratory tract in man, 

 and from the normal human throat. Their possible pathogenic role must be regarded as 

 sub judice ; but their virulence would seem, in any case, to be of a low order. They do 

 not produce a fibrinolysin active on human fibrin. 



Haemolytic Streptococci of Group G. — The streptococci of this group are certainly 

 pathogenic. They have been isolated from tonsilUtis, endocarditis and urinary infections 

 in man (Macdonald 1939, Rantz 1942), from pneumonia in the monkey, and from otitis 

 in the dog. They have also been isolated from the normal human throat. Such evidence 

 as is available suggests that Group G, like Group C strains, have a definitely lower virulence 

 for man than has Str. pyogenes. Group G strains produce a fibrinolysin acting on human 

 fibrin. 



Haemolytic Streptococci of Groups E, H and K. — There is as yet no evidence that the 

 streptococci belonging to these groups are pathogenic. 



Haemolytic Streptococci of Groups L, M, and N. — Organisms belonging to Groups 

 L and M appear to be pathogenic for certain animals, especially the dog. Group N strains 

 are apparently non-pathogenic. 



Variation in the Characters of Streptococci. 



It is not always possible from the records to identify the strain in which parti- 

 cular variations have been observed with one or other of the groups, species or 

 types that have been defined in this chapter. It is, for instance, sometimes im- 

 possible to tell whether the term " haemolytic streptococci " or Str. hcemohjticus, 

 is equivalent to Str. pyogenes, in the sense in which we have used that term. In 

 almost all the instances given below, however, the identity of the strain or strains 

 concerned is not in doubt, and the reservation that it is necessary to make is little 

 more than formal. 



Variations in Str. pyogenes. — There are many reports in the literature of the appearance 

 of non-hsemolytic, or a-hsemolytic, variants in cultures derived from an originally haemolytic 

 strain. These reports have at times been regarded as invahdating hsemolysin production 

 as a differential test ; but in view of our more detailed knowledge of the factors that 

 determine the action of streptococci on red blood corpuscles, it is clearly unnecessary to 

 assume that the appearance, in a culture of a ^-haemolytic streptococcus, of a variant that 

 gives typical a-hsemolysis, or no haemolysis at all, on the surface of an aerobic blood agar 

 plate affords an instance of the mutation oi Str. pyogenes into a streptococcus of the viridans 

 type, or into a completely non-hsemolytic form. 



An illuminating example is given by Todd (19286). By repeated mouse passage he 

 was able to obtain, from a typical /J-hsemolytic strain of streptococcus, a variant that pro- 

 duced no haemolysis at all on the surface of aerobic blood agar plates. When grown 

 anaerobically, this variant maintained full haemolytic activity. Moreover, the aerobicaUy 

 non-haemolytic variant not only inactivated its own haemolysin when exposed to a free 

 supply of oxygen, but, under the same conditions, inactivated the haemolysin produced 

 by the original haemolytic strain, if the latter was grown in symbiosis with it. The appear- 

 ance of non-haemolytic variants in an originally haemolytic strain has, it may be noted, been 

 recorded by many other workers (see p. 567). 



