8 THE SCOPE OF THE ENQUIRY [CH. i 



Gordon (1903-4) isolated from human saliva 300 strains of 

 streptococci. He tested separately the power of these diflFerent 

 strains to ferment various substances, consisting of 14 carbo- 

 hydrates, 13 glucosides, and 6 polyatomic alcohols. Many of 

 these test substances proved of no differential value as regards 

 streptococci, either because they were uniformly attacked by 

 all or because no streptococci could ferment them, but he was 

 led to select a series of seven substances — ^namely saccharose, 

 raffinose, inulin, salicin, coniferin and mannite — as being of 

 special value as tests for streptococci, and to these he added 

 two further tests — the clotting of milk and the reduction of 

 neutral red under anaerobic conditions. By such means he 

 was enabled to distinguish 48 chemical varieties. 



Houston (1903-4) applied the same tests (with the omission 

 of one — the action on coniferin) to 300 strains of streptococci 

 derived from human faeces and was able to distinguish 

 40 chemical varieties amongst them. 



Gordon demonstrated that these chemically different strains 

 were remarkably constant in their reactions and this was 

 confirmed later by the work of Andrewes and Horder (1906), 

 who tested his strains and, in addition, some 200 new strains 

 derived from foci of disease in human beings. " Gordon himself 

 was careful to abstain from claiming specific value for his 

 different chemical types and he did not venture to propose any 

 reasoned scheme of scientific classification based upon his 

 tests." Andrewes and Horder attempted to do this. They 

 collected from various sources particulars of the behaviour of 

 over 1200 different strains of streptococci when subjected to 

 Gordon's tests. As a result they found that these 1200 strains 

 fell into some half a dozen main groups. By adding one 

 further test — the power of growth in gelatin at 20° C. — and by 

 taking into consideration also the morphological characters 

 and pathogenesis, they were able to define five varieties of 

 streptococci which they regarded as of "approximately specific 

 value" though connected by a multiplicity of intermediate 

 varieties. They named these S. cmginosa, 8. salivarius, 

 8. faeccUis, 8. pyogenes and 8. pnevmococcus. 



Though confirming, on the whole, the stability of the 



