134 BACTERIA IN SOIL 



which they could be differentiated from other streptococci. No definite 

 results have as yet been obtained. Houston gives as the general 

 characters of these organisms that they usually grow in short chains, 

 that they produce uniform turbidity in broth, that they give rise to acid 

 and clot in litmus milk at 37 C., and that they are non-pathogenic to 

 mice. The important point is to recognise that streptococci of fairly 

 ordinary types exist in great numbers in human fseces, and that when 

 in any circumstances faecal contamination is suspected the isolation of 

 streptococci strengthens the suspicion. 



We may now give in brief the results at which Houston has 

 arrived by the application of these methods. First of all, un- 

 cultivated soils contain very few, if any, representatives of the 

 b. mycoides, and this is also true to a less extent of the 

 cladothrices. Cultivated soils, on the other hand, do practically 

 always contain these organisms. With regard to the b. coli its 

 presence in a soil must be looked on as indicative of recent 

 pollution with excremental matter. The presence of b. 

 enteritidis is also evidence of such pollution, but from the 

 fact that it is a sporing organism this pollution may not have 

 been recent. With regard to the streptococci, on the other 

 hand, the opinion is advanced that their presence is, on account 

 of their want of viability outside the animal body, to be looked 

 on as evidence of extremely recent excremental pollution. The 

 very great importance of these results in relation to the 

 bacteriological examination of water supplies will be at once 

 apparent, and will be referred to again in connection with the 

 subject of water. 



While such means have been advanced for the obtaining of 

 indirect evidence of excremental pollution of soil, and therefore 

 of a pollution dangerous to health from the possible presence of 

 pathogenic organisms in excreta, investigations have also been 

 conducted with regard to the viability in the soil of pathogenic 

 bacteria, especially of those likely to be present in excreta, namely, 

 the typhoid and cholera organisms. The solution of this problem 

 is attended with difficulty, as it is not easy to identify these 

 organisms when they are present in such bacterial mixtures as 

 naturally occur in the soil. Now there is evidence that bacteria 

 when growing together often influence each other's growth in an 

 unfavourable way, so that it is only by studying the organisms in 

 question when growing in unsterilised soils that information can 

 be obtained as to what occurs in nature. For instance, it has 

 been found that the b. typhosus, when grown in an organically 

 polluted soil which has been sterilised, can maintain its vitality 

 for fifteen weeks, but if the conditions occurring naturally be so 

 far imitated by growing it in soil in the presence of a pure culture 



