BACTERIA IN SOIL 155 



then be picked off for investigation. It is evident lli.it here tlie method 

 must be adopted of taking as a measure of the number of streptococci 

 present the least quantity of the original fluid in which evidence of their 



presence can be detected. 



\\V may now ^ive 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 

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

 always contain thrsr 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. eiiti'ritidis is also evidence of such pollution, but from the fact 

 that this is a sporing organism the 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 

 tWble viability outside the animal body, to be looked on as 

 evidence of extremely recent excremental |x)llution. The very- 

 great importance of these results in relation to the bacterio- 

 logical examination of water supplies will be at once apparent, 

 and will be referred to again in connection with this subject. 



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 

 ir^anisms 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 w r ay, so that it is only by studying the organisms in 

 <iuestion when growing in unsterilised soils that information can 

 l>e 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 of a soil bacterium, it is found that sometimes the 

 typhoid bacillus, sometimes the soil bacterium in the course 

 of a few weeks, or even in a few days, disappears. Further, the 

 character of the soil exercises an important effect on the results ; 

 for instance, the typhoid bacillus soon dies out in a virgin sandy 



