132 BACTERIA IN AIR, SOIL, AND WATER. 



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 bacterio- 

 logical 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 patho- 

 genic 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 iden- 

 tify 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 of one or other certain soil bac- 

 teria, 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 exer- 

 cises an important effect on what happens ; for instance, the 

 typhoid bacillus soon dies out in a virgin sandy soil, even when 

 it is the only organism present. In experiments made by sow- 

 ing cultures of cholera and diphtheria in plots in a field it was 

 found that after, at the longest, forty days they were no longer 

 recognisable. Further, it is a question whether ordinary disease 

 organisms, even if they remain alive, can multiply to any great 

 extent in nature. If we are dealing with a sporing organism 

 such as the B. anthracis, the capacity for remaining in a quies- 

 cent condition of potential pathogenicity is, of course, much 

 greater. 



