156 BACTERIA IN SOIL 



be looked on as evidence of extremely recent excremental 

 pollution. 



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 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 

 soil, even when it is the only organism present. In experiments 

 made by sowing 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 soil under natural conditions. 

 If we are dealing with a sporing organism such as the b. 

 anthracis, the capacity for remaining in a quiescent condition of 

 potential pathogenicity is, of course, much greater. The most 

 important principle to be deduced from these experiments is that 

 the ordinary conditions of soil rather tend to be unfavourable 

 to the continued existence of pathogenic bacteria, so that by 

 natural processes soil tends to purify itself. It must, however, 

 be noted that such an organism as the typhoid bacillus can exist 

 long enough in soil to be a serious source of danger. 



WATER. 



In the bacteriological examination of water three lines of 

 inquiry may have to be followed. First, the number of bacteria 



