BACTERIA IN SOIL 149 



to pathogenic organisms, especially in relation to possible sewage con- 

 tamination, attention is to be directed to three groups of organisms, 

 those resembling the b. coli, the bacillus enteritidis sporogenes, and the 

 streptococcus pyogenes. The characters of the first of these will be 

 found in Chapter XV., of the second in Chapter XVII., of the third in 

 Chapter VII. For the detection of these bacteria the following pro- 

 cedures may be recommended : — 



(a) The B. coli Group. — A third of a gramme of soil is added to 

 10 c.c. of bouillon, is shaken up, and loopfuls are spread on one or more 

 plates of MacConkey's lactose neutral-red agar. After twenty-four hours' 

 incubation in an inverted position any red colonies are picked off and 

 subjected to the tests for the presence of b. coli detailed in Chapter XV. 

 The presence of non-lactose fermenters [e.g., b. typhosus, b. gaertner and 

 its allies), which may have great significance, may claim attention in the 

 examination of such plates, and the method may be employed when the 

 detection of these organisms is the object of special inquiry. 



(b) The Bacillus enteritidis sporogenes. — To search for this organism 

 ' 1 grra. of the soil is thoroughly distributed in 100 c.c. sterile bouillon, 



and of this 1 c.c, '1 c.c, and '01 c.c. is added to each of three sterile 

 milk tubes. These are heated to 80° C. for ten minutes, and then 

 cultivated" anaerobically at 37° C. for forty-eight hours. If the charac- 

 teristic appearances seen in such cultures of the b. enteritidis {q.v. ) are 

 developed, then it may fairly safely be deduced that it is this organism 

 which has produced them. 



(c) Fmcal Streptococci. — The best method to employ is that of Prescott 

 and Winslow modified by Mair. This depends on the fact that when 

 b. coli and streptococci, are growing together in glucose broth, as the 

 medium becomes acid the streptococci tend to outgrow the b. coli. 

 If lactose neutral- red agar plates be made at this stage, the colonies of 

 streptococci, being small and intensely red, can be distinguished from 

 the larger and paler colonies of the b. coli. They can then be picked 

 off for investigation. It is evident that here the 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. 



(d) Anaerobic Bacteria. — A soil may contain such important pathogenic 

 agents as the b. tetani, b. cedematis maligni, etc 



We may now give in brief the results obtained by the applica- 

 tion of such methods. First of all, uncultivated 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 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 feeble viability outside the animal body, to be looked 

 on as Evidence of extremely recent excremental pollution. 



