BACTERIOLOGICAL EXAMINATION OF SOIL. 131 



iarise the worker with the non-pathogenic bacteria usually present. We have 

 referred to these two because of their importance. In regard to pathogenic 

 organisms, especially in relation to possible sewage contamination, attention 

 is to be directed to three groups of organisms, those resembling the B. coli y 

 the bacillus enteritidis sporogenes, and the streptococcus pyogenes. The 

 characters of the first two of these will be found in the chapter on Typhoid 

 Fever; of the third in Chapter VII. For the detection of these bacteria, 

 Houston recommends the following procedure : 



(a) The B. colt group. A third of a gramme of soil is added to 10 c.c. 

 phenol broth (vide chapter on Typhoid Fever) and incubated at 37 C. In 

 this medium very few if any other bacteria except those of the B. coli group 

 will grow, so that if after twenty-four hours a turbidity appears, some of the 

 latter may be suspected to be present. - In such a case a loopful of the broth 

 is shaken up in 5 c.c. sterile distilled water, and of this one or two loopfuls 

 are spread over the surface of a solid plate of phenol gelatin in a Petri's dish 

 either by means of the loop or of a small platinum spatula, and the plate is 

 incubated at 20 C. Any colonies which resemble B. coli are then examined 

 by the culture methods detailed under that organism. Further, all organisms 

 having the microscopic appearances of B. coli, and which generally conform 

 to its culture reactions, are to be reckoned in the coli group. 



($) The Bacillus enteritidis sporogenes. To search for this organism one 

 gramme of the soil is thoroughly distributed in 100 c.c. sterile distilled water, 

 and of this I 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 twenty-four hours. If the characteristic appear- 

 ances 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} The Streptococcus pyogenes. The method here is to pour out a tube 

 of agar into a Petri'g dish, and when it has solidified to spread out .1 c.c. of 

 the emulsion of soil over it and incubate at 37 C. for twenty-four hours. At 

 this temperature many of the non-pathogenic bacteria grow with difficulty, and 

 thus the number of colonies which develop is relatively small. Colonies hav- 

 ing appearances resembling those of the streptococcus (^.7'.) can thus be 

 investigated. 



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 clado- 

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

 always cpntain 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. enteri- 

 tidis sporogenes 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 



