154 BACTERIA IN SOIL 



Sometimes the sheath ruptures, and thus by the extrusion of these 

 dividing .cells and their further division the branching appearance is 

 originated. Reproduction takes place by the formation ot'gonidia in the 

 interior of the terminal cells. These gonidia acquire at one end a bundle 

 of flagella, and for some time swim free before becoming attached and 

 forming a new colony. Houston describes as occurring in the soil another 

 variety, which with similar microscopic characters appears as a brownish 

 growth with a pitted surface and diffuses a Bismarck-brown pigment 

 into the gelatin which it liquefies. 



A few experiments made with an ordinary field soil will, however, 

 familiarise 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 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 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. coli Group. A third of a gramme of soil is added to 

 10 c.c. broth containing '2 per cent, of phenol 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 turbidit} r 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 capsule 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 micro- 

 scopic appearances of b. coli, and which generally conform to its culture 

 reactions, are to be reckoned in the coli group. The media of MacConkey 

 and Drigalski are very useful in connection with the plating and separa- 

 tion of such soil organisms (vide pp. 50, 47). 



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

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

 water, 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 twenty-four 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) FcKcal Streptococci. The method here is to pour out a tube of agar 

 into a Petri capsule, 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 having appearances resembling those of the streptococcus 

 pyogenes (q.v.} can thus be investigated. 



Another method 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 agar plates be made at this stage, 

 the colonies of streptococci, being small and intensely red, can be distin- 

 guished from the larger and less acid colonies of the b. coli. They can 



