BACTERIA IN SOIL 133 



as colourless flocculent growth with an opaque centre, and can be seen 

 under the microscope to send out into the medium apparently branched 

 threads which vary in thickness, being sometimes 2 /j. across. They 

 consist of rods enclosed in a sheath. These rods may divide at any 

 point, and thus the terminal elements may be pushed along the sheath. 

 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 of 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 diffusing 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 Chap. VI. 

 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. 

 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 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 microscopic 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 separa- 

 tion of such soil organisms (v. pp. 42, 43). 



(&) The Bacillus enteritidis 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.e: 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) Faecal 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. Much work has been devoted 

 to the question of these faecal streptococci presenting specific characters by 



