144 LABORATORY EXERCISES IN BACTERIOLOGY. 



part these bacteria in fresh milk are non-pathogenic cocci, although occasionally the 

 mycobacterium of tuberculosis is met with as a serious contamination; and from air 

 or water contamination a variety of more or less serious infections may be met with 

 in older samples. The analysis of milk by culture follows the methods used for water 

 examination. The milk is always diluted and very small quantities of the dilute 

 sample (1: 100 or 1: 1000) employed for diffusion in the nutrient medium, both for 

 the purpose of reducing the number of the bacteria and the confusion of their colonies, 

 and to prevent an annoying turbidity in the medium caused by the fat of the milk. 



Soil Examination. The surface of the ground for a variable depth, depending 

 on its porosity and the amount of surface contamination, is occupied by a great variety 

 of organisms, growing upon the organic matter diffused by drainage through the 

 pores of the ground. These organisms ordinarily are found in profusion in the upper 

 three or four feet, becoming less numerous at greater depths ; and after the intervention 

 of a bed of rock or firm clay they are usually no longer met with. For the most part 

 they are saprophytes of little medical interest, concerned in the conversion of the 

 organic substances in the ground into carbon dioxide, water, and ammonium; but 

 the organisms of tetanus, malignant edema, black-leg, and several other affections 

 are also often encountered, and from recent contamination with effluvia from diseased 



FIG. 43. HARPOON FOR COLLECTING SOIL FROM BELOW THE SURFACE OF THE GROUND. 



individuals the germs of typhoid fever, anthrax, cholera, and a number of other dis- 

 eases may also temporarily exist. One especially important group of soil organisms 

 are the nitrifying bacteria, universally present, but not capable of cultivation upon 

 the ordinary media, and hence not recognized in the usual bacteriologic analysis of 

 soil. They are concerned in the oxidation of ammoniacal compounds in the soil into 

 nitrous and nitric acid and their salts, which are then utilized by the growing vegetation ; 

 hence their importance to the agriculturist from their r61e in the fertilization of the 

 soil. They are grown upon solutions of ammonium sulphate and phosphate of potas- 

 sium, or upon a mixture of these together with magnesium sulphate, calcium chloride, 

 and sodium carbonate added to silicic acid, this mixture producing a mass of gelatinous 

 consistence. 



In examining the soil, specimens are taken from different depths; a convenient 

 harpoon for obtaining the samples is that shown in figure 43. A definite amount of 

 the soil obtained is diffused in a known quantity of sterile water, and a measured 

 quantity of this dilution with the bacteria scattered through it is planted by diffusion 

 in the nutrient medium. This is then plated in one or other of the usual manners 

 and set aside for growth ; from the number of resultant colonies the number of bacteria 

 in the original specimen is calculated; and the further study of their characteristics 

 pursued. 



Material Obtained from Diseased Individuals. In the bacteriologic study 



