268 BACTERIOLOGICAL EXAMINATIONS 



first few inches of ordinary soil contain most of the bacteria, after 

 a depth of two metres no bacteria at all are found and the earth 

 is sterile. 



Soil may be collected in sterile sharp pointed iron tubes, and 

 diluted with sterile water of given quantity and plates poured 

 from it. 



Arable lands may be enriched very much by inoculating them 

 with certain nitrifying bacteria, some of which convert ammonia 

 into nitrous acid, which form in them nitrites; others change nitrites 

 into nitrates (nitrosomonas). Certain of these bacteria are con- 

 cerned in the assimilation of nitrogen from the atmosphere and 

 adding to the nitrogen content of the soil, thus enriching it. On 

 the roots of some plants, alfalfa, beans, peas, and clover, minute 

 tubercles develop. These little growths are caused by the nitrify- 

 ing bacteria, and add to the nutrition of the plant by adding to it 

 ammonia. 



Bacteriology of Cow's Milk. 



Theoretically the milk in the interior of the breasts of nursing 

 women and the udders of cows is sterile. So soon as it leaves the 

 nipple it becomes contaminated with bacteria, and by the time it 

 reaches the pail, in the case of cow's milk, it is far from sterile. 



Bacteria of the air, and dust from the cattle and bedding, at every 

 movement of the cow, and by the agency of flies, find their way into 

 milk and contaminate it. The number of bacteria that develops 

 in the milk depends upon the number that reach it in the first place, 

 the temperature of the air, and the length of time milk is kept at 

 a temperature favorable for their multiplication. Two hundred 

 and thirty-nine different varieties of bacteria have been isolated 

 from milk at different times. 



Pathogenic varieties of bacteria that are found in cow's milk 

 include the tubercle bacillus, Streptococcus pyogenes, Staphylococ- 

 cus aureus, the colon bacillus, typhoid bacillus, the diphtheria bacil- 



