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62 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
be inert. Many bacteria belong to this group, which in- 
cludes some of the udder cocci. Many of the most im- 
portant pathogenic organisms are also inert in so far as 
their effect on milk is concerned, notably the tubercle 
bacillus, the bacillus of typhoid fever, paracoli or para- 
typhus bacilli, and the diphtheria bacillus. The presence 
of these harmful organisms is not indicated by any ap- 
parent change in the milk. 
VARIATIONS IN NUMBER AND KIND OF BACTERIA 
Different samples of market milk may show the 
greatest differences in the number and kind of bacteria 
present. One sample may contain much fewer than 1000 
bacteria per c.c. and another may contain four or five 
million and even more. In one sample, the bacteria may 
be largely of the inert forms and in another the bacteria 
of the gas-forming group may predominate. Market 
milk containing not more than 100,000 bacteria per c.c. 
is considered of good quality. The limit for certified 
milk is 10,000 bacteria per c.c. 
The kind or species of the bacteria must be considered 
as well as the number. Generally, the smaller the num- 
ber of bacteria present the better the milk, but there are 
exceptions to this rule. A few pathogenic organisms 
would be more harmful than a much larger number of 
the common milk bacteria. Between the different groups 
of the latter there are also important differences. Milk 
containing a rather large number of bacteria the greater 
proportion of which belong to the inert or lactic acid 
groups is less objectionable than milk containing a 
smaller number of bacteria with the greater proportion 
belonging to the gas-forming or peptonizing groups. On 
the other hand, while milk containing a large percentage 
