IDENTIFICATION OF BACTERIA IN MILK 605 



reduction in the time necessary to obtain results; milk which con- 

 forms to the standard may be quickly passed. Badly contaminated 

 milk can be detected by simple inspection without even the formality 

 of a count. There are also certain disadvantages. All bacteria which 

 are stainable with methylene blue are visible by this method and 

 dead organisms as well as those which are viable appear in the count. 

 This is a decided source of error in pasteurized milk, where a relatively 

 large proportion of bacteria are killed by heat; the method also does 

 not distinguish sharply between different types of organisms. 



On the whole, the advantages very materially outweigh the disad- 

 vantages and employed judiciously the method is of great practical 

 value in the bacterial control of dairies arid milk supplies. 



The information obtained by the bacterial count is of importance 

 chiefly from the viewpoint of the past history of the milk. Milk pro- 

 duced in cleanly surroundings, handled carefully in sterile utensils, 

 kept cool and delivered promptly, should contain relatively few 

 bacteria. If the milk is handled properly but not kept cool the 

 numbers of organisms usually increase greatly, but as a rule the 

 variety of organisms present will be limited. . Improperly handled 

 milk kept cool will frequently exhibit several types of bacteria, but 

 not necessarily a high total count. A consistent low count with but 

 few types of bacteria usually indicates a satisfactory milk supply. 



Identification of Bacteria in Milk. The bacterial types found in 

 milk may be very varied; the opportunity for contamination does 

 not cease when the milk is drawn from the cow every step in the 

 handling of the milk from the producer to the consumer offers new 

 avenues for infection. A catalogue of all the bacteria which have 

 been isolated from milk would be very extensive, but of little practical 

 value. Of vastly greater importance is the recognition of the patho- 

 genic organisms which may be transmitted to man and the chemical 

 changes which ordinary saprophytic milk-bacteria induce in it. There 

 are relatively few bacteria which are pathogenic both for the cow and 

 for man. Of these, the bovine tubercle bacillus, the unknown virus 

 of foot and mouth disease and the virus of the disease known as trem- 

 bles of cattle are transmissible to man, the latter causing a well-defined 

 symptom complex known as milk sickness. Goats, particularly 

 Maltese goats, infected with the specific organism Micrococcus meli- 

 tensis, transmit the disease Malta fever to man through their milk. 1 



1 The detection of tubercle bacilli in milk has been discussed in the chapters on tuber- 

 culosis and bacillus abortus. Malta fever has been discussed in the chapter on Micro- 

 coccus melitensis and foot and mouth disease in the section relating to filterable viruses. 



