238 MILK HYGIENE 



Numerical determination of bacteria. There is no 

 method known by which the exact number of bacteria in 

 a sample of milk may be determined, and even when the 

 best methods are used, the count is always less than the 

 actual number of bacteria present, for the following 

 reasons : 



a. Many bacteria in process of multiplication are 

 held together by adhesive membranes in pairs, chains 

 or masses. It is for the purpose of separating bacteria 

 thus joined, as well as to obtain an even mixture, that 

 the sample itself and the diluted sample when plating 

 are shaken. This shaking, while it breaks up larger 

 masses and shortens long chains, does not to any great 

 extent break apart the shorter chains, diplococci, etc. 

 Each of these groups of bacteria, when caught in the 

 solid medium, develops as a single colony. 



b. It is impossible to obtain a medium suited to the 

 food requirements of all species or races of bacteria. 



It has been found by experiment that a medium 

 consisting chiefly of a watery extract of raw meat, 

 alkaline to litmus and slightly acid to phenolphthalein, 

 will furnish the best food for the greatest number. 



c. These varying forms of minute vegetable life re- 

 quire varying temperatures for their best development. 

 Many forms which will develop at room temperature 

 will not grow at body temperature. Some require a 

 very high temperature for their best growth. 



d. Some bacteria develop in an atmosphere free 

 from oxygen, some only where oxygen is present ; many 

 are facultative growing under either condition. Bac- 

 teria which require an oxygen- free atmosphere do not 

 develop in plates as generally prepared. Bacteria 

 requiring oxygen, if deep in the medium, develop but 

 slowly, as they obtain oxygen only by diffusion. 



e. Many forms are slow in developing into visible 

 colonies, some requiring three or four days. On the 

 other hand, in plates grown for several days many small 

 colonies are obscured in the growth of larger ones. 



/. Each bacteriujn requires a certain amount of 

 nourishment for development. There are also antag- 

 onistic forms which will not develop in close proximity 



