CHAPTER II 

 EXAMINATION OF MILK 



Milk is exceptionally liable to contamination with bacteria and is a most 

 favorable medium for their development. Consequently practically all of it 

 has an abundant bacterial content. 



Milk that shows 20,000 colonies per cubic centimeter or less is extraordinarily 

 clean. Milk that shows 10,000 colonies per cubic centimeter or less is seldom 

 observed. 



In winter, average city milk shows from 5000 to 500,000 colonies per cubic 

 centimeter, in summer twice as many or more. Milk that shows 1,000,000 

 colonies per cubic centimeter or more is dirty and has been carelessly handled. 



Before examining, milk must be thoroughly shaken to evenly distribute the 

 bacteria. 



Fermentation tests are not made as when examining water, because ferment- 

 ing organisms are known to be always present, most of them are lactic acid bacilli 

 (non-pathogenic) and some are colon bacilli from the cow. 



Various disease-producing bacteria may be contained in milk, including 

 staphylococci, streptococci, diphtheria bacilli, tubercle bacilli and typhoid bacilli; 

 the tubercle bacillus is the most frequently present pathogenic organism and 

 the streptococcus probably comes next. From 6 to 13 per cent, of all market 

 milk contains tubercle bacilli. 



Many observers believe that most of the epidemics of streptococcus throat 

 infections are derived from milk. 



It is practically impossible to isolate tubercle bacilli or diphtheria bacilli 

 from milk, or to observe them in it. For the detection of these organisms in 

 milk inoculations must be made, and these sometimes fail to disclose them. 

 Animal inoculations are generally omitted in the routine examination, opinion 

 as to the bacterial content being based upon examination of sediment for leuco- 

 cytes and bacteria, and plating to determine the number of bacteria per cubic 

 centimeter. 



For the study of leucocytes in milk and observing the presence of staphylo- 

 cocci and streptococci, the best and quickest is Stewart's method. 



When observing the leucocytes one should know whether the milk has been 

 heated or pasteurized because heating apparently increases the number. 



TECHNIQUE 



Shake the sample to evenly distribute contained bacteria. 



Take i cc. of the milk and mix with 9 cc. of sterile distilled water, dilution 

 No. i equals i : 10; take i cc. of dilution No. i and mix with 99 cc. of water, dilu- 

 tion No. 2 equals i : 1000; take i cc. of dilution No. 2 and mix with 9 cc. of water, 

 diutlon No. 3 equals i : 10,000. 



Take i cc. of dilution No. 2 and mix with 99 cc. of water, dilution No. 4 



equals i : 100,000. 



176 



