36 CHANGES IN MILK CAUSED BY BACTERIA. 



The germicidal action is observed in samples drawn so care- 

 fully that none but the micrococci from the interior of the 

 udder are present, as well as in samples of highly contaminated 

 market milk. The fact that the greater decrease in milk con- 

 taining only udder micrococci occurs at 70°F., rather than at 

 40°F. ,' is of interest. The explanation has been advanced very 

 recently (22) that these organisms die off in refrigerated milk, 

 because the temperature is so different from that to which they 

 are accustomed. The theory does not harmonize all the facts 

 observed in connection with the phenomenon. 



The germicidal action of milk is destroyed by heat, being 

 stopped completely by 176"F. for forty minutes. Milk heated 

 to the same temperature for twenty minutes shows no decrease 

 of the milk bacteria freshly implanted in it, and no increase for 

 several hours. Similarly 149°F., for forty minutes, resulted 

 in the restraint of growth for nine hours. Exposure to 131°F. 

 for thirty minutes, or 140 ''F. for twenty minutes, has been 

 shown to destroy the germicidal activity of milk against B. 

 lactis aerogenes, but had no influence upon its action on B. 

 typhosus. 



Fresh milk, drawn with aseptic precautions and planted with 

 pure cultures of various pathogenic organisms, has shown a 

 decrease in colonies in plate cultures, while sterilized milk has 

 shown an immediate increase. Among the organisms tested 

 are the germs of typhoid, cholera, diphtheria, dysentery and 

 B. lactis aerogenes, a common lactic acid producer (35). 



A given sample of milk may show a selective specific action 

 with reference to its restraining action on different organisms. 

 In a given sample one organism may immediately multiply, 

 while another may not. It has been shown that the common 

 lactic acid organisms in market milk usually increase from the 

 beginning even when other forms are disappearing so rapidly 

 that the total numbers are decreasing. It has been suggested 

 that the leucocytes in milk destroy the bacteria by phagocy- 

 tosis. This does occur to some extent, but it has been shown 

 that it is not responsible for the great changes observed (35). 



Some recent studies go to show that the decrease is more 

 apparent than real. Agglutination occurs, and, with the mod- 



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