GERMICIDAL ACTIVITY. 37 



erate agitation of making dilutions for plate cultures,* these 

 clumps would remain undisturbed (35). Thus, the number 

 of colonies on the medium would decrease, while plates made 

 from milk that had been most violently agitated would show 

 an increase. This gives a reason for the discrepancies between 

 the results of various workers, and goes a long way toward 

 explaining the phenomenon. 



It has been generally assumed by writers on the subject, 

 that the decrease in numbers of colonies on plates indicated 

 the death of the organisms concerned. Most writers consid- 

 ered this due to some germicidal property of milk. It has been 

 suggested that many organisms die off in milk because they 

 find there an environment wholly unsuited to them. This is 

 undoubtedly true to some extent, but hardly accounts for some 

 of the latest ob.servations published. Agglutination seems to 

 account for a large part of the apparent decrease in numbers. 

 Even making allowance for this, it is evident that raw milk 

 exerts a restraining influence on the multiplication of bacteria 

 as compared with heated milk. In the light of this view, the 

 phenomenon might be described as similar to the action of a 

 weak antiseptic (35). 



The evidence at hand does not warrant recommending a 

 change in the general practice of rapid cooling of milk. That 

 is, it has not been proven that delay in cooling would improve 

 the keeping quality. It is certain, though, that the phenom- 

 ' enon should be taken into consideration in judging the success 

 of efforts to secure germ-free milk. The length of time after 

 milking, and the temperature, should be taken into consider- 

 ation when various bacterial determinations are compared, and 

 submitted as evidence of success in aseptic milking. For 

 instance, the work reported in Ref. 69, Chap. I, was appar- 

 ently done without recognition of this important factor. 



* The common method of counting bacteria is described in Chap. VIII. 



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