DISEASES COMMUNICABLE IN MILK 67 



butter from sweet cream is most likely to be dangerous for it receives the 

 benefit of no inhibitory principles other than those inherent in itself, 

 namely, that it is a solid medium containing comparatively little mois- 

 ture. Salt is mildly antagonistic to microbial life, but the salting that 

 butter receives is never heavy. 



Effect of Lactic Acid on Disease Germs. The greatest protection 

 is in the acids, and possibly other products, formed by the lactic acid 

 bacteria in souring the cream. Many experiments have been made to 

 measure the germicidal effect that souring cream has on the typhoid 

 bacillus. Some observers have found no such action but the latest 

 experiments indicate that it is very considerable, that it reduces the num- 

 ber of typhoid bacilli but cannot be relied upon to kill them all. 



Krumwiede and Noble found that the typhoid bacillus is gradually 

 killed in sour cream by the acids produced, the rate of destruction being 

 proportional to the degree of acidity and the number of typhoid bacilli 

 present. With moderate contamination the typhoid bacilli are killed 

 in about 4 days, but with heavy contamination, or when initial multipli- 

 cation has taken place, it may take longer. 



Heinemann found that "some acid tolerant cells of B. coli may sur- 

 vive the presence of 0.6 per cent, lactic acid in milk." B. dysenterice, 

 B. typhosus, B. paratyphosus 0, Sp. cholerce, and B. diphtheria in the 

 experiments were destroyed in the presence of 0.45 per cent, lactic acid. 

 It is possible that strains of these bacteria exist which are able to resist 

 a greater amount of lactic acid. Acid tolerant strains of B. coli, B. 

 dysenterice, B. typhosus and B. paratyphosus |8, may multiply in the pres- 

 ence of quantities of lactic acid which are destructive to the majority of 

 cells. The smaller the initial amount of lactic acid, the more likely is 

 the growth of acid-tolerant strains to occur. Therefore, the slower milk 

 sours the greater is the danger of pathogenic bacteria surviving. 



The growth of the test bacteria is influenced to a marked degree by 

 the amount of acid present. Up to a fairly definite amount of acid there 

 is an increase in numbers, followed by a decrease, which becomes more 

 pronounced as the amount of acid increases. The amount of acid may 

 increase owing to the liberation of enzymes, after the number of bacteria 

 has commenced to decrease. 



Possible Infection of Butter in Wrapping. There is opportunity for 

 butter to be infected by those who work it after it is made. It is com- 

 monly shipped from creameries to the large cities in tubs, and is there put 

 up in prints or packages for sale. This necessitates more handling and by 

 another set of employees. Perhaps this last process is not the least im- 

 portant, for the butter reaches the public soon after being wrapped, 

 whereas at other stages in its progress to the consumer, it is held in cold 

 storage and opportunity is given the germs to die out. However, many 

 prints are wrapped by machine and are handled little in the process. 



