Period of Incubation. 



alcohol producers, bacteria with rennet-like action, pigment pro- 

 ducers, slime-forming bacteria, etc. 



The varieties of bacteria which are found in milk under 

 general conditions of production, even when conducted under 

 special provisions for obtaining clean milk with unusual precau- 

 tionary measures (provided that the milk originates from healthy 

 animals and is drawn by healthy milkers), are of special interest 

 to the milk hygienist. 



These bacteria split sugar and proteids, and attack fat. Ac- 

 cording to Fliigge, they are separated into : 



1. Aerobic lactic acid bacteria, which cause spontaneous 

 souring and do not form spores ; 



2. The anaerobic butyric acid bacilli, and 



3. The aerobic peptonizing bacteria, with remarkably 

 resistant spores. 



These bacteria, however, do not propagate uniformly well in 

 milk, but they are subject to influences of the medium, which really 

 constitutes an elective culture medium for some of the species, 

 whereas it is destructive for others. The time during which no 

 increase of bacteria can be noted in milk is known as the period 

 of incubation (Soxhlet). In fact there may be not only no multi- 

 plication of bacteria in the milk, but under certain conditions dur- 

 ing the beginning of the incubation there may even be a diminu- 

 tion of the bacterial number which is first found; the bacteria 

 present in the milk are subject to the injurious influences of the 

 animal secretion; the milk is in the germicidal stage (Koning). 



'Fokker in 1890 was the first to assert that raw milk (he used for his experiments 

 goat's milk) must have germicidal properties. He proved that raw milk when inoculated 

 with lactic acid bacteria resists spoiling for a longer period than was the case with 

 milk that had been boiled. Prior to his investigations however Wolffhiigel and Eiedel 

 found in 1886 that cholera vibrios readily multiply in boiled or sterilized milk, whereas 

 in raw milk their growth is rapidly checked. As a result of these findings the question as 

 to whether milk possesses germicidal properties became the subject of dispute. While 

 Freudenreich, Hesse, Park, Cozzolino, Conn, Schenk, Behring, Eullmann and Trommsdorff, 

 Eosenau and McCoy, Sassenhagen and Bab claim that milk possesses inhibiting, or even 

 destructive properties for bacteria, Richet, Hueppe, Heim, Friedrich, Kitasato, Uffelmann, 

 Weigmann and Zirn, Basenau, Schrank, Schottelius, Moro, Heinemann, Rubinstein, 

 Stocking, Sommerfeld, Klimmer, Knox and Schorer, and Kuntze express their belief 

 against this power in the sense of the bactericidal action of blood serum, and think 

 that the germicidal properties exist only towards certain species of bacteria. They 

 also believe that- the composition of the milk creates favorable conditions for the 

 propagation of some of the bacteria, while for others this is not the case, just as with 

 elective media, some of the less favored species become injured or destroyed by the 

 multiplication of lactic acid bacteria and their products. 



The presence of specific germicidal substances (alexins, ambo- 

 ceptors, leucins) in special kinds of milk, such as colostral and 

 mastitis milk, has been proven by the work of Bauer, Sassenhagen, 

 Rullmann and Trommsdorff, whereas the question of the occur- 

 rence of these in normal milk has not yet been sufficiently demon- 

 strated, although the fact of a diminution of bacteria in freshly 

 drawn normal milk has been established by our methods of 

 counting. 



