GROWTH OF BACTERIA IN NORMAL MILK. 79 



16. The effect of a preliminary icitig of the viilk is also 

 noticeable. When the milk is iced for a period of fifteen ho7irs it 

 produces several important results. The first is a decrease in 

 numbers of bacteria and rapidity of bacteria groivth. This is 

 shozcn, not OJily by the fact that the milk at any particular ho2ir 

 contains fewer bacteria than the similar sample of 7nilk zvhich has 

 not been iced, bjit also by the fact that the number which develops 

 at the time ivhen the milk curdles is very much less in samples of 

 milk which have previously been iced. 



ly. The preliminary icing of milk increases the chayices which 

 the miscellaneotis bacteria have of growiyig in the milk; for milk 

 which has been iced shozvs even in late stages a larger number of 

 77iiscellaneous organis?ns tha?i a siinilar sample of milk zvhich has 

 not been iced. 



18. The effect of the preliminary icing is to postpone the per- 

 iod tvhen the B. acidi lactici develops in abiindance and gains the 

 ascendency over the other species of bacteria; in other words to 

 delay the second period in the ripening. 



SECOND PERIOD. 



ig. The second period is characterized by a very rapid devel- 

 opment of total rmmbers of bacteria which increase more rapidly 

 than in the first period. This great increase is due chiefiy to 

 the developmeyit of lactic organisms. B. acidi lactici I. in par- 

 ticular develops now zvith marvellous rapidity, and the B . acidi 

 lactici II. is also found, in many cases, in great abundance. B. 

 aerogencs, in our milk, rarely becomes very numerous. The acid 

 organisms that develop dtiring this seco?id period sometimes 

 reach pp per cent, of the total bacteria, though commonly not 

 so high as this. 



20. Parallel zvith the increase in lactic organisms there is a 

 decrease in the 7^elative munbers of all other species. Not only do 

 the other species decrease in relative numbers, but most of them 

 decrease in absolute numbers. The liguefiei^s freqtiently disappear 

 entirely, and the Streptococcus group, zvhich is one of the most 

 persistent, seems to disappear entirely in many samples of milk 

 during the second period. The miscellaneous organis^ns certainly 

 disappear from the analyses; a?id this may mean that they have 

 been destroyed or merely that they are in such small munbers that 

 they fail to appear in highly diluted plates. 



