104 GROWTH OF BACTERIA IN MILK. 



we have classed together as producing no noticeable action 

 on the milk, although liquefiers may be abundant. The lactic 

 bacteria are usually very few. Indeed, it frequently happens 

 that in clean milk the number of lactic bacteria is so small 

 that they cannot be found by ordinary methods of bacterial 

 analysis (see Chapter IX.). Sometimes, however, the lactic 

 bacteria are more numerous even at the outset, due probably 

 to extra secondary contamination. 



After the initial germicidal period of three to thirty 

 hours a multiplication begins which, hour by hour, affects 

 nearly all of the bacteria present. But they do not all 

 increase with equal rapidity. Some develop slowly, and 

 during this first period only multiply two- or threefold. 

 Others seem to multiply not at all, or very slightly, while 

 others grow much more rapidly. The result is that, while there 

 is an actual increase in numbers of nearly all species, some few 

 those which grow the fastest comprise each hour a larger 

 and larger per cent, of the whole. The liquefiers frequently, 

 though not always, increase quite rapidly and form a larger 

 per cent, than at the outset. The group which produces no 

 effect visible in the milk also increases very rapidly, and at 

 the close of the first period they may be vastly more numer- 

 ous than at first. But the most striking fact is the growth of 

 the lactic bacteria. These, if present at the outset in small 

 numbers, steadily increase in numbers and percentage. The 

 Bact. lactis acidi, which may not have been detected at the 

 outset because of their small numbers, become numerous 

 enough in a few hours to form an appreciable percentage, at 

 first perhaps 2% to 4%. The other types of lactic bacteria 

 also frequently increase in percentage, although this does 

 not occur with such constancy as in the case of Bact. lactis 

 acidi. The lactic bacteria increase with absolute constancy 

 during the whole of the first period. 



The length of time during which this period lasts depends 

 upon the temperature and upon the number and kinds of 



