192 AGRICULTURAL BACTERIOLOGY. 



It is interesting and suggestive to find that the number of 

 lactic bacteria which get into the milk from different cows is 

 by no means the same. Sometimes, for wholly unexplained 

 reasons, milk from certain cows, when carefully collected, will 

 not sour. If the milking from the different cows of a large 

 herd be collected in such a way as to avoid contamination from 

 falling dirt and dust, it is quite common to find the milk from 

 some of the cows without any representatives of lactic bac- 

 teria. If a large series of samples are thus collected from 

 different cows and immediately protected from dust by cotton 

 plugs, many of the samples fail to sour, since they contain no 

 lactic bacteria. Indeed, milk direct from the milk duct does 

 not commonly contain the ordinary lactic bacteria. 



It is clear from these facts that the reason why lactic bac- 

 teria are regarded as preeminently dairy organisms, and the 

 reason why milk is so sure to sour, are not because of the 

 excessive number of lactic bacteria in the barn and dairy. 

 Fresh milk frequently contains other species in just as great 

 abundance, but the lactic bacteria find the conditions in milk 

 more favorable for their growth, so that at the end of twenty- 

 four hours they have gone ahead of all the rest. 



The souring and curdling of milk is a phenomenon so nearly 

 universal that in past years it has been regarded as a normal 

 change due to the character of milk, just as clotting is a 

 characteristic of blood. But this has been wholly disproved 

 by showing that it is perfectly possible to preserve milk sweet 

 indefinitely if bacteria can be kept out of it. But the bacteria 

 are so common around barns that they are practically sure to 

 get into the milk and, finding the milk an exceptionally good 

 medium for growth, they develop until they produce a suffi- 

 ciency of acid to throw the casein out of solution. Any acid 

 added to milk will curdle it, since the casein is only dissolved 

 in an alkaline medium, and the curdling inevitably follows the 

 production of lactic acid. 



