BACTERIA IN MILK.. 75 



BACTERIA IN MILK. 



S. C. Keith, Jr., Boston, Mass. 

 Ladies and Getitlemen : 



Many people are under the impression that the bacteria are 

 bugs and animals of a size only a little smaller than can be seen 

 with the naked eye, and that they possess digestive organs, legs, 

 etc. Many also think that these so-called bugs are always of such 

 a nature that we should give them a wide berth. As a matter of 

 fact, however, the bacteria are not animals but are very minute 

 plants ; in fact, they are the smallest of all living things and their 

 images require to be magnified more than five hundred times before 

 they can be seen at all. And in order to see them well, we must 

 view them with a microscope having a power of from one thou- 

 sand to fifteen hundred diameters. Many millions of them can 

 easily be taken up on the point of a needle at one time and it has 

 been estimated that one hundred times the population of London 

 could be spread in a single layer over a square inch of surface 

 without crowding. The bacteria have very simple forms and can 

 all be classified under three heads — ball shaped, rod shaped and 

 bent or spiral shaped. They reproduce by simple division, one 

 bacterium gradually splitting into two, these two again making 

 four, and so on. In this way under favorable conditions from a 

 single bacterium millions may grow in a few hours, as it takes 

 only about a half an hour ordinarily to complete one generation. 



I can very well demonstrate this method of multiplication by 

 means of a sausage, which might well represent a giant bacterium 

 of the rod-shaped variety, and at the same time convey to your 

 minds a clearer notion of their infinitesimal size by comparing the 

 sausage bacterium to some known object magnified corresponding- 

 ly. If, for instance, I should be the object in question, my height 

 would be some sixty miles, and the giant hand holding the bac- 

 terium the size of a sausage would be three miles wide. By 

 twisting the sausage so that the envelope becomes narrowed at a 

 point near the middle of the sausage and by twisting until the 

 division is complete, we very nearly represent the division taking 

 place in the bacterium cell, only in the case of the bacterium the 

 division into two parts takes place by simply drawing together in- 

 stead of the twisting process as I have shown you here. It per- 

 haps might be represented more nearly by tying a string around 



