BACTERIA IN MILK 1029 



remain until delivered to the consumer. In consequence of such 

 measures, it is possible for cities to be supplied with milk containing 

 no more, and often less, than fifty thousand bacteria to the cubic 

 centimeter. A standard of cleanliness has been established in various 

 towns by the introduction of the so-called "certified milk," which, 

 by the New York Milk Commission, is required to contain no more 

 than thirty thousand bacteria per cubic centimeter. Great stress 

 is laid upon such numerical counts simply in that they are ap- 

 proximate estimates of cleanliness. Most of the bacteria, however, 

 contained in milk are non-pathogenic, and numbers much larger 

 than the maximum required for certified milk may be present with- 

 out actual disease or harm following its consumption. 



The various species of bacteria which may be found in milk 

 include almost all known varieties. Whether there are special, 

 so-called milk bacteria or not is a question about which investigators 

 have expressed widely differing opinions. It is probable that many 

 of the species, formerly regarded as specifically belonging to milk, 

 are there simply by virtue of their inhabitual presence in fodder, 

 straw, or bedding, or upon cattle. It is likely, furthermore, that 

 some of these species are found with great regularity because of their 

 power to outgrow other species under the cultural conditions offered 

 them in milk. 



Under normal conditions, milk always undergoes a process which 

 is popularly known as souring and curdling. This is due to the 

 formation of lactic acid from the milk sugar and is the result of 

 the enzymatic activities of several varieties of bacteria commonly 

 found in milk. Most common among these bacteria is the so-called 

 Bacillus lactis aerogenes, an encapsulated bacillus closely related 

 to the colon- bacillus group. (See page 637.) The transformation 

 of the lactose into lactic acid may occur either directly by hydrolytic 

 cleavage : 



C 12 H 22 U + H 2 = 4 C 3 H 6 0,j 

 or indirectly through a monosaccharid, 



C 12 H 22 O lt + H 2 O = 2 G 6 H 12 6 = 4 C 3 H 6 O 8 . 



Other microorganisms which may cause lactic-acid fermentation in 

 milk are the so-called Streptococcus lacticus, the common pyogenic 

 streptococcus, the Staphylococcus aureus, Bacillus coli communis and 

 communior, and many other species. Most commonly concerned in 



