BACTERIA IN MILK 701 



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 approximate 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 without actual disease or harm following its consump- 

 tion. 



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 habitual 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 forma- 

 tion 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 451.) The transformation of the lactose into lactic acid may occur 

 either directly by hydrolytic cleavage: 



or indirectly through a monosaccharid, 



C B H r2 It + H 2 O = 2 C 6 H 12 6 = 4 3 H 6 O 3 . 



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 lactic- 

 acid production, however, according to Heinemann, 1 are the two first- 

 mentioned, Bacillus lactis aerogenes and Streptococcus lacticus. The 

 secret of the regularity with which some of these bacteria are present in 

 sour milk is probably found in the ability of these varieties to withstand 

 a much higher degree of acidity of the culture medium than other species. 



1 Heinemann, Jour, of Inf. Dis., 3, 1906. 



