42 MILK AND THE PUBLIC HEALTH chap. 



milk culture 24 to 48 hours old will kill a guinea-pig by sub- 

 cutaneous injection within about 24 hours. The changes in 

 nailk grown anaerobically are the diagnostic characters relied 

 upon for its identification. They are most characteristic after 

 two days' growth. Gas is abundantly formed, the cream being 

 torn so that the surface of the medium is covered by stringy, 

 pinkish -white masses of coagulated casein. Beneath is a 

 colourless, clear, or slightly turbid whey, with some entangled 

 casein. The whey is markedly acid, has a pronounced butyric 

 acid odour, and contains numerous bacilli, but in the non- 

 sporing stage. 



B. hutyricus closely resembles B. enteritidis sporogenes. It 

 shows slight cultured differences, but its essential distinction 

 is its non-pathogenicity to rodents. It is not infrequent in 

 milk. 



There is no evidence that either of these organisms are 

 harmful to man, but B. enteritidis sporogenes is of importance 

 in that it is a valuable means of measuring the manurial 

 pollution of milk, since the spores of this organism are 

 prevalent in manure and in dust, while they are absent 

 from milk collected under conditions of great cleanliness. 



Of the aerobic bacilli included in this group the most 

 commonly met with include B. subtilis, B. mesentericus mdgatiis, 

 B. mycoides, B. lacticola. There are also a number of other 

 less well-deiined organisms which have been isolated, but the 

 differentiation of the included organisms is not very satis- 

 factory. 



B. subtilis, and most of the others, are abundant in hay, 

 straw, etc., and obtain access to milk directly or indirectly from 

 such sources. Their presence in milk indicates outside con- 

 tamination. 



They are non -pathogenic, but they decompose and pep- 

 tonise milk. This whole group — aerobic and anaerobic — are 

 very heat resistant, and constitute most of the bacilli which 

 are left in imperfectly sterilised milk. 



Pliigge isolated a number of such spore -bearing bacilli, 

 and to one group of peptonising bacilli he ascribed pathogenic 

 effects. 



