BACTERIA 39 



than the Streptococci, while they form notable amounts of acetic 

 acid besides. They are more aerobic in character than the 

 Streptococci, forming as a rule large surface colonies which are 

 often coloured yellow, orange or pink. Most of them liquefy 

 gelatine, though generally slowly, and they will therefore also 

 coagulate milk, though the quantity of acid formed is seldom 

 sufficient to accomplish this. Several of the liquefying species 

 probably play some part in the ripening of certain cheeses, for 

 example Tetracoccus liquefaciens 1 , which forms white colonies 

 and produces dextro lactic acid. The Tetracocci are found in 

 great numbers in cow dung and earth, whence they find their way 

 into dust ; they are well able to stand desiccation and common 

 salt, and many of them will also stand heating to over 70 C. 

 They are common skin bacteria, some giving rise to pustules 

 and others to inflammations, which are generally not of a 

 dangerous character. According to Beijerinck, certain lactic- 

 acid-producing sarcinae which occur in soil, but are of no import- 

 ance in dairy practice, produce large amounts of carbon dioxide 

 and hydrogen. 



The Pseudo Lactic Acid Bacteria. These are motile 

 or non-motile, Gram-negative short rods with rounded ends, 

 which seldom form chains or threads of any length. They do not 

 require organic nitrogen, and in stab cultures show profuse surface 

 growth. As a rule they do not liquefy gelatine, and are mainly 

 intestinal and excremental bacteria. The gas which they produce 

 from sugars consists of carbon dioxide and hydrogen in widely 

 varying proportions ; according to certain American investigators, 

 the composition of the gas formed affords a basis for the classifica- 

 tion of these organisms 2 . 



The aerogenes bacteria are non-motile rods which produce large 

 amounts of gas, being able to convert most of the sugar present 

 into gas, especially if the acid which is produced is neutralised. 

 The gas may contain up to three times as much carbon dioxide as 

 hydrogen, while aerogenes forms are known which may even 

 produce carbon dioxide alone. In dairy bacteriology two types 

 may specially-be distinguished. The one forms much slime which 

 gives rise to outstanding colonies with the shiny appearance of 

 porcelain ; appreciable amounts of alcohol are formed, but not 

 acid enough to coagulate milk. If the acid is neutralised by chalk 

 as fast as it is formed, the milk will gradually be converted into a 

 thick slime. To this type belong certain pathogenic bacteria, such 



1 The author ("Landwirt. Jahrbuch der Schweiz," 1904, pp. 349 and 369) 

 has described this organism under the name Micrococcus casei liquefaciens. 



2 L. A. Rogers, W. Mansfield Clark, Brooke J. Davis and Alice C. Evans 

 (" Journ. of Infectious Diseases," vols. 14, 15 and 17). 



