112 MICRO-ORGANISMS AND FERMENTATION. 



milk in 24 hours at 35 C., ferments lactose, and produces 

 volatile acids, especially acetic acid. Its growth is, however, 

 soon arrested by the true lactic acid bacteria, and, in 

 particular, by the development of Bact. lactis acidi (Streptococcus 

 lacticus), which causes a vigorous formation of acid. 

 According to Weigmann, the lactic acid is then gradually 

 displaced by acid-consuming fungi or by bacteria producing 

 alkali, and the peptonising or casease bacteria come into 

 activity ; in particular, the species producing cheesy 

 aroma. 



The special characters of different kinds of cheese are 

 due to special micro-organisms. The particular part played 

 by the lactic acid bacteria in the process is to prevent the 

 peptonising bacteria from getting too great a hold, and thus 

 producing too quick a decomposition of the curd. The con- 

 version process is thus regulated by means of the lactic acid 

 organisms. According to 0. Jensen, they further stimulate 

 the action of pepsin derived from rennet. Lastly, their 

 importance in the ripening of cheese depends to a great extent 

 upon the fact that certain species effect a further transforma- 

 tion of the products of decomposition, especially of the 

 albumoses and peptones produced in milk, and in the early 

 stages of the ripening of cheese by peptonising bacteria. In 

 particular, the production of volatile acids, such as propionic 

 acid and acetic acid, detected by O. Jensen, is to be attributed 

 to this cause. Quite recently Jensen has discovered a special 

 propionic acid ferment in Emmenthaler cheese, which he 

 believes to be a variety of Bact. lactis acidi. According to 

 Jensen the carbon dioxide produced by this species forms the 

 " eyes " in Emmenthaler cheese. 



Amongst the casein-digesting bacteria must be classed the 

 aerobic Tyrothrix species, minutely described by Duclaux, 

 thread-forming bacteria, which secrete an enzyme resembling 

 trypsin, and belong to the group of hay bacilli. To this group 

 belongs Bacillus nobilis, discovered by Adametz, and, lastly, 

 Paraplectrum foetidum, detected by Weigmann in Limburger 

 cheese, which occurs in milk as thick rods, and at the tempera- 

 ture of the incubator (30-40 C.) assumes mallet shapes, and 

 quickly forms spores which are twice as long as they are wide. 



