BACTERIA. 



61 



These three species are characterised as follows : Bacterium 

 aceti (HANSEN) (Fig. 14) forms a slimy smooth film on "double 

 beer " (top-fermentation beer, rich in extract, containing 1 per 

 cent, of alcohol) at a temperature of 34 and in the course of 

 24 hours. The slime is not coloured by any iodine solution. 

 The cells of this film are hour-glass shaped rod-bacteria, 

 arranged in chains ; occasionally longer rods and threads occur, 

 with or without swellings. At 40-40JC. long thin threads 

 develop. In plate-cultures with wort-gelatine, this bacterium 

 at 25C. forms colonies with sharply defined edges, or, more 

 rarely, stellate colonies, which appear grey by reflected light, 

 bluish by transmitted light ; they mainly consist of single 

 rod-bacteria. In meat-water peptone-gelatine the colonies 

 are surrounded by milky zones, separated from the colony by 

 bright zones ; they may later become iridescent. On sowing 

 drops on wort-gelatine, flat, rosette-shaped colonies are formed 

 at 25 C C. in the course of 18 days. In "double beer" the 

 temperature maximum for growth is 42 C., the minimum 

 4-5 C. 



This species is of common occurrence both in high- and low- 

 fermentation beers. 



Bacterium Pasteurianum (HANSEN) (Fig 15) forms a dry 

 film on "double beer" at 34 C., which soon becomes wrinkled and 

 pleated. In young, vigorous films on beer or wort, at favourable 

 temperatures, the slime surrounding the cells is coloured blue 

 by any iodine solution. The 

 cells of the film form long 

 chains and are usually larger, 

 especially thicker than in the 

 previous species. The thread- 

 form at 40-401 C. is also 

 a little thicker than that of 

 B. aceti. In plate-cultures, 

 with wort-gelatine at 25 C., 

 the colonies resemble those of 

 the previous species, but are a 

 little smaller, and consist 



chiefly of chains. In meat-water peptone-gelatine the colonies 

 are similar to the previous species. On sowing drops on 



^ 



~ Bacterium Pasteurianum - (Aftcr 



