60 MICRO-ORGANISMS AND FERMENTATION. 



specks of Bact. aceti have a tendency to assume stellated 

 forms. From a morphological standpoint the two species 

 behave alike. Spores have not been observed. A fact of 

 practical interest is the observation that pure cultures of the 

 two species of acetic acid bacteria in beer do not exercise 

 any influence on the colour or brightness of the liquid. 

 It is therefore possible, to a certain extent, to ascertain 

 whether or not these organisms are alone present, since other 

 bacteria, when present in beer, produce turbidity in the liquid. 

 In order to develop vigorously, Bact. aceti not only requires 

 a very plentiful supply of oxygen, but also a fairly high 

 temperature. Hansen found that a temperature of about 

 33 C. was the most favourable when Carlsberg lager beer 

 was used. In a well-conducted store-cellar ( 1 to 3 C.) there 

 is therefore nothing to fear from Bact. aceti. But as soon as 

 the beer leaves the cellar, and is exposed to higher temperatures, 

 there is always a danger. 



In leaven, especially when it has become old and extremely 

 sour, Peters recently discovered an acetic acid bacterium 

 which distinctly differs from Bact. aceti and B. Pasteurianum. 

 The colonies in ordinary plate-cultures are circular in shape, 

 of a homogeneous appearance, and, when seen in transmitted 

 light, of a strong brown colour ; the surface colonies are 

 largely expanded. The single individuals are 1*6 /* long, 

 0-8 /* broad, truncated at one end, tapering at the other; 

 they occur singly or in pairs, rarely in groups of four. The 

 bacterium does not exhibit any motion. In yeast-water 

 containing 5 per cent, of alcohol it first produces turbidity, 

 then a thin film forms on the surface, and gradually becomes 

 viscous. This bacterium is perhaps identical with the species 

 described by Duclaux. 



The Bacillus ethaceticus discovered by Percy Franldand 

 induces a vigorous fermentation in various substances (e.g. 

 mannite), the chief products of which are ethyl-alcohol and 

 acetic acid. 



Pasteur has shown that, by the oxidation of alcoholic 



