ACETIC ACID BACTERIA. 95 



Hansen's researches are among the first which proved that a 

 definite fermentation is not induced by one species of bacterium 

 only, but by several ; these researches also furnish some of the 

 earliest experimental evidence of the fact that one and the same 

 species can occur in very different shapes ; the correctness of 

 his results was later confirmed by Zopf, de Bary, and A. J. 

 Brown. By means of his staining experiments with Bacterium 

 (Mycoderma) aceti (1879), he discovered that at least two 

 distinct species are hidden under this name, of which the one, 

 like most other bacteria, is stained yellow by iodine, whilst the 

 other assumes a blue coloration with the same reagent. For 

 the former he retained the old name Bact. aceti, whilst the one 

 stained blue he named after Pasteur Bact. Pasteur ianum. The 

 film formations on wort and beer, and likewise the growths 

 on wort-gelatine, give a fine blue colour 

 with tincture of iodine, or iodine dis- 

 solved in a solution of potassium 

 iodide, whilst the growths which de- 

 velop on yeast-water and on broth 

 with peptone and gelatine are coloured 

 yellow ; even very old films on beer 

 show a yellow reaction. It is the slime 

 formation secreted from the cell-wall 

 that is coloured blue. At a later period Fi s- v --Bacterium 



,. , , . , . (after Hansen.) 



Hansen discovered a third species. 



These three species are characterised as follows : Bacterium 

 aceti (Hansen) (Fig. 17) 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 C., and in the course of 

 24 hours. The slime is not coloured by iodine. The cells 

 of this film consist of rod-bacteria, hour-glass-shaped, and 

 arranged in chains ; occasionally longer rods and threads 

 occur, with or without swellings. At 40-40-5 C. long thin 

 threads develop. In plate-cultures with wort-gelatine at 25 C. 

 these bacteria form 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 peptone-gelatine broth the colonies 

 are surrounded by milky zones, separated from them by 



