386 ACETIC FERMENTATION. 



210. Morphology of the Acetic Acid Bacteria. 



Strictly speaking, Pasteur's publication did not advance our 

 knowledge of the morphology of the organisms in question beyond 

 the discoveries made by Kiitzing ; and the case remained in statu 

 quo for another fifteen years, until taken up by Emil Christian 

 Hansen, whose researches on the acetic acid bacteria not only 

 threw new light upon the organisms themselves, but were also 

 and that in a dual sense important to the subject of Fermentation 

 Physiology generally. 



Until then the opinion was current that any given fermenta- 

 tion was carried through by merely a single species of ferment. 

 HANSEN (VI.), however, showed in 1878 that, in the spontaneous 

 souring of beer at least two different species of bacteria can come 

 into action, one of which he named Mycoderma aceti and the 

 other Mycoderma Pasteurianum, in honour of his predecessor. At 

 the suggestion of W. Zopf he afterwards changed these names 

 to Bacterium aceti and Bacterium Pasteurianum respectively. 

 This important discovery was subsequently extended, partly by 

 HANSEN (VII.) himself who afterwards introduced into the litera- 

 ture of the subject a third species under the name of Bacterium 

 Kiitzingianum and partly by A. J. BROWN (I.), W. PETERS (L), 

 A. ZEIDLER (L), WERMISCHEFF (I.) and the author. Of all these 

 species, only those described by Hansen have been thoroughly 

 investigated morphologically, and for this reason they alone will 

 be more closely considered in the following lines. 



When inoculated into lager-beer or the so-called " doppel-bier " 

 a Danish high fermentation beer rich in extract and poor in 

 alcohol and kept at a temperature of about 34 C., these three 

 species provided air is freely admitted will develop on the 

 surface of the beer (which remains bright) to a pellicle within 

 twenty-four hours. In the case of B. aceti, this skin is moist 

 and mucinous, smooth and veined, but in B. Pasteurianum is, 

 on the other hand, dry, and soon develops fine implications. 

 That of B. Kiitzingianum resembles the first species, but differs 

 therefrom in raising itself high above the surface of the liquid 

 by gradually climbing up the walls of the vessel. Fresh differ- 

 ences make their appearance when a small portion of the skin is 

 examined under the microscope. Whilst the cells of ]3. Kiitzing- 

 ianum are, for the most part, single, and are only rarely seen joined 

 together as chains, those of the other two species are seldom found 

 as separate cells. The cells of B. aceti (Fig. 81) are somewhat 

 more slender, and frequently exhibit the sand-glass or figure 8 

 form ("en huit") noticed by Pasteur. In B. Pasteurianum 

 (Fig. 82) they are mostly rather longer and considerably broader 

 (plumper) than those of the other two species. 



These bacterial pellicles are true zoogloea, i.e. the individual 

 ells are attached together by a mucinous envelope, formed by the 



