296 ACETIC FERMENTATION 



applied the specific name Mycoderma aceti, first employed by Thomson. Of 

 course, at that time, PASTEUR (XIV.) was not in a position to prepare or use 

 pure cultures, consequently the results of his experiments cannot now be credited 

 with more than the single value of having unimpeachably proved the dependence 

 of acetic fermentation on the vital activity of certain micro-organisms. Pasteur 

 did not determine to what group of living organisms Mycoderma aceti belongs, 

 the botanical, and especially the morphological side of the question concerning 

 him but little ; only, in one place in his treatise, he states that he cannot regard 

 the organism as a bacterium, as was done by Stack in 1863. Nevertheless, we, 

 at the present day, must agree with the opinion of the last-named : the cause 

 of acetic fermentation studied and described by Pasteur can only have been 

 a fission fungus. 



The property of forming mucinous skins on the surface of liquids is not 

 peculiar to the acetic acid bacteria alone, but, on the contrary, is a very general 

 vital phenomenon among fungi. It is particularly noticeable among a group 

 (which will be considered in the second volume) of budding fungi, which have 

 been named, according to the nature of the medium in which they are found, 

 Mycoderma cerevisite, Mycoderma vini (Fr. fleur de la biere and fleur du vin 

 respectively). Pasteur denied that any of these skin-forming budding fungi 

 have the power of producing acetic acid, but the author refuted this opinion 

 by proving, in 1893, the existence of at least one such species endowed with this 

 faculty. More particulars concerning this will be given in one of the chapters 

 of the second volume. At present we are only concerned with the fact that 

 acetic fermentation is a vital manifestation not peculiar to fission fungi alone. 



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 fermentation 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 literature of the subject a third species under the name 

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

 A. ZEIDLER (I.), 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 B. Pasteurianum is, on the other hand, dry, 



