388 ACETIC FERMENTATION. 



the attention of Hansen to the existence of two species of acetic 

 acid bacteria. The chemical composition of the envelope has 

 not yet been determined, but that it is not cellulose must be 

 concluded from the negative results obtained from the tests made 

 with various reagents (iodosulphuric acid, zinc iodochloride) for 

 that substance. Already in this characteristic these three species 

 differ from the acetic acid bacterium introduced into the literature 

 of the subject by A. J. Brown under the name of Bacterium 

 xylinum. The tough, leathery skin of zoogloea (measuring as 

 much as one inch in thickness), formed by this bacterium on the 

 surface of the nutrient solution, and generally known in England 

 as the vinegar-plant, consists principally of the extensively de- 

 veloped mucinous envelope of the cells. If the contents be 

 extracted by suitable means, a mass is left which answers to the 

 cellulose reactions (e.g. solubility in ammoniacal copper oxide) and 

 on ultimate analysis exhibits a composition agreeing with the= 

 formula (C 6 H 10 5 ) ? , 



Moreover, the three Hansen species differ notably in the 

 appearance and development of their colonies, prepared by the 

 transference of droplets (rich in cells) from a pure culture 

 grown at 25 C. on to solid nutrient media (wort gelatin or 

 doppel-bier gelatin). Those from B. aceti assume the form of 

 exceedingly pretty, many-rayed stars or rosettes j those from B. 

 Pasteurianum have an almost perfectly smooth periphery (without 

 dentations) and exhibit convolutions of the surface resembling 

 those of the brain ; whilst those of B. Kutzingianum are readily 

 recognisable by the absence of both the stellar form and con- 

 volutions. 



211. The Morphological Influence of Temperature. 



Hansen's researches into the acetic acid bacteria also afford an 

 important support to the theory of bacterial pleomorphism, as- 

 will now be shown. The cell forms described and illustrated in 

 the previous paragraph are not the only ones assumed by the 

 fission fungi under consideration. On the contrary, the pleomor- 

 phic variations are exceedingly plentiful, though they may all 

 be grouped under three main types, viz., chains of short rods (as 

 already described), long threads, and, finally, distended or bulged 

 forms. The conditions ascertained by HANSEN (VII.) as influ- 

 encing the development of one of these forms, its gradual conver- 

 sion into the others, and, finally, its restoration to the original 

 shape, will now be briefly referred to. It must be premised that 

 the minimum limit of temperature at which development can 

 proceed is for B. aceti, 4-5 C. ; for B. Pasteurianum, 5-6 C. ; 

 and for B. Kutzingianum, 6-7 C., the maximum being about 

 42 C., and the optimum temperature about 34 C. 



Cultures of Bacteria Pasteurianum on doppel-bier have shown. 



