298 ACKTIC FERMENTATION 



varies from green to bluish green, according to the proportion of the matrix. 

 It was this difference in the behaviour of the mucinous envelopes which, how- 

 ever, is noticeable only in young and vigorous pellicles that first directed 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 zooglrea 

 (measuring as much as one : nch 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 developed 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 exceed- 

 ingly pretty, many-rayed stars or rosettes ; those from B. Pasteur iannm have an 

 almost perfectly smooth periphery (without dentations) and exhibit convolutions 

 of the surface resembling those of the brain ; whilst those of B. Klitzingi- 

 anum are readily recognisable by the absence of both the stellar form and 

 convolutions. 



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 

 pleomorphic 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 influencing the development of one of these forms, its gradual 

 conversion 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. Kiitzingianum, 6-7 C., the maximum 

 being about 42 C., and the optimum temperature about 34 0. 



Cultures of Bacterium Pasteurianum on doppel-bier have shown that, at all 

 temperatures higher than 5 C. (but not greatly exceeding 34 C.), chains of 

 short rods develop, which, when grown at temperatures below 15 C., often 

 attain extraordinary dimensions, especially in the direction of the breadth. The 

 formation of chains proceeds most abundantly at about 34 C., the individual 

 short rods then having the ordinary form and being filled with firm, slightly 

 lustrous plasma. 



If a small portion of such a skin, cultivated at 34 C., be transferred to a 

 fresh nutrient medium, and maintained at 4O-4O.5 C., a morphological altera- 

 tion of the cells (Fig. 84) occurs, and is already noticeable at the end of a few 

 hours. The short rods about 2 p. long and j p broad, of which the chains of the 

 seed were composed, begin to elongate, and at the end of eight to nine hours 



