62 MICRO-ORGANISMS AND DISEASE. [CHAP. 



about 0.0015 to 0'003 mm. long, constricted in the centre ; 

 they form short chains, or even zoogloea, and they are motile. 

 They produce the lactic acid fermentation, in the course of 

 which lactic sugar is transformed into lactic acid ; they are 



FIG 32. BACTEKIUM LACTIS. 



anaerobic. Lister, 1 by means of pure cultures, established 

 experimentally their causal relation to the lactic fermentation 

 or souring of milk. 



(c) Bacterium aceti (Mycoderma aceti) is a little smaller than 

 bacterium lactis, being about 0'0015 mm. in length, and often 

 forms chains, and also pellicles, on the surface of the fluid ; it 

 is motile. Pasteur maintains that it is the ferment of the 

 acetic acid fermentation. Cohn 2 found it in enormous masses 

 in beer that had become sour ; it forms dumb-bells, seldom 

 chains of four, and sometimes a pellicle on the surface. Pure 

 cultivations have not been made with it, and before deciding 

 whether it is the real cause of the acetic acid fermentation, 

 experiments with such pure cultures, i.e. inoculations of 

 alcoholic fluids with it, are required. 



3. Pigment bacteria. Two kinds have been described : 

 Bacterium xanthinum and Bacterium aeruginosum. 



(a) Bacterium xanthinum 3 is a bacterium, about 0*007 to 

 O'Ol mm. long, motile, single, also in dumb-bells, or short 

 chains. It produces the yellow colour of yellow milk. Its 

 pigment is soluble in water, and insoluble in alcohol or ether. 

 When introduced into boiled milk of neutral reaction, it 

 multiplies with great rapidity ; the milk coagulates after 

 twenty-four hours ; it is soon teeming with them and turns 

 yellow. The reaction of the yellow milk is at first acid, but 

 soon becomes alkaline, and the alkalinity gradually increases. 



(&) Bacterium aeruginosum. In green pus Schroeter dis- 

 covered a bacterium, Bacterium aeruginoswn* The pigment 



1 Pathological Soc. Transactions, 1878. 



2 Biol. d. Pflanzen, ii. p. 173. 



3 Schroeter, Biol. d. Pjlanzcn, ii p. 120; Vibrio tynxanthus, EhretiLerg. 



4 Lot. cit. p. 122. 



