240 BACTERIAL ORIGIN OF GUMS OF ARABIN GROUP, 



Groivth upon saccharose-potato-agar. — It will be remembered 

 that the slime of the arabin bacteria Bad. acacice and Bact. 

 metarahinum was first obtained in quantity upon a medium made 

 with saccharose, tannin and potato-extract. At a later date, 

 when the extract had been obtained from a second lot of potatoes, 

 the bacteria refused to produce the slime. That this was 

 probably not due to a deterioration of the gum-forming faculty 

 of the bacteria was inferred from the formation of a galactan 

 slime of Bact. sacchari upon the old potato medium after this 

 organism had been subcultivated in the laboratory for over a year. 

 The potato-extract was apparently at fault, and experiments which 

 were made showed that when a smaller quantity of the extract 

 was used in the agar medium the bacteria again produced their 

 slimes. At a still later date a third lot of potatoes were bought 

 and the extract prepared. Upon media prepared with this 

 extract no slime could be obtained, and it made no difference how 

 much the raw juice was diluted. I was of the opinion that the 

 behaviour of the second lot of potatoes might have been due to 

 the fact that they were new, the first quantity having been 

 obtained in the winter and the second in the spring. But as the 

 third purchase made was in the following winter, and to all 

 appearance was similar to the first, the quality as determined 

 by winter and spring tubers could not explain the difference in 

 the slime-forming effect. In Australia there is no great difference 

 between potatoes, such as there is in the British Isles, for the climate 

 forbids their storage, and they reach the Sydney market during 

 the year from localities at different elevations and in different 

 latitudes. 



A number of experiments were made with the third potato- 

 extract medium, such as varying the tannin and the acidity, but 

 to no purpose. However, about this time the infl,uence of 

 dextrose in depressing the slime-forming faculty was discovered, 

 and it appeared to be extremely probable that an excess of 

 dextrose in the extract or juice was the real reason for the non- 

 production of gum by the bacteria. 



