390 BACTERIAL ORIGIN OF GUMS OF ARABIN GROUP, 



older. The change from buff to green results from the alteration 

 in the reaction of the medium from neutral or faintl}/ alkaline to 

 decidedly alkaline, as is shown by pressing litmus paper against 

 parts of the under side of the plate — i.e., the agar in contact with 

 the bottom of the petri-dish. The greenish, mottled slime is 

 converted to a buff colour by the addition of a few drops of dilute 

 hydrochloric acid. 



When the culture becomes dark-coloured there is not so much 

 slime obtained; what has been formed appears to have condensed 

 and to have become less soluble in water, so that the culture forms 

 a suspension with water, while the earlier buff cultures form a 

 stiff slime under similar conditions. 



Further experiments with the tannin medium showed that the 

 slime was formed more readily by growing the bacteria at 15° — 

 the laboratory temperature at the time — than at 22°. The most 

 successful method consisted in growing the culture at 15° for 

 three days, and then scraping the slime from the plates. In 

 another two days a further quantity can be removed, and jDossibly 

 still another in two days later. The slime is acidulated with a 

 few drops of dilute hydrochloric acid and treated with alcohol, 

 when either white stringy flakes or white floccules are precipitated 

 according to the alcohol which has been added. The stringy, 

 cohesive flakes are changed to floccules by strong alcohol. The 

 alcoholic mother liquor is coloured a bright yellow from the 

 lipochromes of the bacterial cells. The floccules of slime and 

 bacteria are white, and on treatment with water swell up, forming 

 a stiff paste like that made from flour. The opalescence is caused 

 by the bacteria, and to eliminate them the paste was treated with 

 2 to 5 drops of dilute sulphuric acid, and heated in the autoclave 

 at 3 atmospheres' pressure for 15 minutes. This treatment had 

 been found very useful in separating bacteria from slime on a 

 former occasion. By this treatment a faintly opalescent, gummy 

 fluid, which could easily be separated by filtration through paper 

 from the precipitated bacteria, was obtained. The faintly opal- 

 escent fluid was easily clarified with aluminium hydrate. The 

 difference between slime and gum appears to be caused by the 



