BACTERIA OCCURRING IN SEWACxE 65 



sewage, are, though exceedingly active, comparatively few. 

 The facultative anaerobes, on the other hand — those that can 

 live either with or without oxygen — are much more numerous, 

 as being the ones most suited to a liquid which contains little 

 or no oxygen, but may at any time become oxygenated. Thus 

 yeast, which was classed by Pasteur as " both an aerobian 

 and an anaerobian " — i.e., as facultatively anaerobic — when in 

 presence of excess of oxygen multiplies vigorously, but does 

 not act as a ferment ; whereas in sugar solutions containing 

 no oxygen it multiplies with less activity, but the fermentive 

 character is most marked, the yeast attacking the sugar, and 

 obtaining any oxygen it requires from it or from the water 

 present. Boussingault found that normal fermentation could 

 be carried on in vacuo, and was greatly promoted by removing 

 the carbonic acid and alcohol as fast as they were formed, and 

 thus preventing their retarding action. In the same way with 

 bacteria a better result is attained when the liquid products 

 are continuously removed, as in the bacterial tanks of Cameron 

 and Moncrieff and the nitrifying trays of the latter, than where 

 periods of stagnation occur, as in the intermittent system. 



In order to preserve pure cultures and to identify growths 

 obtained in the different plates, as soon as the colony is 

 sufficiently developed it is carefully examined under a low 

 power with the microscope, and minute portions transferred 

 with a sterilized platinum needle to various culture media, and 

 the development of these subcultures noted from time to time. 



3. Streak Cultures. — A tube of melted gelatine or agar is 

 allowed to solidify in a slanting position so as to expose a long 

 surface ; the tube is then inverted, the cotton-wool plug care- 

 fully removed, and the surface of the jelly lightly scratched 

 with the infected platinum wire. The plug is then singed and 

 quickly replaced. Streak cultures are specially adapted for the 

 development of pigments, which generally require free access of 

 air for their production. 



4. Stah Cultures. — The tube is held horizontally, the inocu- 

 lated wire plunged steadily nearly to the bottom, withdrawn, 

 and the cotton-wool plug replaced. Certain ramifying growths 

 show themselves better under this method, and, moreover, the 

 occurrence of a growth in the deep layers will often reveal the 

 presence of facultative anaerobic organisms, which can after- 

 wards be dealt with. 



5. Shake Cultures. — The fluid gelatine or agar is inoculated 



