800 METHODS OF INVESTIGATING BACTEEIA. 



tube is continually rotated in a horizontal position, and 

 thus as the material cools it solidifies in the form of an 

 equally thin layer over the whole wall of the tube. The 

 best way of doing this is to close the tube by means of a 

 caoutchouc cap, let it swim on cold water, and give it a 

 slight rotatory motion with the right hand, holding the 

 orifice of the tube loosely in the left, and keeping it in 

 the horizontal position. In order to count the colonies 

 the outer surface of the tube may be divided into larger 

 or smaller portions by ink lines. One great advantage 

 of this method is, that in the case of bacteria which 

 grow very slowly an opportunity is afforded for their 

 growth in these tubes, in which contamination cannot 

 occur. It is more difficult to examine accurately the 

 individual colonies, and to inoculate from them, than in 

 the case of the plate cultivations, and it is only in special 

 cases that this method would be employed when further 

 cultivations were required. 



Where it is important to obtain as complete know- 

 ledge as possible of all the species of bacteria which are 

 present, we must vary the nutrient conditions as far as 

 we can, more especially we must vary the amount of 

 sugar, the degree of alkalinity, the temperature, and the 

 amount of oxygen ; in the case of numerous bacteria the 

 conditions necessary for their artificial cultivation are 

 not as yet known, and hence it is desirable to vary the 

 Cultivation of conditions as much as possible. For cultivating the 

 bacteria anaerobic bacteria Liborius recommends deep layers of 

 nutrient agar containing 2 per cent, of dextrose. For 

 this purpose test tubes are filled to a height of about 10 

 cm. with the nutrient material, and the bacteria to be 

 investigated are mixed with this material while still fluid 

 and at a temperature of 40 C. ; the result is that isolated 

 colonies of the anaerobes grow in the deeper layers. 

 Instead of this method we may employ vessels contain- 

 ing nutrient substrata from wilich all the air has been 

 expelled by means of hydrogen. Vessels of the form 

 shown in fig. 142 are best suited for this purpose, and 

 these are filled with the nutrient agar up to the level of the 

 lateral tube; the material to be tested is then introduced 



