112 BACTERIOLOGY 



position a little inclined from the horizontal, with the neck up, 

 for twenty-four hours, so that the agar may adhere to the wall 

 of the tube. 



By the plate-method as originally devised by Koch, instead of using 

 Petri dishes, the gelatin was poured upon a sterile plate of glass. This plate 

 of glass was laid on another larger plate of glass, which formed a cover for a 

 dish of ice-water, the whole being provided with a leveling apparatus. The 

 plate was kept perfectly level until it had solidified, which took place rapidly 

 on the cold surface. The glass plates were placed on little benches enclosed 

 within a sterile chamber. The more convenient Petri dish has now displaced 

 the original glass plate. 



Streak Method of Isolating Bacteria. The isolation of bac- 

 teria may sometimes be effected by drawing a platinum wire 

 containing material to be examined rapidly over the surface of a 

 Petri dish containing solid gelatin or agar; or over the surface of 

 the slanted culture-medium in a test-tube; or by drawing it over 

 the surface of the medium in one test-tube, then, without steril- 

 izing, over the surface of another, perhaps over several in succession. 

 This method is ordinarily less reliable than the regular plating 

 method. 



Veillon's Tall-tube Method. Three to six tubes of glucose 

 agar, the agar being at least 6 cm. deep, are liquefied and cooled 

 to 45 C. in a water-bath. A small amount of the material to 

 be examined is placed in the first tube by means of the platinum 

 loop, and carefully mixed. From this dilutions are made in series 

 to tubes, 2, 3, 4, 5 and 6, each being carefully mixed without intro- 

 ducing air bubbles. The tubes are quickly solidified by immersion 

 in cold water, and are incubated at 37 C. These culture tubes 

 offer the contained bacteria a wide range of oxygen supply. 

 This is abundant at and near the top, and gradually diminishes 

 lower in the tube until near the bottom almost perfect anaerobic 

 conditions obtain. The method is very useful in isolating B. 

 bifidus from feces of infants, and in studying the oxygen require- 

 ments of other bacteria. When energetic gas-forming bacteria 

 are present in considerable number, the method is of no value. 



