Methods of Determination of Pathogenic Microbes. 135 



Physical and chemical characters of cultures. 



Physical characters. — When germs proliferate in 

 artificial media their cultures assume characters 

 which vary with the nature of these germs and with 

 the media. 



In cultures on gelatin plates the colonies assume 

 diflerent aspects. Sometimes these colonies are rep- 

 are that the heating be done carefully in order to avoid breakage 

 and that the whole extent of the tube be suiEciently heated to 

 prevent condensation of the alcohol vapor. As soon as the alco- 

 hol is completely volatilized the open end of the tube is sealed ; an 

 abrupt curve near the point insures its breaking at the desired 

 place when the tubes are to be filled. A large number of these 

 tubes can be made at one time and kept in stock. In making 

 bouillon cultures the fluid contained in test-tubes, or (for this 

 purpose) better, in small homeopathic vials, is boiled, allowed to 

 cool sufficiently, and inoculated with the material from which the 

 culture is to be made. The point of the anaerobic tube, after 

 quick flaming, is broken off under the fluid by contact with the bot- 

 tom of the vial. When the tubes have been properly made, only a 

 very small bubble of air should be included with the fluid w^hich 

 rushes in. The point is then sealed in the flame, during which 

 process a drop or two of the bouillon is necessarily expelled. By 

 the use of gelatin or the agar and gelatin mixture, and proper di- 

 lution in the usual way, these tubes are quite well adapted for 

 isolation of species. The colonies which develop in the substance 

 of the solid medium never become large nor show as character- 

 istic appearances as by the plate method, but they remain isolated 

 and admit of pure cultures being obtained from them. In exam- 

 ining with the microscope or making sowings from these colonies, 

 I break the tube near the middle, quickly flame one of the seg- 

 ments, and carefully heat it at the closed extremity ; the cylinder 

 of solid nutrient medium is slowly expelled and received in a 

 sterilized Petri dish, from which the " fishing" can be performed 

 in the usual way. In tubes prepared in this way strictly anaero- 

 bic species can be cultivated, while aerobic species, such as the 

 hay bacillus, refuse to grow. — D.] 



