80 MICROBIOLOGY 



Fermentation tubes usually require a cleaning mixture be- 

 cause of the inability to use the test tube brush.* 



The above has reference to the cleaning of new or empty 

 glassware; but it is necessary to use again the glassware and 

 consequently the tubes and Petri dishes containing cultures 

 must be dealt with. The tubes, flasks and Petri dishes con- 

 taining cultures should be placed in a water bath, covered 

 with water to which a little salsoda is added and boiled for at 

 least 20 minutes. The tubes can then be emptied of their 

 contents and again boiled in clean soap and water after which 

 they are treated as in the beginning. Cultures of spore-bear- 

 ing pathogenic bacteria such as anthrax should be sterilized 

 by heating them in an autoclave at a temperature not less than 

 110 Q C. for at least a half hour in order to kill the spores. 



Culture media and their preparation. In the cultiva- 

 tion of bacteria it is necessary to have a medium or soil in 

 which they can multiply. A large number of natural sub- 

 stances and compounds are used. Like the higher plants 

 bacteria are somewhat selective in the choice of media in 

 which to grow. In their artificial cultivation it is necessary 

 therefore to simulate as nearly as possible the natural re- 

 quirements of the organism. In the preparation of media it 

 is well to keep in mind that they must contain nourishment 

 in an available form and in suitable quantities for the bac- 

 teria and that their chemical reaction must be right, as bac- 



* It is sometimes desirable to use the cleaning mixture. In this 

 case after the tubes are washed with soap and water they are filled 

 with the cleaning mixture and stood in a glass jar (aquarium). 

 After it has acted for from 10 to 20 minutes, it is poured k>ack from 

 the tubes into the bottle originally containing it. The tubes are 

 then to be thoroughly rinsed, and dried as before. 



Formula for chromic acid cleaning mixture. Dissolve 80 grams 

 of potassium dichromate (K 2 Cr 4> O ) in 300 cc. of warm water; when 

 all of the K^Cr^O^ is dissolved and the solution cooled, add to it 

 slowly with constant stirring 460 cc. of concentrated sulphuric acid. 

 A lead lined iron kettle is the best receptacle in which to mix the 

 ingredients. Store the mixture in a glass-stoppered bottle. The 

 liquid will be quite thick with small crystals. When the crystals are 

 used up me liquid should be discarded. 



