CULTIVATION OF MICROORGANISMS 35 



('ommercial extract of beef may be used in place of fresh 

 meat, and is widely employed on account of its convenience. 

 Suuars of various kinds, glycerin, and other substances may 

 be added to the broth to adapt it to the needs of specific 

 organisms. This simple culture 'medium will permit the 

 LiTowth of the great majority of organisms in which the 

 bacteriologist is interested. 



Milk, which contains a mixture of sugars, proteins, and 

 mineral substances, is admirably adapted to the cultivation 

 of bacteria. 



Materials that are naturally solid or that are solidified 

 by heat are used as media. Prominent among these are 

 slices of various vegetables. Potato is most commonly used, 

 as are the mixture of the white and yolk of the egg, and 

 blood scrum. 



Liquefiable solid media. It is necessary to have a me- 

 dium that is liquid at high temperatures and that becomes 

 solid on cooling for purposes of isolation, as will be ex- 

 plained later. Such a medium can be obtained by the ad- 

 dition of 10 per cent, of gelatin to the beef broth. This 

 medium will remain solid up to 77 F. Since some bacteria 

 do not grow at such low temperatures, gelatin can not be 

 used for their cultivation. Certain bacteria have the power 

 of digesting or liquefying it, an advantage or a disadvan- 

 tage, depending on the service the medium is to yield. 



The broth may also be made into a liquefiable-solid me- 

 dium by the addition of from 1 to 1.5 per cent, of agar, a 

 substance obtained from certain seaweeds found in Japan 

 and China. The medium to which the agar is added ex- 

 hibits the peculiar property of melting at 98 C. (208 F.), 

 but of not solidifying until it is cooled to 38 C. (100 P.). 

 This enables the bacteria to be mixed with it while it is at 

 a temperature that will not injure them, and for the cul- 

 tures to be kept at any desired temperature without becom- 



