Microbes Must Eat 79 



In brief, we autoclave all liquids except those which are thermo- 

 labile. Such fluids are usually filtered or sterilized by inter- 

 mittent selective heating over a period of days. All glassware that 

 must be dry after sterilization and all oils and greases should be 

 oven-sterilized. Glassware that need not be dry before use may be 

 autoclaved, which is a shorter process than the oven treatment. 



Chemical Methods of Sterilization 



Chemical sterilization of bacteriological media is impractical. 

 Any chemical used to free a medium of organisms will nullify the 

 usefulness of that substrate for the subsequent cultivation of cells. 

 Selected chemicals are being employed for the sterilization of 

 surgical instruments which cannot undergo the rigors of heat treat- 

 ment, but the choice of chemical and the length of the contact 

 period are prime considerations. In a later chapter the use of 

 chemicals as antiseptics and as disinfectants will be discussed, 

 pointing out the limitations as well at the strong points in favor of 

 representative chemical agents. 



PREREQUISITE V: DESIRED PHYSICAL PROPERTIES 

 Every medium blended in the laboratory has a definite use, and 

 the physical properties of that medium should help to fulfill specific 

 requirements. In studying motility of organisms the culture should 

 be grown in a liquid medium which encourages flagella to develop. 

 Organisms picked from a colony growing on a solid medium cannot 

 be expected to exhibit active motility, if they show locomotion at 

 all. In nutritional studies where development of turbidity is to be 

 the criterion of growth, it is imperative that the culture medium be 

 as clear as possible before inoculation with the test organisms. If 

 characteristics of colonies are to be studied, our microbes must be 

 cultivated on a medium containing some solidifying agent. In 

 brief, a liquid medium, a clear medium, or a solid medium is pro- 

 vided depending upon the ultimate use to which the substrate is to 

 be put. The effect of such factors as surface tension, concentration 

 of ingredients, etc., will be discussed in a later chapter. 



