MICRO-ORGANISMS 25 



tervals until a method that prolongs the life expectancy of the culture 

 is substituted. 



The type of action that is taken, at least as a preliminary, always 

 includes transfer of the culture to a fresh nutrient medium, followed 

 by incubation under conditions suitable for its normal development. 



Choice of a medium and a set of conditions for growing a new acces- 

 sion usually is not difficult. Some donors describe the medium upon 

 which they have grown the culture and some even suggest an incuba- 

 tion temperature. More often, however, a name — more or less com- 

 plete — is all the information that is with a culture when it is received. 

 Selection of a medium and conditions for growing a culture is then up 

 to the recipient. 



An experienced microbiologist can usually make a fairly good guess 

 as to whether the name on an incoming agar or broth culture is reliable 

 and, if not, which genus is most likely. With this much information 

 he can generally select a suitable medium. Most common bacteria will 

 grow on nutrient agar slants of 0.3-percent beef extract, 0.5-percent 

 peptone, 2-percent agar, and water, all adjusted to pH 7. Oxygen 

 relationships will be discovered by visual inspection. Incubation at 

 25° to 30° C, although not necessarily optimal, probably will be satis- 

 factory. 



Of course, with fresh isolates the necessary information will have 

 been acquired during isolation, purification, and identification. How- 

 ever, there are many fastidious bacteria that require different media, 

 restricted access to air, higher or lower incubation temperatures, or 

 incubation in the light. A similar situation applies to other micro- 

 organisms, where one or two media and sets of growth conditions suf- 

 fice for the growth of most types, but where special media and condi- 

 tions are needed for large numbers of less common types. 



Once the culture has been successfully cultivated on laboratory 

 media, there are several possible methods of prolonging viability. The 

 most common is to store a mature plate, slant, or broth culture in the 

 refrigerator as long as is safe from the standpoint of longevity. Re- 

 frigerated cultures of many bacteria and yeasts will remain viable for 

 several months, but it is usually recommended that no more than 3 or 

 4 months be allowed to elapse before making fresh transfers for con- 

 tinued storage. Sporulated cultures of actinomycetes and many molds 

 will also remain viable for several months if kept refrigerated. 



With many bacteria the period of storage may be extended by de- 

 pressing the cotton plug and using a rubber stopper in the tube to 

 reduce evaporation. In tubes protected only by cotton plugs a culture 

 may lose its viability because the medium dries up ; whereas if evapora- 

 tion is reduced, many cultures will continue to be viable for longer 

 periods. 



In the past such methods of maintaining stock cultures were the 

 most common, and they still are widely used. However, if there are 

 many cultures, these methods are tedious, time consuming, and there- 

 fore costly. The greatest drawback is that continued serial transfer 

 often leads to degeneration of cultures. Sometimes the degeneration 

 is quantitative, as with 2-ketogluconic acid-producing strains of Pseu- 

 domonas reptilivora Cald. Olivers. During the 2 to 3 years when 

 one of the cultures was being transferred at 3- to 4-month intervals, 

 the yield of acid declined a third. 



