ROUTINE TESTS FOR THE IDENTIFICATION OF BACTERIA 147 



INCUBATION 



Cultures should be incubated at or near the optimum temperature of the 

 organism or organisms under investigation. As a rule it is not necessary, 

 however, to know the exact optimum temperature of each organism. If 

 the laboratory is equipped with a series of incubators running at 20, 25, 

 30, and 37°C, the temperature requirements of practically all bacteria 

 except the thermophilic forms can be very satisfactorily met. Room 

 temperature is sometimes used in place of 25° but is not to be recom- 

 mended because of its uncontrollable variations. 



Length of incubation varies and is specified on the chart under many of 

 the tests. In cases where it is not specified, one should observe the follow- 

 ing general rule: On the day when good growth first appears, the proper 

 descriptive terms on the chart should be underlined. Any changes 

 occurring and noted in subsequent study should also be recorded on the 

 chart. The meaning of the terms given in this section of the chart will in 

 general be made clear by consulting the glossary (Chap. XIII) . 



VARIATION 



In using these methods it must be remembered that among bacteria, 

 the individual members of any species may differ from one another in 

 respect to both physiology and morphology, thus making it difficult to 

 define the limits of the species ; also that any individual culture in repeated 

 examinations may produce variable results in connection with some test 

 even when studied under apparently constant conditions. For these 

 reasons it is important that single determinations shall never be used for 

 characterizing any culture that has been studied or much less for charac- 

 terizing any species or type that is being described. Determinations must 

 be repeated at different times and under different conditions in order to 

 learn definitely the physiological characteristics of a culture. Whenever 

 possible, an effort should be made to correlate the variations in physiology 

 and serology with colony type and to list separately the physiological 

 characteristics of the "smooth," ''rough," ''mucoid," "opaque," "trans- 

 lucent" strains, etc. When an organism shows any tendency to "dissoci- 

 ate" into "phase variants," its description is incomplete if it applies to 

 only one phase or to a culture containing a mixture of two phases or more. 

 In such case the phase variants should be separated by plating methods or 

 otherwise and a separate chart should be used for each individual strain 

 studied. The individual charts may be filed for the investigator's infor- 

 mation, but it must be insisted that results of such work should not be 

 published for the use of other bacteriologists until repeated determinations 



