144 MANUAL OF MICROBIOLOGICAL METHODS 



transfers. Such adaptations may be necessary to permit a study of an 

 unknown culture in terms of previously described cultures. 



After these growth peculiarities are determined, it is possible to proceed 

 with the study of an organism under optimum conditions. Space is left 

 on the chart under all the procedures listed where the medium used and 

 the temperature of incubation can be recorded. As far as possible the 

 same uniform set of conditions should be used throughout the entire study 

 of one organism. If, for example, one set of tests is made on solid media 

 at 25°C, the other tests should be made likewise. Leaving out those 

 organisms referred to above which require special conditions of study and 

 other organisms of ''special" growth requirements, such as the thermo- 

 philic bacteria, there are four different sets of conditions that will suit 

 practically all bacteria, namely, liquid media at 37°, solid media at 37°, 

 liquid media at 21-25°, and solid media at 21-25°. 



Space is provided on the Standard Chart for recording optimum 

 medium and temperature. This does not ordinarily mean that one must 

 determine the one best medium for the growth of the culture or the exact 

 degree of temperature at which it grows most rapidly. In the first blank 

 one may record such terms as ''organic, solid"; "organic, liquid"; 

 "inorganic, solid"; etc., unless it be known that there is one particular 

 medium specially adapted to the organism in question. Under the 

 second blank one may record temperature in general terms, as: "20-25°," 

 "35-40°," "45-50°," or "over 55°." 



It is also important to remember that certain organisms (frequently 

 facultative anaerobes) which do not grow in either solid or true liquid 

 media will grow in a "semisolid" medium (that is, a nutrient solution in 

 which 0.05-0.1 per cent of agar has been dissolved). It is, of course, 

 important that such organisms be studied under optimum conditions, and 

 for their study the procedures given in this manual should ordinarily be 

 modified by using media containing 0.05-0.1 per cent of agar instead of 

 the usual liquid or solid media. 



Thermal death point, as called for under "Temperature Relations" on 

 the back of the chart, is undoubtedly best determined with the use of 

 capillary tubes. Short pieces of thin-walled tubing having an internal 

 diameter of 1-1.5 mm are filled with the culture (consisting mostly of 

 spores if it is a sporeformer) and are heated for varying periods of time at 

 the temperatures under investigation. After heating, each tube is broken 

 into a tube of a medium in which the organisms grow well. A tabulation 

 of results gives a good idea of the thermal death point. This procedure 

 requires careful attention to detail, and one should consult the description 

 of it by Magoon (1926). Results are most valuable if the length of time 

 before death is recorded, in which case, this becomes a test for thermal 

 death time. 



