144-10 PURE CULTURE STUDY OF BACTERIA 



phases, due to the fact that it was first studied in an unstable form 

 and dissociation was taking place during the course of the study. On 

 the other hand, some of the methods employed in the hopes of induc- 

 ing phase variation may actually cause contamination and be in- 

 correctly interpreted. Some of these points are very adequately dis- 

 cussed by Frobisher (1933). 



The third source of error above mentioned (variation in methods) 

 also needs emphasis. When a species is described in such terms as 

 one frequently encounters in published descriptions — e.g. "Produces 

 acid (without gas) from glucose and lactose but not from sucrose; 

 does not reduce nitrates" — one has to guess at the answers to such 

 questions as these: What basal medium was used in each instance.'' 

 What indicator of acid production was employed? How thorough a 

 study was made to show the absence of any acid from sucrose, or of 

 any reduction of nitrate .f* Or, in the latter instance, is it safe to as- 

 sume that the author of the species merely failed to find nitrite in 

 some nitrate medium? Unless such questions are answered cor- 

 rectly, the description is meaningless, the attempt to identify an un- 

 known culture with such a description may well give misleading 

 results. 



With all these pitfalls to avoid, it is easy to see how the same set of 

 data, no matter how carefully prepared, can be differently interpreted 

 by two different bacteriologists. As a result extreme caution is urged, 

 both in determining the identity of a culture and in deciding whether 

 or not to pronounce it a new species. 



Practical Hints 



Determining the characteristics of a culture: One should always, if 

 possible, make a complete study of a culture promptly after its first 

 isolation while it is in vigorous condition. When a culture has be- 

 come attenuated in the laboratory, it should be restored to vigor by 

 growth under conditions well suited for its invigoration. When this 

 is done, however, the possibility should always be recognized that 

 by such "invigoration" dissociation may be induced so that the 

 phase subsequently studied may be quite different from the original 

 isolation. Whenever distinct evidence of dissociation is observed, 

 each phase should be studied and recorded separately; and efforts 

 should be made to reverse the change or to obtain the same change 

 with other strains until the possibility of impure cultures seems to be 

 out of the question. No importance should ever be attached to a 

 single determination, unless supported by a duplicate or even by 

 triplicates giving the same results. In describing morphology, one 



