6 MANUAL OF MICROBIOLOGICAL METHODS 



1953-4 M. J. Pelczar,! R. C. Bard, G. W. Burnett, H. J. Conn, E. E. Evans M. W. 



Jennison, H. C. Lichstein, L. S. McClung, A. P. McKee, A. J. Riker, J. 



Warren, O. B. Weeks, F. A. Weiss. 

 1955-7 M. J. Pelczar,! R. C. Bard, G. W. Burnett, H. J. Conn, R. D. DeMoss, 



E. E. Evans, M. W. Jennison, A. P. McKee, A. J. Riker, J. Warren, O. B. 



Weeks, F. A. Weiss. 



GENERAL CONSIDERATIONS 



Pitfalls to Be Avoided by the Student 



In studying microbial cultures with the object of identifying them or 

 describing them, the student is apt to run into certain pitfalls. Some of 

 these apply specifically to certain types of work and are therefore best 

 taken up in the various chapters of this book where they seem properly to 

 fit. Others are more general; some, in fact, are well known even to 

 beginning students in bacteriology. However, as others are less fully 

 appreciated, a few words concerning some of these pitfalls seem called 

 for here — even at the risk of repeating cautions that may seem too ele- 

 mentary. These pitfalls arise primarily from three sources: (1) the 

 danger of impure cultures, (2) confusing results because of variation of 

 bacterial species, (3) differences in methods of study. 



The danger in impure cultures is, of course, thoroughly understood. 

 Unfortunately, however, the second consideration just mentioned makes 

 it more important to emphasize the danger of impure cultures today than 

 was the case before 1920. In those days bacteriologists quite generally 

 accepted the idea of monomorphism, and whenever a culture was observed 

 to be noticeably abnormal in either morphology or physiology, it was 

 promptly discarded as a contaminant. When, however, it began to be 

 learned that even the most strictly guarded pure cultures might show 

 changes in morphology during their life history, and then later when it 

 was realized that the same organism might occur in two or more phases 

 showing distinctly different cultural and physiological characteristics, the 

 old ideas of monomorphism were decidedly upset. As a result of the 

 changing point of view, it is very easy for a careless student today to 

 believe that he is observing two phases of the same pure culture when, 

 actually, one of his ''phases" is a contaminant. This makes constant 

 checking as to purity of cultures even more important than it was before 

 dissociation into phase variants was generally accepted by bacteriologists. 



Accepting the idea of dissociation presents other difficulties to the 

 student. Without exhaustive study, it is sometimes very easy to 

 describe two phases of the same species as though they were different 

 organisms. It is also easy to prepare a description of some culture w^hich 

 is an illogical jumble of the characteristics of two or more phases, due to 



* Chairman,, 



