126 Microbes and You 



viruses, and the number of animals that can be used for such 

 studies is more hmited. Mice are susceptible to the action of some 

 viruses, rhesus monkeys to others, and ferrets to still others, but 

 many animals are completely refractory to some viruses. 



IDENTIFICATION OF PURE CULTURES 



After primary isolation procedures have assured that the cul- 

 tures so obtained are pure (only one species), the next step is to 

 subject the pure culture to an orderly sequence of morphological, 

 cultural, and physiological tests. 



Trying to determine the accepted name of an organism involves 

 an elimination procedure, each test narrowing down the possibili- 

 ties. Bergeijs Manual of Determinative Bacteriology is the stand- 

 ard reference work used in the final identification of bacteria. 



Since the number of organisms studied in a course of this nature 

 is usually not very extensive, many institutions incorporate into 

 their laboratory manuals simplified reference charts to which stu- 

 dents may refer, rather than have the beginning students labori- 

 ously thumb through bergey's manual. By employing a chart 

 similar to the one which follows, all pertinent information for the 

 organisms under investigation can be tabulated in an orderly 

 fashion for handy reference. It seems appropriate to discuss in 

 some detail the theory and the significance of some of these tests 

 commonly used in describing the growth and activities of micro- 

 organisms. This should aid the student to understand better the 

 why and the wherefore of what might appear to be hocus-pocus 

 practiced in bacteriology laboratories. 



MORPHOLOGY 

 Gram Stain 



A carefully prepared gram stain will immediately place the or- 

 ganism in question into one of two major groups— the gram posi- 

 tive bacteria or the gram negative bacteria. This is the first im- 

 portant step in the elimination process. The value of a reliable 

 gram stain cannot be over-emphasized. Much time and effort can 

 be needlessly wasted by trying to identify an organism that doesn't 



