Cultivation and Identification of Bacteria 135 



diagnostic test. A typical strain of Escherichia coli, therefore, is 



IMViC -{ — I , while a typical Aerohacter aerogenes is IMViC 



1--|-. There are fourteen other combinations of these four 



reactions, and organisms exhibiting these in-between reactions are 

 called intermediates. The significance of these intermediates will 

 be discussed in the chapter on water. 



Urea Test 



Many organisms have the power of converting urea (the most 

 prominent nitrogen compound present in urine) to ammonium 

 carbonate and finally to free ammonia and carbon dioxide. The 

 enzyme capable of attacking urea is urease. This reaction is an 

 important one in the soil where fertility is dependent to a large 

 measure on microbial activity. By growing bacteria in a highly 

 buffered urea broth with phenol red as a pH indicator, if urease 

 is produced by the test organisms, the liberated ammonia will raise 

 the pH of the medium to the point where the indicator will turn 

 deep red— a positive urea test. In studying the gram negative rods 

 we find, conveniently enough, that of the fermenting organisms 

 only the Proteus genus is capable of giving a positive urea test, 

 and this is an important physiological reaction employed in screen- 

 ing cultures of Proteus (usually considered to be non-pathogenic) 

 from pathogenic gram negative rods. 



SEROLOGY 



Even after running through the usual morphological, cultural, 

 and physiological reactions, some closely related organisms cannot 

 be separated from each other unless serological tests are employed. 

 This situation would not ordinarily exist in an introductory course 

 in microbiology the way it might in advanced pathogenic bac- 

 teriology. Serology is a study of the test tube (in vitro) reactions 

 using blood serum containing antibodies in an attempt to deter- 

 mine the nature of some chemical components in the cell. Prac- 

 tical applications of serology will be considered in the discussion of 

 blood types and blood groupings later in the book, but serology is 



