130 Microbes and You 



on these media may exhibit characteristic cultural appearances, 

 some of which are of diagnostic significance to a trained eye. 



PHYSIOLOGY 



In simple terms, physiology involves the enzyme systems pos- 

 sessed by bacteria and the effects these enzymes have on the sub- 

 strates employed for the cultivation of these organisms. The re- 

 actions to be discussed represent only a few of the fundamental 

 considerations in the identification of bacteria. 



Fermentation 



In advanced courses in microbiology it is not uncommon to 

 study the fermentation of a dozen or more substances. Two sugars 

 you can expect to find in all such lists are glucose (also known as 

 dextrose) and lactose (milk sugar). Glucose is a simple sugar— 

 a monosaccharide, while lactose is more complex— a disaccharide. 



Different species of bacteria attack some sugars and not others, 

 and the type of physiological reaction produced is also variable. 

 The fermentation process may produce, among other things, vari- 

 ous acids, and to detect the presence of acid it is customary to in- 

 corporate a pH indicator dye in the broth to which the sugar has 

 been added. The indicator chosen varies from one laboratory to 

 another depending upon individual preferences. Brom thymol 

 blue, the same indicator employed in the pH adjustment of stand- 

 ard nutrient agar, is commonly used in fermentation studies. 

 Andrade's indicator is another. As the growing organisms attack 

 the sugar in the broth, the acid produced depresses the pH level 

 to the point where color changes are brought about in the dye 

 indicators. 



In addition to acids formed, various gases, notably carbon 

 dioxide and hydrogen, may be evolved. A gas trap is included in 

 these fermentation tubes by placing a small test tube, or vial, in an 

 inverted position within the tube containing the sugar broth. Gas 

 being liberated during the breakdown of the sugar rises in the 



