132 Microbes and You 



Methylene Blue Milk 



Methylene blue dye imparts a robin's egg blue color to milk. 

 In contrast to brom cresol purple, brom thymol blue, and other 

 indicators, methylene blue provides no information about pH. 

 But this dye does respond to different levels of oxygen tension. 

 If methylene blue milk has its dissolved oxygen removed, either 

 mechanically with the aid of a vacuum pump or biologically by 

 organisms which produce the enzyme reductase, the indicator will 

 change from blue to its white form. The latter is termed leuco 

 methylene blue. 



The color change with methylene blue dye is a reversible one. 

 That is, if we shake a tube of reduced methylene blue milk and 

 reincorporate oxygen, the milk will revert to its former blue color 

 and will remain blue until the critical low oxygen level is once 

 more attained, at which time the blue color will fade into the leuco 

 form. 



Gelatin Liquefaction 



While nutrient gelatin is not a satisfactorv medium for routine 

 streak platings because of the disadvantage outlined in the chapter 

 on media making, gelatin does serve a useful function in the study 

 of physiological reactions of organisms. If the bacteria secrete 

 the enzyme gelatinase, this digestive enzyme can alter gelatin to 

 the extent that even though the medium is chilled below its nor- 

 mal solidifying point (about 23° C), the gelatin will remain liquid. 

 Since some organisms produce the enzyme gelatinase and others do 

 not, we have one more reaction in the battery of tests employed to 

 identify microbial species. The speed and the degree of liquifac- 

 tion are other variables between organisms. When a bacteriolo- 

 gist refers to a physiological reaction as being rapid, moderate, or 

 slow, this usuallv means the reaction takes place in one day, two 

 days, or longer than two days, respectively. 



Indole Reaction 



This test is based upon the ability of certain microorganisms to 

 produce an enzyme capable of attacking a substance called trijpto- 



