178 Microbes and You 



certain seasons of the year, particularly summer, appear to favor 

 the activities of these curious species. 



All three morphological types of bacteria— rods, spheres, and 

 spirals— have been found to possess photogenic properties. A great 

 deal of pure research on this interesting phenomenon needs to be 

 done, and for a good review of the topic, interested students are 

 referred to the book. Living Light, by Harvey ( 1940). 



CHROMOGENESIS {PIGMENT PRODUCTION) 



Most bacterial colonies are white, gray, or nearly transparent, 

 but a few organisms produce pigments with colors extending over 

 the entire range of the visible spectrum. Some of these pigments 

 are confined within the cell (intracellular) and others are secreted 

 through the cell wall into the surrounding medium (extracellular). 

 Chemical analyses have shown that these pigments are similar, if 

 not identical, with those found in flowers and in vegetables. 

 Yellow and shades of yellow, followed in descending numbers by 

 reds, blues, violets, greens, browns, and blacks are the colors found 

 among microbes. 



With few exceptions, such as the pigments in the green and 

 purple sulfur bacteria, pigments serve no known useful function 

 for the organisms. In fact, they are generally considered to be 

 waste products of metabolism. Bacteriochlorophyll, while closely 

 related to the chlorophyll of higher plants, is not identical with this 

 vital material which higher green plants depend upon for their 

 survival. 



The solubility of bacterial pigments is the basis for schemes 

 of classification, including the following: 



1. Those pigments soluble in water. (Including the pigment 

 produced by Pseudomonas aeruginosa, the etiological agent in 

 green pus. ) 



2. Those pigments soluble in alcohol but not in water. (This in- 

 cludes most bacterial pigments, and Serratia marcescens is an 

 example of such a chromogenic species. ) 



3. Those pigments insoluble in either alcohol or water. {Micro- 



