Effects of Bacterial Growth on the Environment 179 



coccus citreus is an example of an organism producing this type 

 of pigment.) 



Chromogenesis occurs only under specific conditions of the 

 environment, such as the presence of specific constituents in the 

 medium, pH of the substrate, incubation temperature, light in- 

 tensity, oxygen supply, etc. This last factor is particularly im- 

 portant, and its effect can be demonstrated by examining colonies 

 growing in the depths of a solid medium and comparing them with 

 surface colonies on the same culture plate. Best pigment produc- 

 tion occurs when there is an abundant supply of oxygen. 



Pigments vary in amount and in intensity of color even within 

 a given species; variations (temporary changes) and mutations 

 ( permanent changes ) may be demonstrated. The amount of color- 

 ing matter within a single cell is too minute to be detected under 

 the microscope, but masses of cells, especially surface colonies on 

 culture plates, provide enough pigment to be seen with the unaided 

 eye. Pigmentation often does not occur until a culture is in the 

 latter stages of growth, which is further evidence to some workers 

 that pigments are accumulations of waste products. 



The "bloody bread" of Biblical times was undoubtedly caused 

 by Serratia marcescens, the red pigment-producing bacterium, 

 growing on the sacrament wafers. Blue milk, red milk, and other 

 pigments in dairv products are the result of microbial growth on 

 these media, and they can be very troublesome. 



A pigment called cytochrome has been found to be present in 

 most, if not all, living cells, and it functions in respiration. Cyto- 

 chrome, however, is not a pigment in the same sense as the color- 

 ing matter in cells described above. Unless cytochrome is con- 

 centrated, its color is not detectable. In other words, cytochrome 

 does not cause bacterial colonies to appear pigmented. 



The red-purple coloring matter of a select group of bacteria 

 allows the organisms to assimilate carbon dioxide with the aid of 

 sunlight. This pigment is bacteriopurpurin, and it provides a 

 type of metabolism unique among bacteria which ordinarily do not 

 thrive in the presence of sunlight. 



