226 LABORATORY EXERCISES IN BACTERIOLOGY. 



the precise relationship of any bacterium to a given standard culture medium must 

 in the study of these chemical phenomena be recorded to give any observations their 

 full value for the classification of the organism. 



1. Pigment Production. Various pigments, probably analogous to the coloring 

 principles of higher plants, are developed by a number of bacteria (known as chromo- 

 genic bacteria). These coloring substances are in same instances probably retained 

 within the bacterial body ; in others, are diffused more or less in the medium of growth. 

 They are of varied hues yellow, red, blue, violet, black, green, fluorescent (reflecting 

 colors different from the hue of the culture itself), or iridescent (manifesting a play 

 of colors as of a rainbow). Little is known of these pigments other than the few pecu : 

 liarities mentioned and their solubility in different reagents, as water, alkaline or 

 acid solutions, carbon disulphide, alcohol, ether or chloroform. They are of complex 

 organic composition, iron being probably a more or less important factor in most, 

 its absolute absence from the medium of growth being usually followed by failure 

 of chromogenesis. They are best seen in cultures grown in media containing carbo- 

 hydrates, as potato. In the ordinary bacteria the coloring matter is not visible in 

 the individual cells as examined with the microscope ; there are a few instances, ap- 

 parently approaching the lower algae, in which a slight green tint is to be noticed, 

 perhaps of the nature of chlorophyll ; and in the thiobacteria and rhodobacteria, granules 

 of a reddish or violet color are found (sulphur or bacterio-purpurin) . 



Note. Here should be demonstrated a number of the common types of chromogeris 

 in gross colony. 



Exercise 56. Grow Bacillus prodigiosus upon potato at room tempera- 

 ture, and another similar culture at 37 C. Note the red color of the first 

 and the absence of color from the second. Likewise plant this organism 

 on agar free from sugar and carry it through several generations to observe 

 loss of chromogenesis. 



Exercise 57. Plant Pseudomonas pyocyanea upon two tubes of agar. 

 Close one of these tubes with rubber stopper (or put in anaerobic jar), 

 leaving the other protected only by the cotton plug. Grow in incubator 

 and compare appearance of growth at close of each twenty-four hours for 

 several days. 



Pour into the second tube, showing a beautiful blue-green color diffused 

 through the agar, a little chloroform ; allow it to stand for some minutes 

 and observe the solution in the fluid of the blue color (pyocyaniri). In a 

 potato tube of the same organism note the yellowish-brown color of the 

 culture. Press upon the surface of the growth with the sterilized platinum 

 needle, and observe that in a few minutes a green color has succeeded the 

 brown (chameleon reaction of Pseudomonas pyocyanea}, later fading back to 

 the original brown tint. 



2. Photogenic Power. The phenomenon of light production belongs to a small 

 group of bacteria and is apparently a characteristic of the vital activities of the germs 

 rather than due to any chemical product of the organisms. The light is of course 

 not visible in the lighted room, but is apparent at night or in a darkened room ; and 



