MORPHOLOGY AND PHYSIOLOGY OF BACTERIA. 25 



which cut off injurious rays. Duclaux has investigated the same 

 subject, and observed that micrococci were more sensitive to sun- 

 light than the spore-bearing bacilli. Engelmann has described a 

 bacterium whose movements cease in the dark, and Zopf states that 

 in his cultures of Beggiatoa roseo-persicina the growth was much 

 more strongly developed on the side of the vessel facing the light. 

 Arloing, Marshall Ward, and Dieudonne have studied the effect 

 of the sun's rays on anthrax spores, and on chromogenic and 

 other bacteria, and maintain that they are bactericidal. The 

 effect is due chiefly, if not entirely, to the blue rays. 



Chemical Reagents. — Many substances, such as carbolic acid, 

 corrosive sublimate, chlorine, bromine, have a marked effect upon 

 the growth of bacteria. This will be more fully described in 

 another chapter. In several cases the bacteria themselves secrete 

 a substance which is injurious to their future development. 



Products of Growth. 



Bacteria may be grouped together according to the changes pro- 

 duced in the media in which they grow. Thus wo have pigment- 

 forming, phosphorescent, fermentative, putrefactive, nitrifying, and 

 disease-producing bacteria. 



Chromogenic or pigment-forming bacteria elaborate during their 

 growth definite colour stuffs. Such species are exemplified by Bacillus 

 violaceus, which produces a striking purple growth ; Bacillus 

 pyocyaneus, which secretes pyocyanin, a substance which has been 

 isolated and obtained in a crystalline form ; Micrococcus prodigiosus, 

 which produces a pigment allied to fuchsine ; Beggiatoa roseo-per- 

 sicina, which is characterised by the presence of bacterio-purpurin ; 

 Sarcina lutea, Bacillus cyanogenus, and many others. 



Photogenic, or light-producing, bacteria are found more especially 

 in sea-water. There are several species of phosphorescent bacilli, 

 and according to Beyrinck the best medium for their cultivation is 

 fish-broth made with sea-water. Photographs can be obtained of 

 cultures by their own light. 



Zymogenic or ferment bacteria produce their changes in non- 

 nitrogenised media. Bacterium aceti, by its growth produces the 

 acetic fermentation in wine by which alcohol taking up atmospheric 

 oxygen is converted into vinegar : — 



C^H'O + 0= = C'H^O'' + H^O. 



The fermentation of urine, by which urea is converted into carbonate 

 of ammonia, can be brought about by several micro-organisms, but 



