CH. vi.] MORPHOLOGY OF BACTERIA. 55 



Engelmann 1 describes a bacterium photometricum, the 

 motility of which directly depends on light ; it ceases in 

 the dark, Duclaux found that exposure to direct sunlight 

 injures the life and growth of some bacteria, both septic and 

 pathogenic. 



Some bacteria require free access of oxygen, and are 

 called aerobic (Pasteur) ; others grow without free oxygen, 

 and are anaerobic (Pasteur). All require for their growth 

 certain nourishing material containing carbon and nitrogen. 

 Water is an essential element for them, and a certain temper- 

 ature is in many instances a stimulant of their growth. Most 

 pathogenic bacteria require for their propagation a temperature 

 varying in the different cases between 18 and 40 C. The 

 bacteria obtain their nitrogen from organic compounds 

 some are capable of obtaining it from compounds as simple 

 as ammonium tartrate ; others, especially pathogenic organ- 

 isms, require much more complex combinations, such as 

 occur in the animal body. Carbon they obtain likewise 

 from organic compounds, such as carbohydrates, amongst 

 which sugar is the chief, and vegetable acids combined as 

 salts are also to be mentioned. It is essential for all that 

 certain inorganic salts, phosphates, potassium and sodium 

 salts, should be present, since their own substance contains a 

 large percentage of it 4 to 6 per cent. 



While all are capable of disintegrating organic combina- 

 tions containing nitrogen, they in their turn help to produce 

 certain chemical products, which in some cases are definite 

 for a definite species (see below). Such is the case with the 

 various bacteria connected with the fermentations producing 

 lactic acid, butyric acid, and acids belonging to the aromatic 

 series. On many bacteria connected with putrefaction, and 

 also on some pathogenic organisms, these chemical products 

 1 Unters. aus. d. physiol. Labor, Utrecht, 1882. 



