184 BACTERIOLOGY. 
What are facultative parasites ? 
Saprophytic bacteria may occasionally become parasitic and are then 
called facultative parasites. Parasitic bacteria may in turn occur as 
saprophytes and are called facultative saprophytes. 
What are necessary factors in the growth and nutrition of bacteria ? 
A certain amount of heat and moisture is absolutely essential. 
They require carbon compounds and nitrogen, which must be presented 
to them in the form of decomposable organic compounds. They 
develop best where their surroundings are neutral or slightly alkaline 
in reaction. Some varieties require oxygen, while to others its pres- 
ence is harmful. Wherever the above conditions are present these 
organisms will be found. 
What is the difference between zrobic and anzrobic bacteria ? 
The former variety can live and grow only in the presence of oxy- 
gen, while for the growth and development of the latter class oxygen 
must be absent. The majority of bacteria may be placed in a so-called 
facultative class, for they are able to develop whether oxygen is present 
or not. 
What are the products of bacteria? 
Ptomaines, pigment of different colors, gases, odors and phosphor- 
escent material. ; 
What are ptomaines ? 
They are substances of peculiar composition excreted by some forms 
of bacteria. They are extremely poisonous, as larger animals may be 
killed by an infinitesimal amount of this toxic material. It is to the 
action of ptomaines that the injurious effects of the pathogenic bacteria 
are thought to depend. 
What colors may be produced by growing bacteria ? 
The color-producing bacteria, or, as they are called, the chromogenic 
bacteria, are most of them harmless. The colors produced may be of 
all shades, black, blue, white, orange, red, yellow, brown, green, etc. 
Some forms possess the property of producing a phosphorescent glow 
of considerable distinctness in the dark. Just how these colors and 
this phosphorescence are produced is not well understood. 
What may be said of the gases produced by bacteria ? 
When a solid culture medium is employed, certain kinds of bacteria 
growing in its substance possess the power to generate gases which 
