EFFECT OF OXYGEN. 141 
which the cultures were grown and the use of silk 
threads or cover-glasses. In all these experiments, of 
course, it should be previously determined that in 
spore-bearing species there are no spores present. 
Behavior Toward Oxygen and Other Gases. As already 
noted under the nutritious substances required by bac- 
teria, it is customary to divide bacteria into three classes, 
according to their behavior toward oxygen. 
1. Aerobic Bacteria. Growth only in the presence of 
oxygen; the slightest restriction of air inhibits devel- 
opment. Spore-formation especially requires the free 
admission of air. 
2. Anaerobic Bacteria. Growth and spore-formation 
only in the total exclusion of oxygen. Among this class 
of bacteria are the bacillus of malignant cedema, the 
tetanus bacillus, the bacillus of symptomatic anthrax, 
and many soil bacteria. Exposed to the action of oxy- 
gen, the vegetative forms of these bacteria are readily 
destroyed; these spores, on the contrary, are very re- 
sistant. Anaérobic bacteria being deprived of oxygen— 
the chief source of energy supplied to the aérobic spe- 
cies, by which they oxidize the nutritive substances in 
the culture media—they are dependent for their nutri- 
tion upon decomposable substances, such as grape- 
sugar, which on separating into two smaller molecules 
—alcohol and carbonic acid—give out energy or heat. 
Anaérobic bacteria, therefore, require for their cultiva- 
tion, as a rule, media containing 1 to 2 per cent. of 
glucose or some equivalent. 
3. Facultative Aerobic and Facultative Anaerobic Bac- 
teria. The greater number of aérobic bacteria, including 
most of the pathogenic species, are capable of withstand- 
ing, without being seriously affected, some restriction 
