BIOLOGY OF BACTERIA. 45

favorable or detrimental to their existence becomes a
necessity.

Conditions influencing1 the Growth of Bacteria.—

(a) Oxygen.—The majority of bacteria grow best when
"exposed to the air. Some develop better when the air is
withheld; some will not grow at all where the least
amount of oxygen is present. Because of these pecu-
liarities bacteria are divisible into the

Aerobic bacteria, those growing in oxygen.

Anaerobic bacteria, those not growing in the presence
of oxygen.

As, however, some of the aerobic forms will grow
almost as well without as with oxygen, the term optional
(facultative) anaerobics has been applied to the special
class made to include them.

As examples of strictly aerobic bacteria the Bacillus
subtilis and the Bacillus aerophilus may be given. These
forms will not grow if oxygen is denied them. The
staphylococci of suppuration and the bacilli of typhoid
fever, pneumonia, and anthrax, as well as the spirillum
of cholera, will grow almost equally well with or with-
out oxygen, and hence belong to the optional anaerobics.
The bacillus of tetanus and of malignant edema, and the
n on-pathogenic forms, the Bacillus butyricus, Bacillus
muscoides, and Bacillus polypiformis, will not develop
at all where any oxygen is present, and hence are
strictly anaerobic.

(K) Nutriment.—The bacteria do not seem able to derive
their nourishment from purely inorganic matter. Pros-
kauer and Beck, however, have succeeded in growing the
tubercle bacillus in a mixture containing ammonium
carbonate 0.35 per cent, potassium phosphate 0.15 per
cent, magnesium sulphate 0.25 per cent., glycerin 1.5
per cent. They grow best where diffusible albumins are
present. The ammonium salts are rather less fitted to
support them than their organic compounds. The in-
dividual bacterium varies very widely in the nutriment
which it requires. Some of the water-microbes can live