_Bacilli_ are rod-shaped bacteria, usually at least twice as long as
they are broad (Fig. 4). Some multiply by fission, others by
sporulation. Some forms are motile, others are non-motile. Tuberculosis,
tetanus, anthrax, and many other surgical diseases are due to different
forms of bacilli.

_Spirilla_ are long, slender, thread-like cells, more or less spiral or
wavy. Some move by a screw-like contraction of the protoplasm, some by
flagellæ. The spirochæte associated with syphilis (Fig. 36) is the most
important member of this group.

#Conditions of Bacterial Life.#--Bacteria require for their growth and
development a suitable food-supply in the form of proteins,
carbohydrates, and salts of calcium and potassium which they break up
into simpler elements. An alkaline medium favours bacterial growth; and
moisture is a necessary condition; spores, however, can survive the want
of water for much longer periods than fully developed bacteria. The
necessity for oxygen varies in different species. Those that require
oxygen are known as _aërobic bacilli_ or _aërobes_; those that cannot
live in the presence of oxygen are spoken of as _anaërobes_. The great
majority of bacteria, however, while they prefer to have oxygen, are
able to live without it, and are called _facultative anaërobes_.

The most suitable temperature for bacterial life is from 95° to 102° F.,
roughly that of the human body. Extreme or prolonged cold paralyses but
does not kill micro-organisms. Few, however, survive being raised to a
temperature of 134½° F. Boiling for ten to twenty minutes will kill all
bacteria, and the great majority of spores. Steam applied in an
autoclave under a pressure of two atmospheres destroys even the most
resistant spores in a few minutes. Direct sunlight, electric light, or