hers 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
flagellae. The spirochaete 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 _aerobic bacilli_ or _aerobes_; those that cannot
live in the presence of oxygen are spoken of as _anaerobes_. The great
majority of bacteria, however, while they prefer to have oxygen, are
able to live without it, and are called _facultative anaerobes_.

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 134.5 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
even diffuse daylight, is inimical to the growth of bacteria, as are
also Rontgen rays and radium emanations.

#Pathogenic Properties of Bacteria.#--We are now only concerned with
pathogenic bacteria--that is, bacteria capable of producing disease in
the human subject. This capacity depends upon two sets of factors--(1)
certain features peculiar to the invading bacteria, and (2) others
peculiar to the host. Many bacteria have only the power of living upon
dead matter, and are known as _saphrophytes_. Such as do nourish in
living tissue are, by distinction, known as _parasites_. The power a
given parasitic micro-organism has of multiplying in the body and giving
rise to disease is spoken of as its _virulence_, and this varies not
only with different species, but in the same spec