#General Characters of Bacteria.#—­A bacterium consists of a finely granular mass of protoplasm, enclosed in a thin gelatinous envelope.  Many forms are motile—­some in virtue of fine thread-like flagella, and others through contractility of the protoplasm.  The great majority multiply by simple fission, each parent cell giving rise to two daughter cells, and this process goes on with extraordinary rapidity.  Other varieties, particularly bacilli, are propagated by the formation of spores.  A spore is a minute mass of protoplasm surrounded by a dense, tough membrane, developed in the interior of the parent cell.  Spores are remarkable for their tenacity of life, and for the resistance they offer to the action of heat and chemical germicides.

Bacteria are most conveniently classified according to their shape.  Thus we recognise (1) those that are globular—­cocci; (2) those that resemble a rod—­bacilli; (3) the spiral or wavy forms—­spirilla.

Cocci or micrococci are minute round bodies, averaging about 1 [micron] in diameter.  The great majority are non-motile.  They multiply by fission; and when they divide in such a way that the resulting cells remain in pairs, are called diplococci, of which the bacteria of gonorrhoea and pneumonia are examples (Fig. 5).  When they divide irregularly, and form grape-like bunches, they are known as staphylococci, and to this variety the commonest pyogenic or pus-forming organisms belong (Fig. 2).  When division takes place only in one axis, so that long chains are formed, the term streptococcus is applied (Fig. 3).  Streptococci are met with in erysipelas and various other inflammatory and suppurative processes of a spreading character.

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 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.