III. Family Spirillaceae

Cells twisted spirally like a corkscrew, or representing sections of the spiral. Division only transverse to the long diameter.

1. Spirosoma

Rigid; without flagella.

2. Microspira

Rigid; having one, two, or three undulating flagella at the ends.

3. Spirillum

Rigid; having from five to twenty curved or undulating flagella at the ends.

4. Spirochoeta

Serpentine and flexible. Flagella not observed; probably swim by means of an undulating membrane.

B. Sub-order: Trichobacteria (Higher Bacteria)

IV. Family Mycobacteriaceae

Cells forming long or short cylindric filaments, often clavate-cuneate or irregular in form, and at times showing true or false branchings. No endospores, but formation of gonidia-like bodies due to segmentation of the cells. No flagella. Division at right angles to the axis of rod in filament. Filaments not surrounded by a sheath as in Chlamydo-bacteriaceae.

1. Myobacterium

Cells in their ordinary form, short cylindric rods often bent and irregularly cuneate. At times Y-shaped forms or longer filaments with true branchings may produce short coccoid elements, perhaps gonidia. (This genus includes the Corynebacterium of Lehmann-Neumann.) No flagella.

2. Actinomyes

Cells in their ordinary form as long branched filaments; growth coherent, dry, or crumpled. Produce gonidia-like bodies. Cultures generally have a moldy appearance, due to the development of aerial hyphae. No flagella.

V. Family Chlamydobacteriaceae

Forms that vary in different stages of their development, but all characterized by a surrounding sheath about both branched and unbranched threads. Division transverse to the length of the filaments.

1. Cladothrix

Characterized by pseudodichotomous branchings.

Division only transverse. Multiplication by the separation of whole branches. Transplantation by means of polar flagellated swarm-spores.

2. Crenothrix

Cells united to form unbranched threads which in the beginning divide transversely. Later the cells divide in all three directions of space. The products of final division become spheric, and serve as reproductive elements.

3. Phragmidiothrix

Cells at first united into unbranched threads.

Divide in three directions of space. Late in the development, by the growth of certain of the ceils through the delicate, closely approximated sheath, branched forms are produced.

1The spirocheeta and some closely related forms are now thought to be more properly classified among the protozoa than among the bacteria.

4. Thiothrix

Unbranched cells inclosed in a delicate sheath. Non-motile. Division in one direction of space. Cells contain sulphur grains.

II. Order: Thiobacteria (Sulphur Bacteria). I. Family Beggiatoaceae

Cells united to form threads which are not surrounded by an inclosing sheath. The septa are scarcely visible. Divide in one direction of space only. Motility accomplished through the presence of an undulating membrane. Cells contain sulphur grains. There are two families, numerous subfamilies, and thirteen genera in this order. They are all micro-organisms of the water and soil, and have no interest for the medical student.

A more common but less accurate method of classification divides bacteria into:

1. Bacillus

A rod-shaped organism that is not curved or twisted upon itself, one diameter being distinctly greater than the other.

2. Micrococcus Or Coccus

A minute spheric organism.

(a) Diplococcus, when occurring in twos.

(b) Streptococcus, when occurring in chains.

(c) Staphylococcus, when in bunches like grapes.

(d) Tetracocci, when division takes place in two directions, and the individuals remain attached in groups of four.

(e) Sarcina, when dividing in three directions, giving rise to bale-like packages.

(/) Zoöglea, when grouped in irregular masses.

3. (A) Spirillum

An organism twisted like a corkscrew and rigid; usually has polar flagellae.

(b) Spirochoeta, when the organism is long, slender, flexible, and without flagella.

(c) Vibrio - A short organism, bent like a comma, usually with a single end-flagellum.

Structure

Bacteria are composed of a small amount of cytoplasm surrounding a large but indistinctly defined nucleus. In this cytoplasm may be found granules of fat, pigment, sulphur, etc. Each cell is surrounded by a distinct cell-wall, and sometimes there is present a peculiar gelatinous material forming a capsule.