These organisms are of interest in that they may be parasitic upon and within the body of man, of the lower animals, and other plants. The less important ones will be presented first, the bacteria being discussed more fully later; these latter being especially important on account of their relation to disease and their bearing upon general hygiene and preventive medicine.

The Yeasts

The yeasts, blastomycetes or saccharomycetes, are unicellular fungi which multiply by budding-, in which naked asci (spore cases) are formed freely on the mycelium. The yeast cell is, as a rule, oval, but among the wild-yeasts, or torulae, spheric forms are common. Great variations occur in size, yeasts measuring usually from 10 to 20 in length, with a width of about one-half or two-thirds of the long diameter. They possess a well-defined, doubly contoured cell-membrane, composed chiefly of cellulose, and the cytoplasm, unlike that of the bacteria, shows definite structure. These organisms multiply by budding, at which time the mother-cell sends out a small globular projection of the cell membrane into which maternal cytoplasm flows. This bud gradually enlarges until it becomes about the same size as that of the original cell. Finally, by the gradual narrowing of the isthmus connecting the two, the daughter-cell becomes complete and free. When the surrounding conditions are unfavorable most yeasts are able to form spores. These, called "ascospores," are formed within the yeast cell itself, each spore forming a separate membrane of its own, but all of them lying well protected within the original cell-membrane.

In this family Besson includes the following parasitic yeasts: the Endomyces albicans (Oïdium albicans), the parasite of thrush; the Cryptococcus dermatitis {Blastomyces dermatitis), the cause of a form of chronic dermatitis; and the Saccharo-myces tumefaciens.

The Molds

In this group, the hyphomycetes, may be included many organisms having in common the formation of a well-marked mycelium, but differing so greatly in other respects as to be placed in widely separated groups in the systematic arrangement of the fungi. The characteristic feature of this class is the formation of long, interlacing filaments or threads, known as mycelia. From these there extend branches called hyphae. In this class may be placed the A chorion schönleini (the parasite causing favus), the Trichophyton and Microsporon, the Mucor, the A spergillus, and Eurotium. These parasitic organisms are described in detail under the headings of the diseases caused by them.

The Higher Bacteria

This group occupies an intermediate position between the true bacteria and the molds. These organisms are characterized by filamentous forms with real or apparent branchings. The filaments are usually divided transversely, appearing as if composed of bacilli. The free ends only seem to be endowed with the ability to reproduce, and they develop peculiar elements that differentiate the higher from the other bacteria, whose cells are all equally free and independent.

Leptothrix

These comprise long threads which do not branch, and are at times separated with difficulty from chains of bacilli. They rarely cause trouble, but have been observed in connection with inflammations of the mouth and pharynx, particularly along the edges of the tonsillar crypts, where they grow with the formation of persistent white patches. Cultivation of the leptothrix is difficult.

Cladothrix is a thread-like form in which false branching may be recognized, an appearance resulting from the fragmentation of the threads. The terminal cell breaks away from the main stem, is set at an angle by the elongation of the thread itself, and, as both continue to divide, the simulation of true branching is produced. This type is probably not pathogenic; most of the cases ascribed to this class were likely due to streptothrix infection.

Streptothrix denotes forms with numerous true branches and spores which usually appear in chains. Numerous cases of disease have been reported as being caused by these organisms.

Actinomyces is characterized by the formation of club-shaped ends and the radiating arrangement of the threads. This organism causes a specific disease of the lower animals, sometimes transmitted to the human being.

Bacteria

Bacteria are minute unicellular organisms, probably belonging to the vegetable kingdom, the schizomycetes. It is difficult to classify them, but probably the best arrangement is a modification of Migula's method as follows:

Classification Of The Bacteria

1. Order: Eubacteria (True Bacteria) A. Sub-Order: Haplobacteria (Lower Bacteria) I. Family Coccaceae

Cells globular, becoming slightly elongate before division. Division in one, two, or three directions of space. Formation of endospores very rare.

(A) Without Flagella. 1. Streptococcus

Division in one direction of space, producing chains like strings of beads.

2. Micrococcus

Division in two directions of space, so that tetrads are often formed.

3. Sarcina

Division in three directions of space, leading to the formation of bale-like packages.

(B) With Flagella. 1. Planococcus

Division in two directions of space, like micrococcus.

2. Planosarcina

Division in three directions, like sarcina.

II. Family Bacteriaceae

Cells more or less elongate, cylindric, and straight. They never form spiral windings. Division in one direction of space only, transverse to the long axis of the cell.

(A) Without Flagella. 1. Bacterium

Occasional endospores.

(B) With Flagella. 2. Bacillus

Flagella arising from any part of the surface. Endospore formation common.

3. Pseudomonas

Flagella attached only at the ends of the cell.

Endospores very rare.