The greater number of bacteria are non-motile, but many possess the power of motility as a result of the presence of flagella. Most of the cocci are non-motile. According to the presence or absence of flagella, the following classification of bacteria has been made:

Types of Micro organisms.

Fig. 79. - Types of Micro-organisms.

i, Coccus; 2, streptococcus; 3, staphylococcus; 4, capsulated dip-lococcus; 5, biscuit-shaped coccus; 6, tetrads; 7, sarcina form; 8, types of bacilli (1 to 8 are diagrammatic); 9, non-septate spirillum (X 1000); 10, ordinary spirillum: (a) comma-shaped element; (b) formation of spiral by comma-shaped elements (X 1000); 11, types of spore formation; 12, flagellated bacteria; 13, changes in bacteria produced by plasmolysis (after Fischer); 14, bacilli with terminal protoplasm (Bütschli); 15, (a) Bacillus composed of five protoplasmic meshes; (b) protoplasmic network in micrococcus (Bütschli); 16, bacteria containing metachromatic granules (Ernst Neisser); 17, Beggiatoa alba - both filaments contain sulphur granules - one is septate; 18, Thiothrix tenuis (Winogradski); 19, Lepto-thrix innominata (Miller); 20, Cladothrix dichotoma (Zopf); 21, Strep-tothrix actinomyces (Boström): (a) colony under low power; (b) filament showing true branching; (c) filament containing coccus-like bodies; (d) filament with club at end.

1. Gymnobacteria, forms without flagella. 2. Trichobacteria, forms with flagella.

(a) Monotricha, a single flagellum at one end.

(b) Leptotricha, a bundle of flagella at one end.

(c) Amphitricha, a flagellum at each end.

(d) Peritricha, flagella arising from all parts of the surface of the organism.


Bacteria are so minute that a special unit has been adopted for their measurement. This is the micromillimeter (u), or one-thousandth part of a millimeter, known as the micron. It is equivalent to the one-twenty-five-thousandth part of an inch. The size of bacteria vary from a fraction of a micromillimeter to 20 or even 40 micromillimeters.

Reproduction. Fission

The most common method in which the organisms divide into two. This occurs very rapidly if there is enough nutritive material present and the surrounding conditions are favorable, the length of a generation varying from fifteen to forty minutes.

Sporulation occurs when the conditions do not favor multiplication. There are then formed small, round or oval, highly refracting bodies called spores, which are capable of resisting very unfavorable surroundings. They differ from bacteria in being able to withstand evaporation and exposure to quite high degrees of heat. Few adult bacteria can resist temperatures above 700 C, but spores are uninjured by such heat and may even resist the temperature of boiling water (100° C.) for a short time. If the spore develops within the bacterium in the middle, or at one or other of the ends, it is called an endo-spore. If the spore is so large as to cause a bulging of the organism it is called a Clostridium. These forms occur in the bacilli. Among the micrococci there are times when the entire organism is transformed into a spore, an arthrospore.

Germination Of Spores

When conditions become favorable the spore may develop into an adult organism. Its contents, which have been clear and transparent, become granular, the body increases slightly in size, the capsule becomes less distinct, and in the course of time splits open to allow the escape of the young organism. This begins to increase in size, develops its characteristic capsule, and presently begins to multiply by fission.

Growth Of Bacteria

In the cultivation of bacteria there are many conditions that can influence the growth favorably or unfavorably. There are, however, certain factors that are really essential.


Bacteria grow best when diffusible albumins are present, but carbohydrates will do. It has been found that the food requirements differ very greatly with the different kinds of organisms. Some will live in water to which an extremely small amount of organic matter has been added. Others require a concentrated medium such as blood-serum. Then, too, the addition of certain substances, such as glucose or glycerin, may exert a very favorable influence.


'All micro-organisms must have oxygen in order to continue to live, but it may be present either in the free or in a combined condition. Those organisms which grow in the presence of free (uncombined) oxygen are known as aerobes. Those which will not grow in the presence of free oxygen are the anaerobes. There are, however, some of the aerobic type which will grow about as well without free oxygen as with it; these are the optional (facultative) anaerobes.

Moisture to some degree is an absolute necessity, but it may be present in very slight amount. Unless some is present nearly all organisms will dry up and cease to multiply, but spores may be formed first and persist more or less indefinitely. In making up artificial culture-media there should be present at least 80 per cent, of water.

Temperature of a proper degree is of the greatest importance. Every micro-organism grows best at some definite degree of heat, and shows variations in its activity as the temperature changes. The organisms, however, may be able to endure extreme degrees of cold without being destroyed - -some can be placed in liquid air and yet undergo multiplication when the temperature is raised. They cannot, as a rule, stand the higher temperatures as well, although a few varieties of organisms may thrive at high degrees (65° - 70° C). They are called thermophilic, and are found in manure piles and hot springs.

The temperature at which micro-organisms grow best is known as the optimum; the lowest temperature at which they continue to multiply, as the minimum; the highest at which they remain active, the maximum. With pathogenic or disease-producing organisms the optimum temperature is that normal to the body (37° C).

A temperature of from 500 to 6o° C. will weaken and finally destroy nearly all forms. If they are exposed to steam or boiling water at the temperature of 100° C. all fully developed bacteria will be killed in a few minutes, but their spores may be able to resist this heat for a longer time. When dry heat is used a higher temperature is required. The spores may withstand 1500 C. for an hour or 1750 C. for five to ten minutes.


Most true bacteria grow best in neutral or feebly alkaline media, although some grow well in strong acids and others in marked alkalinity.

Many chemical bodies will restrain the growth or destroy the bacteria. These substances may be produced by the bacteria in their growth or they may be artificially introduced. Those which will restrain the growth but not kill are called antiseptics; those that kill, germicides.

Light, particularly the direct rays of the sun, will retard bacterial growth and in many cases kill the organisms. Certain colors distinctly retard growth, blue being the most effective. A weak, diffused light seems most favorable, but various organisms react differently, certain bacteria producing color only when exposed to the ordinary light of a room, while others will produce color only in the dark.


Bacteria apparently grow best when there is an absence of motion.

Electricity and x-rays do not seem to have any constant effect upon bacteria.

Symbiosis, or the association of one organism with another, may cause an increase in its activity, as the growth of the tetanus bacillus in the presence of other bacteria that use up the supply of oxygen. A ntibiosis is the condition in which the association may be detrimental to one of the organisms.

Products Of Bacterial Growth

According to the substances formed as a result of their growth bacteria may be divided into:

Zymogens, bacteria of fermentation.

Saprogens, bacteria of putrefaction.

Chromogens, bacteria which produce colors.

Photogens, phosphorescent bacteria.

Aërogens, gas producers.

Pathogens, bacteria which produce disease.

Bacteria through their activity split up complex organic substances into simple compounds.

Fermentation is the splitting up of carbohydrates by the activity of the micro-organisms. This is the process that takes place in the formation of alcohol as a result of the action of yeast. Other forms of fermentation are those in which acetic, lactic, or butyric acids result.

Putrefaction is the breaking up of nitrogenous compounds by micro-organisms that can live only in dead organic substances. The albumins are first transformed into peptones, which split up into gases, acids, bases, and salts.

The albumins may become changed to toxalbumins or into alkaloidal substances called ptomains, which are "chemical compounds, basic in character, formed by the action of bacteria upon organic matter." Ptomains are generally formed outside of the living body and cause harm only when introduced within it.

Toxins and toxalbumins are poisonous substances elaborated by bacteria during growth, and it is upon them that the disease-producing power of the organism rests.

The bacterioproteins also belong to this same group. These bacterial products are destroyed by sunlight, by heating to 6o° to 8o° C, by long keeping, and by the gastric juices. Tuberculin is an exception, in that it remains unaltered at a temperature of 100° C. The poisonous bodies may be either soluble or insoluble, and are generally peculiar to the variety of organism by which they are formed. Certain ones select definite cells upon which they act, and are called specific. Others, having no special selective powers, are non-specific.

Chronto Genesis

Bacteria that produce colored colonies or give a color to the medium in which they grow are called chromogenic; those producing white or no color, non-chromo-genic. Most chromogenic organisms are saprophytic and nonpathogenic; but some of the pathogenic forms may produce color. Almost all known colors may be formed by different bacteria, and sometimes one organism will form two or more colors. The formation of the pigment probably depends upon the presence of oxygen.


During fermentation and decomposition various gases are given off, such as carbon dioxide, sulphureted hydrogen, ammonia, etc.

Other enzymes formed by bacteria may cause the coagulation of milk and the liquefaction of gelatin. Some bacteria liquefy the gelatin in such a peculiar and characteristic manner as to make the appearance a valuable guide for the differentiation of species.


Those micro-organisms which cause disease are called pathogenic; those that do not, non-pathogenic. There is, however, no sharp line between the two, as under adverse cultural conditions the pathogens may lose their ability to produce disease. On the other hand, those that are usually harmless may be made virulent.