Bacteriology is the study of bacteria. Of primary interest to Hospital Corpsman is medical bacteriology, which deals with the bacteria that cause disease in man.

Bacteria are prokaryotic microorganisms of the kingdom Protista. They reproduce asexually by transverse binary fission in which the cell divides into two new cells. Bacteria are found almost everywhere, and the human body harbors vast numbers. Many bacteria are beneficial and essential to human life; only a few are harmful to man.

BACTERIA CLASSIFICATION
Since there are thousands of types of bacteria, a method of classification is essential. Bacteria are classified according to their respective

disease-producing ability,
growth requirements,
morphologic characteristics,
colonial morphology,
toxins produced, and
Gram's stain reaction.

Disease-Producing Ability
The disease-producing ability of bacteria is referred to as either pathogenic or nonpathogenic. Pathogens are bacteria that cause diseases, and nonpathogens are harmless bacteria. Bacteria that are essential to our body are, in their proper environment, called common or normal flora. For example, alpha streptococcus in the throat is common flora, but when it is found elsewhere (such as in the blood stream, possibly as a result of tooth extraction), it may cause diseases such as septicemia and endocarditis.

Growth Requirements
The four growth requirements for bacteria are
temperature,
oxygen,
nutrition, and
moisture.
TEMPERATURE REQUIREMENTS.-
Temperature requirements are divided into the following three categories.

Psychrophilic-bacteria that reproduce best at 15EC to 20EC

Mesophilic-bacteria that reproduce best at 20EC to 45EC
Thermophilic-bacteria that reproduce best at 50EC to 55EC
OXYGEN REQUIREMENTS.-The amount of oxygen needed for an organism to grow or reproduce varies with the type of organism. Aerobes are organisms that reproduce in the presence of oxygen. Obligate aerobes are organisms that grow only in the presence of free oxygen. Anaerobes are organisms that do not reproduce in the presence of oxygen, and obligate anaerobes are organisms that grow only in the absence of free oxygen and are killed if exposed to free oxygen. Facultative organisms are organisms that grow in the presence of free oxygen and in an oxygen-free atmosphere. Microaerophilic organisms are organisms that grow only in low amounts of free oxygen.

NUTRITION REQUIREMENTS.-Nutrition requirements for the various types of bacteria depends on what their particular environment provides. Autotrophic bacteria are self-nourishing, and heterotrophic bacteria are not self-sustaining.

MOISTURE REQUIREMENTS.-Moisture is indispensable for bacterial growth.

Morphologic Characteristics
The structural (or morphologic) characteristics of bacteria are based on three distinct shapes or categories:

Coccus (pl. cocci)-spherical, appears singly, in pairs, chains, clusters, or packets.

Bacillus (pl. bacilli)-rod-shaped, appears singly, in chains, or in palisades.
Spirillum (pl. spirilla)-spiral-, corkscrew-, or comma-shaped, appearing singly only.

Three special structures, present on some bacteria, aid in the classification process of bacteria. The special structures are the capsule, the spore, and the flagellum. The capsule is a gummy, gelatinous, or mucoid structure surrounding certain bacteria. The spore is an inactive, resting, and resistant form produced within the organism, usually as a result of unfavorable environmental conditions. The third and final special structure is the flagellum, a hairlike structure that provides motility.

Colonial Morphology
A colony is a cohesive mass composed of many millions of bacterial cells, growing on or in a medium (such as blood agar, a gel enriched with blood that is used in the preparation of solid culture media for microorganisms) as a result of the multiplication and division of a single cell. The size, color, shape, edge, topography, consistency, and odor of the colony vary with each organism.

Toxins Produced
Generally, toxins produced are waste products of metabolism in a bacterial cell. Some bacteria produce toxins that attack red blood cells in a culture medium such as blood agar. Examples of toxins produced by bacteria are listed below:

Alpha hemolysin-produces partial hemolysis (the disruption of the integrity of the red cell membrane causing release of hemoglobin) and changes the medium to a green color.

Beta hemolysin-completely lyses the RBC, leaving a clear zone of hemolysis.

Endotoxin (low potency)-comprises part of the cell wall and is released as the bacterial cell spontaneously destroys itself with self-generated enzymes (a process known as autolysis).

Exotoxin (high potency)-derives from the bacteria during its growth but is found outside the bacterial cell in the surrounding medium. Exotoxins are highly poisonous, soluble, and protein in nature.

Gram's Stain Reaction
To differentiate and identify bacteria, you must make them visible by staining. The staining procedure, devised by Dr. Hans Christian Joachim Gram, stains microorganisms such as bacteria with crystal violet, treats them with 1:15 dilution of strong iodine solution, decolorizes them with ethanol or ethanol-acetone, and counterstains them with a contrasting dye, usually safranin. Microorganisms that retain the crystal violet stain (a dark blue-black color) are said to be gram-positive, and those that lose the crystal violet stain by decolorization but stain with counterstain (a deep pink or reddish color) are said to be gram-negative.

COMMON BACTERIA
Bacteria are named by genus and species. The first word (capitalized) indicates the genus; the second word (not capitalized) indicates the species, a subdivision of the genus. For example:

Table 7-3 will familiarize you with commonly encountered bacteria. This table lists the bacteria's morphologic shape, Gram stain response, genus and species, and the type of infection it produces.