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 o x y g e n - f r e e a t m o s p h e r e . 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: GENUS SPECIES Neisseria gonorrhoeae 7-26