presence of free oxygen.Anaerobes are organismsthat do not reproduce in the presence of oxygen, andobligate anaerobes are organisms that grow only inthe absence of free oxygen and are killed if exposed tofree oxygen. Facultative organisms are organismsthat grow in the presence of free oxygen and in ano x y g e n - f r e e a t m o s p h e r e .Microaerophilicorganisms are organisms that grow only in lowamounts of free oxygen.NUTRITION REQUIREMENTS.—Nutritionrequirements for the various types of bacteria dependson what their particular environment provides.Autotrophic bacteria are self-nourishing, andheterotrophic bacteria are not self-sustaining.MOISTURE REQUIREMENTS.—Moisture isindispensable for bacterial growth.Morphologic CharacteristicsThe structural (or morphologic) characteristics ofbacteria are based on three distinct shapes orcategories:Coccus (pl. cocci)—spherical, appears singly, inpairs, chains, clusters, or packets.Bacillus (pl. bacilli)—rod-shaped, appearssingly, in chains, or in palisades.Spirillum (pl. spirilla)—spiral-, corkscrew-, orcomma-shaped, appearing singly only.Three special structures, present on some bacteria,aid in the classification process of bacteria. Thespecial structures are the capsule, the spore, and theflagellum. The capsule is a gummy, gelatinous, ormucoid structure surrounding certain bacteria. Thespore is an inactive, resting, and resistant formproduced within the organism, usually as a result ofunfavorable environmental conditions. The third andfinal special structure is the flagellum, a hairlikestructure that provides motility.Colonial MorphologyA colony is a cohesive mass composed of manymillions of bacterial cells, growing on or in a medium(such as blood agar, a gel enriched with blood that isused in the preparation of solid culture media formicroorganisms) as a result of the multiplication anddivision of a single cell. The size, color, shape, edge,topography, consistency, and odor of the colony varywith each organism.Toxins ProducedGenerally, toxins produced are waste products ofmetabolism in a bacterial cell. Some bacteria producetoxins that attack red blood cells in a culture mediumsuch as blood agar. Examples of toxins produced bybacteria are listed below:Alpha hemolysin—produces partial hemolysis(the disruption of the integrity of the red cellmembrane causing release of hemoglobin) andchanges the medium to a green color.Beta hemolysin—completely lyses the RBC,leaving a clear zone of hemolysis.Endotoxin (low potency)—comprises part ofthe cell wall and is released as the bacterial cellspontaneously destroys itself withself-generated enzymes (a process known asautolysis).Exotoxin (high potency)—derives from thebacteria during its growth but is found outsidethe bacterial cell in the surrounding medium.Exotoxins are highly poisonous, soluble, andprotein in nature.Gram’s Stain ReactionTo differentiate and identify bacteria, you mustmake them visible by staining. The stainingprocedure, devised by Dr. Hans Christian JoachimGram, stains microorganisms such as bacteria withcrystal violet, treats them with 1:15 dilution of strongiodine solution, decolorizes them with ethanol orethanol-acetone, and counterstains them with acontrasting dye, usually safranin. Microorganismsthat retain the crystal violet stain (a dark blue-blackcolor) are said to be gram-positive, and those that losethe crystal violet stain by decolorization but stain withcounterstain (a deep pink or reddish color) are said tobe gram-negative.COMMON BACTERIABacteria are named by genus and species. The firstword (capitalized) indicates the genus; the secondword (not capitalized) indicates the species, asubdivision of the genus. For example:GENUSSPECIESNeisseriagonorrhoeae7-26
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