LYCOS RETRIEVER 
Tetracycline: Bacteria
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Tetracycline interferes with bacterial protein synthesis. It is bacterioststic and has broad spectrum action. It blocks the binding to aminoacyl transfer RNA to messenger RNA. It has some immunosuppressive actions and is anti-inflammatory affecting the lymphocyte function, as well as neutrophil chemotaxis. It inhibits lipase and collagenase.
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A Tetracycline disk may be used to determine microbial susceptibility to drugs in the Tetracycline class. If the Kirby-Bauer method of disk susceptibility testing is used, a 30 mcg Tetracycline disk should give a zone of at least 19 mm when tested against a Tetracycline susceptible bacterial strain. Microorganisms may be considered susceptible if the MIC (minimum inhibitory concentration) is not more than 4.0 mcg/mL and intermediate if the MIC is 4.0 to 12.5 mcg/mL.
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The first tetracycline-resistant bacterium, Shigella dysenteriae, was isolated in 1953.Tetracycline resistance now occurs in an increasing number of pathogenic, opportunistic, and commensal bacteria. The presence of tetracycline-resistant pathogens limits the use of these agents in treatment of disease. Tetracycline resistance is often due to the acquisition of new genes, which code for energy-dependent efflux of tetracyclines or for a protein that protects bacterial ribosomes from the action of tetracyclines. Many of these genes are associated with mobile plasmids or transposons and can be distinguished from each other using molecular methods including DNA-DNA hybridization with oligonucleotide probes and DNA sequencing. A limited number of bacteria acquire resistance by mutations, which alter the permeability of the outer membrane porins and/or lipopolysaccharides in the outer membrane, change the regulation of innate efflux systems, or alter the 16S rRNA. New tetracycline derivatives are being examined, although their role in treatment is not clear.
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