Neomycin is an aminoglycoside antibiotic found in many topical medications such as creams, ointments, and eyedrops. The discovery of neomycin dates back to 1949. It was discovered in the lab of Selman Waksman, who was later awarded the Nobel Prize in Physiology or Medicine in 1951. Neomycin belongs to aminoglycoside class of antibiotics that contain two or more aminosugars connected by glycosidic bonds. Due to the inherent oto- and nephrotoxicity of these substances, systemic use has declined, as safer alternatives have become available.
Uses
Neomycin is typically used as a topical preparation, such as Neosporin. It can also be given orally, where it is usually combined with other antibiotics. Neomycin is not absorbed from the gastrointestinal tract and has been used as a preventive measure for hepatic encephalopathy and hypercholesterolemia. By killing bacteria in the intestinal tract, it keeps ammonia levels low and prevents hepatic encephalopathy, especially prior to GI surgery. It has also been used to treat small intestinal bacterial overgrowth. It is not given intravenously, as neomycin is extremely nephrotoxic (causes kidney damage), especially compared to other aminoglycosides. The exception is when neomycin is included, in very small quantities, as a preservative in some vaccines â€" typically 0.025 mg per dose.
Molecular biology
Neomycin resistance is conferred by either one of two aminoglycoside phosphotransferase genes. A neo gene is commonly included in DNA plasmids used by molecular biologists to establish stable mammalian cell lines expressing cloned proteins in culture; many commercially available protein expression plasmids contain neo as a selectable marker. Non-transfected cells will eventually die off when the culture is treated with neomycin or similar antibiotic. Neomycin or kanamycin can be used for prokaryotes, but geneticin (G418) is, in general, needed for eukaryotes.
Spectrum
Similar to other aminoglycosides, neomycin has excellent activity against Gram-negative bacteria, and has partial activity against Gram-positive bacteria. It is relatively toxic to humans, and many people have allergic reactions to it. See: Hypersensitivity. Physicians sometimes recommend using antibiotic ointments without neomycin, such as Polysporin. The following represents MIC susceptibility data for a few medically significant Gram-negative bacteria.
- Enterobacter cloacae: >16 μg/ml
- Escherichia coli: 1 μg/ml
- Proteus vulgaris: 0.25 μg/ml
Composition
Standard grade neomycin is composed of a number of related compounds including neomycin A (neamine), neomycin B (framycetin), neomycin C, and a few minor compounds found in much lower quantities. Neomycin B is the most active component in neomycin followed by neomycin C and neomycin A. Neomycin A is not a unique structure, but rather a compositional degradation product of the C and B isomers. The quantities of these components in neomycin vary from lot-to-lot depending on the manufacturer and manufacturing process.
Safety
In 2005â€"06, neomycin was the fifth-most-prevalent allergen in patch test results (10.0%).
History
Neomycin was discovered in 1949 by the microbiologist Selman Waksman and his student Hubert Lechevalier at Rutgers University. It is produced naturally by the bacterium Streptomyces fradiae. Synthesis requires specific nutrient conditions in either stationary or submerged aerobic conditions. The compound is then isolated and purified from the bacterium.
DNA binding
Aminoglycosides such as neomycin are known for their ability to bind to duplex RNA with high affinity. The association constant for neomycin with A-site RNA has been found to be in the 109 Mâˆ'1 range. However, more than 50 years after its discovery, its DNA-binding properties were still unknown. Neomycin has been shown to induce thermal stabilization of triplex DNA, while having little or almost no effect on the B-DNA duplex stabilization. Neomycin was also shown to bind to structures that adopt A-form structure, triplex DNA being one of them. Neomycin also includes DNA:RNA hybrid triplex formation.
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