18.01.4.2 Aminoglycosides

The aminoglycosides bind to the bacterial ribosome and interfere with the binding of the tRNA. This leads to the wrong amino acid being added to the peptide. Gentamicin and neomycin are examples of aminoglycoside antibiotics.

The entry of gentamicin into bacterium is by oxygen-dependent active transport. Thus, gentamicin is much more effective against aerobic (oxygen-requiring) than anaerobic (not requiring oxygen) bacteria. Gentamicin is more effective against aerobic Gram-negative than Gram-positive bacteria. This is a positive, as Gram-negative bacteria are usually more difficult to treat than Gram-positive bacteria. Gentamicin is used in the treatment of serious Gram-negative bacterial infections (e.g. septicaemia), and in cystic fibrosis and bronchiectasis, where there is an increased risk of infection in damaged lungs. Resistance to gentamicin is increasing, and this resistance is due to induction of metabolising enzymes in the bacteria, that metabolise and inactive the gentamicin.

Gentamicin is highly polar (ionized) and, consequently, it is not absorbed from gastrointestinal tract. Gentamicin is used intraventously (iv) or intramuscularly (im) for systemic infections. Gentamicin is inhaled when it is used in respiratory infections associated with cystic fibrosis or bronchiectasis. Gentamicin is predominantly eliminated from the kidney, and consequently it is necessary to reduce the dose in patients with renal impairment to prevent the plasma concentration rising to toxic levels. Gentamicin is reserved for serious infections that are not easily treated with other antibiotics because it cannot be used orally, and because it can cause serious toxicity. The serious toxicity with gentamicin includes irreversible damage to the ear (ototoxicity) leading to the loss of balance, or even deafness, and damage to the kidney (nephrotoxicity) which fortunately is reversible.

Neomycin is used as a broad spectrum aminoglycoside antibiotic. Neomycin is used topically for the treatment of infections of the skin and mucous membranes e.g. impetigo (‘school sores’) and boils. Neomycin is also used in some anti-acne lotions, to treat acne that is associated with a bacterial infection.