Friday, 1 July 2011

The Tetracyclines

Chlortetracycline, oxytetracycline and tetracycline. These are very closely related substances which, for practical purposes, have an identical and wide range of activity. This is offset to some extent by their action being bacterio-static and not bactericidal. The oral dose for an adult is 250-500 mg given at six-hourly intervals and taken before meals.
Chlortetracycline is used for the local treatment of skin infections as it does not cause cutaneous sensitization.
Demethylchlortetracycline and methacycline attain higher and more prolonged blood levels than the other tetracyclines and hence may be given less frequently.These advantages, however, are counter-balanced by the high degree of binding to plasma proteins.
The tetracyclines inhibit the growth of a wide range of Gram-positive and Gram-negative organisms and are particularly useful in the treatment of brucellosis and exacerbations of chronic bronchitis. Their value in chronic bronchitis has been limited recently by a significant increase in tetracycline-resistant pneumococci and to a lesser extent H. influenzae.
The tetracyclines are also active against the following organisms which are intermediate between bacteria and viruses:
(a) Rickettsia (typhus fevers)
(b) Coxiella burneti (Q-fever)
(c) Mycoplasma pneumoniae (atypical pneumonia)
(d) Chlamydiae (lymphogranuloma venereum, psittacosis).
The tetracyclines are also employed systemically in the treatment of acne vulgaris and rosacea where their beneficial effect is almost certainly not due only to their antibacterial action.
Many of the normal saprophytes of the alimentary tract are sensitive to the tetracyclines and are inhibited when these antibiotics are administered. Insensitive organisms then multiply and cause ‘super-infection‘. This occurs particularly with fungi, such as Candida albicans, which cause stomatitis, glossitis, diarrhea or a distressing anal or vulvar pruritus. In hospital, superinfection with resistant staphylococci may occur and the ensuing enteritis, septicaemia or pneumonia can be fatal. Fortunately this is a rare complication of tetracycline therapy. Superinfection following the use of the tetracyclines emphasises that an antibiotic with a wide range of activity should not be prescribed if an appropriate ‘narrow-spectrum’ antibiotic is available. The tetracyclines quite commonly cause mild diarrhoea.
Pregnant women with pyelonephritis treated with tetracycline intravenously may develop severe liver damage. Staining of growing teeth and bone occurs and this could interfere with their growth and development in infants. Accordingly the use of tetracyclines is contraindicated in pregnancy and in childhood. The tetracyclines can also exacerbate renal failure and should not be given to patients with renal disease.

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