one of a group of drugs that includes the *penicillins and the *cephalosporins. All have a four-membered beta-lactam ring as part of their molecular structure. Beta-lactam antibiotics function by interfering with the growth of the cell walls of multiplying bacteria. Bacteria become resistant to these antibiotics by producing beta-lactamases, enzymes (such as *penicillinase) that disrupt the beta-lactam ring. To counteract this, beta-lactamase inhibitors (e.g. *clavulanic acid) may be added to beta-lactam antibiotics. For example, co-amoxiclav is a mixture of *amoxicillin and clavulanic acid.
Antibiotic is the term used to describe any antibacterial agent derived from micro-organisms, although most of them are now prepared synthetically. Such agents destroy or inhibit the growth of other micro-organisms: examples are penicillin, cephalosporin, amino-glycosides, streptomycin, and tetracycline.
Penicillin was the ?rst antibiotic to be discovered and used in the 1940s. The discovery and isolation in 1958 of the penicillin nucleus, 6-amino penicillanic acid (6-PNA), allowed many new penicillins to be synthesised. These are now the largest single group of antibiotics used in clinical medicine. Most staphylococci (see STAPHYLOCOCCUS) have now developed resistance to benzylpenicillin, the early form of the drug, because they produce penicillinases – enzymes which break down the drug. Other types of penicillin such as cloxacillin and ?ucoxacillin are not affected and are used against penicillin-resistant staphylococci.
The cephalosporins are derived from the compound cephalosporin C, which is obtained by fermentation of the mould cephalosporium.
The cephalosporin nucleus 7 amino cephalosporanic (7-ICA) acid has been the basis for the production of the semi-synthetic compounds of the cephalosporin nucleus. The ?rst semi-synthetic cephalosporin, cephalothin, appeared in 1962; it was followed by cephaloridine in 1964. The original cephalosporins had to be given by injection, but more recent preparations can be given by mouth. The newer preparations are less readily destroyed by betalactamases and so they have a much broader spectrum of antibacterial activity. The newer cephalosporins include cephalexin, cefazolin, cephacetrile, cephapirin, cefamandole, cefuroxine, cephrodine, cefodroxil and cefotaxine. Inactivation of beta-lactamase is the basis of bacterial resistance both to the penicillins and to the cephalosporins, so that attempts to prepare these antibiotics with resistance to betalactamase is of great importance. A synthetic inhibitor of beta-lactamase called clavulanic acid has been synthesised; this is used in combination with the penicillins and cephalosporins to prevent resistance. The cephamycins are a new addition to the beta-lactam antibiotics. They are similar in structure to the cephalosporins but are produced, not by fungi, but by actinomycetes.
Overuse and misuse of antibiotics have resulted in many bacteria becoming resistant to them. Hospitals, in particular, have problems with METHICILLIN-RESISTANT STAPHYLOCOCCUS AUREUS (MRSA). Combinations of antibiotics are needed to combat resistant strains of bacteria, another example being Mycobacterium tuberculosis.... antibiotics
(beta-adrenergic receptor blocker) a drug that prevents stimulation of the beta *adrenoceptors at the nerve endings of the sympathetic nervous system. Blockade of ?1 receptors causes a decrease in heart rate and force; blockade of ?2 receptors causes constriction of the airways and the arteries. Beta blockers include *acebutolol, *atenolol, *bisoprolol, *oxprenolol, *propranolol, and *sotalol; they are used to control abnormal heart rhythms, to treat angina, and to reduce high blood pressure (although they are no longer regarded by some experts as the first choice of drug for treating hypertension in the absence of heart disease, being less effective than newer antihypertensive drugs). Beta blockers that block both ?1 and ?2 receptor sites cause constriction of air passages in the lungs, and these drugs should not be used in patients with asthma and bronchospasm. Other beta blockers are relatively selective for the heart (cardioselective) and are less likely to constrict the airways. Some beta blockers (e.g. *carteolol, *levobunolol, and *timolol) reduce the production of aqueous humour and therefore the pressure inside the eye; they are taken as eye drops in the treatment of *glaucoma.... beta blocker
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