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 ﬁrst 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 ﬂucoxacillin 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 ﬁrst 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.