Although there are several classification schemes for antibiotics, based on bacterial spectrum (broad versus narrow) or route of administration (injectable versus oral versus topical), or type of activity (bactericidal versus bacteriostatic), the most useful is based on chemical structure. Antibiotics within a structural class will generally show similar patterns of effectiveness, toxicity, and allergic potential.
PENICILLINS The penicillins are the oldest class of antibiotics and have a common chemical structure that they share with the cephalosporins. Classed as the betalactam antibiotics, the two groups are generally bacteriocidal, which means that they kill bacteria rather than simply inhibit its growth. The penicillins can be further subdivided. The natural penicillins are based on the original penicillin G structure; penicillinase-resistant penicillins, notably methicillin and oxacillin, are active even in the presence of the bacterial enzyme that inactivates most natural penicillins. Aminopenicillins such as ampicillin and amoxicillin have an extended spectrum of action compared with the natural penicillins; extended spectrum penicillins are effective against a wider range of bacteria. These generally include coverage for Pseudomonas aeruginosa.
CEPHALOSPORINS Cephalosporins and the closely related cephamycins and carbapenems, like the penicillins, contain a beta-lactam chemical structure. Consequently, there are patterns of cross-resistance and cross-allergenicity among the drugs in these classes. The "cepha" drugs are among the most diverse classes of antibiotics and are themselves subdivided into first, second, and third generations. Each generation has a broader spectrum of activity than the one before. In addition, cefoxitin, a cephamycin, is highly active against anaerobic bacteria, which offers utility in treatment of abdominal infections. The third generation drugs, cefotaxime, ceftizoxime, ceftriaxone, and others, cross the blood-brain barrier and may be used to treat meningitis and encephalitis. Cephalosporins are the usually preferred agents for surgical prophylaxis.
FLUOROQUINOLONES The fluoroquinolones are synthetic antibacterial agents and not derived from bacteria. They are included here because they can be readily interchanged with traditional antibiotics. An earlier, related class of antibacterial agents, the quinolones, drugs that were not well absorbed, could be used only to treat urinary tract infections. The fluoroquinolones, which are based on the older group, are broad-spectrum bacteriocidal drugs that are chemically unrelated to the penicillins or the cephalosporins. They are well distributed into bone tissue and so well absorbed that in general they are as effective by the oral route as by intravenous infusion.
TETRACYCLINES Tetracyclines got their name from the fact that they share a chemical structure that has four rings. They are derived from a species of Streptomyces bacteria. Broad-spectrum bacteriostatic agents, the tetracyclines may be effective against a wide variety of microorganisms, including rickettsia and amoebic parasites.
MACROLIDES The macrolide antibiotics are derived from Streptomyces bacteria. Erythromycin, the prototype of this class, has a spectrum and use similar to penicillin. Newer members of the group, azithromycin and clarithromycin, are particularly useful for their high level of lung penetration. Clarithromycin has been widely used to treat Helicobacter pylori infections, the cause of stomach ulcers.
OTHERS Other classes of antibiotics include the aminoglycosides, which are particularly useful for their effectiveness in treating Pseudomonas aeruginosa infections, and the lincosamide drugs clindamycin and lincomycin, which are highly active against anaerobic pathogens. There are other, individual drugs which may have utility in specific infections.