ANTIBIOTICS - PENICILLINS (other topics)
Beta-lactam antibiotics are among the most commonly prescribed drugs, grouped together based upon a shared structural feature, the beta-lactam ring. The classification, spectrum of activity and pharmacology of one group of beta-lactam antibiotics, the penicillins, will be reviewed here. The mechanisms of action and resistance and major adverse reactions of the beta-lactam antibiotics are discussed separately. The cephalosporins and novel beta-lactam drugs which constitute the other beta-lactams are also discussed separately.
Penicillin, any one of a group of antibiotics derived from the fungus Penicillium or created by using partially artificial processes. The action of natural penicillin was first observed in 1928 by British bacteriologist Sir Alexander Fleming, but another ten years passed before penicillin was concentrated and studied by German-British biochemist Ernst Chain, Australian pathologist Sir Howard Florey, and other scientists.
Penicillins can be classified into the following categories: Penicillin G Antistaphylococcal penicillins (nafcillin, oxacillin, cloxacillin and dicloxacillin) Broad spectrum penicillins
Second generation (ampicillin, amoxicillin and related agents)
Third generation (carbenicillin and ticarcillin)
Fourth generation (piperacillin)
Penicillin acts both by killing bacteria and by inhibiting their growth. It does not kill organisms in the resting stage but only those growing and reproducing. Penicillin is effective against a wide range of disease-bearing microorganisms, including pneumococci, streptococci, gonococci, meningococci, the clostridium that cause tetanus, and the syphilis spirochete. The drug has been successfully used to treat such deadly diseases as endocarditis, septicemia, gas gangrene, gonorrhea, and scarlet fever.
Despite the effectiveness of penicillin in curing a wide range of diseases, infections caused by certain strains of staphylococci cannot be cured by the antibiotic because the organism produces an enzyme, penicillinase, capable of destroying the antibiotic. In addition, enterococci and other bacteria known to cause respiratory and urinary tract infections were found intrinsically resistant to the action of penicillin. Appropriate chemical treatment of a biological precursor to penicillin, isolated from bacterial cultures, resulted in the formation of a number of so-called semisynthetic penicillins. The most important of these are methicillin and ampicillin—the former is remarkably effective against penicillinase-producing staphylococci and the latter is not only active against all organisms normally killed by penicillin, but also inhibits enterococci and many other bacteria.
The strength and dosage of penicillin are measured in terms of international units. Each of these units is equal to 0.0006 g of the crystalline fraction of penicillin called penicillin G. In the early days of penicillin therapy, the drug was administered every three hours in small doses. More recently, a preparation called benzathine penicillin G has been produced that provides detectable levels of antibiotic for as long as four weeks after a single intramuscular injection; it is useful for treatment of syphilis and strep throat. Bacterial resistance to some penicillins has increased over the years, creating a need for alternative therapies.
Penicillin G is highly active against: Gram-positive cocci (except penicillinase-producing staphylococci, penicillin-resistant pneumococci, enterococci, and oxacillin-resistant staphylococci) (See "Resistance of Streptococcus pneumoniae to beta-lactam antibiotics"). Gram-positive rods such as Listeria Gram-negative cocci such as Neisseria sp (except penicillinase-producing Neisseria gonorrhoeae) Most anaerobes (with certain exceptions, such as Bacteroides)
Antistaphylococcal penicillins (nafcillin, oxacillin, cloxacillin and dicloxacillin) inhibit penicillinase-producing staphylococci but are inactive against oxacillin-resistant staphylococci.
The broad spectrum penicillins are distinguished by their activity against gram-negative bacilli. These agents have been stratified into the second-generation penicillins (ampicillin, amoxicillin and related agents), the third-generation penicillins (carbenicillin and ticarcillin), and the fourth-generation penicillin piperacillin. None of the broad spectrum penicillins is effective against penicillinase-producing staphylococci.
Second generation — Ampicillin, amoxicillin, and closely related antibiotics are able to penetrate the porin channel of gram-negative bacteria but are not stable to beta-lactamases. These antibiotics are active against the majority of strains of Escherichia coli, Proteus mirabilis, Salmonella, Shigella, and Haemophilus influenzae.
Amoxicillin and ampicillin have an identical spectrum of activity, but amoxicillin is better absorbed from the intestine when administered orally and yields higher blood and urine levels. Amoxicillin is available generically and is preferable to ampicillin for oral use except in the therapy of Shigella infections sensitive to ampicillin.
Carbenicillin and ticarcillin also can penetrate the porin channel of gram-negative bacteria in high doses, but they are less active than ampicillin on a weight basis. However, the carboxy group on the side chain of these antibiotics expands the spectrum of activity by rendering them more resistant to the chromosomal beta-lactamases of certain organisms, such as indole-positive Proteus species, Enterobacter species, and Pseudomonas aeruginosa. Third and fourth generation penicillins are most useful in infections caused by these organisms.
Piperacillin is a derivative of ampicillin. It covers much the same spectrum as carbenicillin and ticarcillin but is more active in vitro on a weight basis. In addition, it has some activity against strains of Klebsiella, although cephalosporins remain the preferred agents. It is more active than carbenicillin or ticarcillin against enterococci and Bacteroides fragilis, but other agents are preferred for the treatment of these organisms as well.
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