Meds and you

17 November 2009

Beta lactam antibiotics

This is the largest group of antibiotics.

This is the largest group of antibiotics. They all share a structural similarity, the beta-lactam ring.

The various chemicals attached to the beta lactam ring are what give the different members of this group their different properties.

Penicillin was the first of the beta lactams, and after the discovery of the beta lactams nucleus, modifications were made to alter its spectrum of activity (the range of organisms that it is active against).

Later on, cephalosporins were discovered – also the product of a type of fungus, and again, modifications of the various chemical structures around the beta lactams ring in cephalosporins allowed for changes in antibacterial activity. Cephalosporins were followed by the monobactams, and then the carbapenems.

Mechanism of action

These antibiotics all act by inhibiting cell wall synthesis. They bind to enzymes in the cell wall, which are responsible for putting the components of the cell wall together.

By inhibiting these enzymes, the organism is unable to synthesise the cell wall properly, and cannot replicate.

The various side chains on the beta lactams ring are what allow the antibiotic to either gain access to the enzymes in the cell wall, or else to resist the effect of certain resistance mechanisms of the bacteria, and this is how the modifications of the side chains change the antibacterial activity of the antibiotic.


Although some of these agents are only available by injection, many beta lactams (the penicillins and cephalosporins) can be given orally as well as intravenously or intramuscularly.

This versatility means that they are commonly used in hospitals as well as in the community (ie in homes etc). Both aztreonam and the carbapenems are only available intravenously.

Spectrum of action

Beta lactams are active against a wide range of bacteria, and can therefore be used to treat a wide variety of infections.

Generally speaking, penicillins are active against Gram-positive organisms, although some penicillins have been modified (such as amoxicillin) or combined with other agents (such as clavulanic acid) to make them more active against Gram negative organisms.

The cephalosporins are active against both Gram positive and Gram-negative organisms, but the range of organisms varies depending on the type of cephalosporin.

Aztreonam is really only effective against Gram-negative organisms, while carbapenems are active against most bacteria (Gram positive and Gram negative).

However, carbapenems are only available intravenously, and are very expensive. They are thus only used in hospital settings to treat very severe infections, or to treat organisms that are very resistant to antibiotics.

Examples of infections commonly treated with beta lactams are:

  • Pneumonia (penicillin or amoxicillin)
  • Meningitis (ceftriaxone)
  • Ear infections (amoxicillin or amoxicillin/clavulanic acid)
  • Streptococcal pharyngitis (penicillin or amoxicillin)
  • Urinary tract infections (amoxicillin/clavulanic acid or cefuroxime)
  • Cellulitis (cloxacillin)
  • Syphilis (penicillin)
  • Gonorrhoea (ceftriaxone - used to be penicillin, but resistance to this agent is becoming more common)

Side effects

These antibiotics are generally very safe. The most important side effect is allergic reactions, especially to penicillin. This can take several forms – ranging from a mild skin reaction to life threatening anaphylaxis (acute constriction of the airways and falling blood pressure which can be fatal if untreated).


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