Macrolides

These agents have also been around for many years. Newer macrolides have been introduced relatively recently. Erythromycin was probably the first such agent, and the newer ones include azithromycin and clarithromycin.

Mechanism of action

These antibiotics also kill bacteria by inhibiting protein synthesis. This is done by binding to a part of the ribosome (similar to the aminoglycosides, but macrolides bind to a different site on the ribosome).

Route of administration

The macrolides are all available both orally and intravenously – but the intravenous form is often very expensive. Topical formulations are also available.

Spectrum of action

Erythromycin was often used as an alternative to beta lactams (for example in patients allergic to penicillin). It is primarily active against Gram positive bacteria (various streptococcal species, Staphylococcus aureus). However, it is active against a variety of other bacteria, and can be used to treat the following conditions:

• Diphtheria (Corynebactreium diphtheriae)
• Whooping cough (Bordetella pertussis)
• Legionnaire’s disease (Legionella pneumophila)
• Pneumonia due to Mycoplasma pneumoniae

The newer macrolides are also active against some other common organisms causing respiratory tract infections – notably Haemophilus influenzae, thus they are also being promoted as agents for respiratory tract infections. As with the newer quinolones, there is some debate regarding their use in this setting.

Clarithromycin is the drug of choice (in combination with others) when treating infections with some mycobacterial species. These organisms are related to Mycobacterium tuberculosis, and most commonly cause infection in immunocompromised patients.

Side effects

The commonest side effects reported are gastro-intestinal disturbances, particularly with erythromycin. Clarithromycin and azithromycin reportedly cause less severe gastro-intestinal disturbances.