Spectrum of activity

Antibiotics are used to kill bacterial infections. However, antibiotics differ in their mechanism of action and the species of bacteria they target. Some antibiotics are effective against a very wide range of bacteria and these are known as broad-spectrum antibiotics. Broad-spectrum antibiotics are often used if the patient requires treatment, but the isolated species has not yet been determined — this is often known as empirical therapy.

The determination of which antibiotics are used in empirical therapy is often based on the following considerations:

• Initial diagnosis
• The type of microbes that cause infection at a particular site
• Antibiotic penetration into the infected site
• Local patterns of antimicrobial sensitivity
• Side effect profile
• Patient factors

Examples of broad-spectrum antibiotics are ampicillin (a penicillin), doxycycline (a tetracycline-class drug), imipenem (a carbapenem) and some aminoglycosides. This list is not exhaustive and there are many other broad-spectrum antibiotics that are often used in empirical therapy.

Definitive or directed therapy is used when the species of pathogen is known and the antimicrobial agent targets that particular bacterial species and few other targets. To use definitive therapy:

• Clinical samples are obtained as soon as possible and isolates identified (preferably before any antibiotics are initiated)
• Change from a broad, empiric regimen (if initiated) to a narrow, pathogen-directed regimen is called de-escalation, or streamlining