∞ generated and posted on 2016.03.20 ∞
The range of targets against which a drug is biologically active, particularly in terms of clinical effectiveness.
Spectrum of activity, for antibiotics, is the range of bacterial types against which the antibiotic is effective, where these types often are measured in terms of inherent sensitivities. Bacteria, that is, may or may not inherently display targets for antibiotic action and an antibiotic's spectrum of activity to a first approximation will include all bacteria that typically do possess those antibiotic targets.
Inherent sensitivities are rather than as can be a consequence of the acquisition or evolution of antibiotic resistance. Bacteria, that is, can obtain resistance either through the loss of targets or other means of evading antibiotic activity, thereby resulting, in practice, in reductions in a drug's spectrum of activity.
A related issue is that an antibiotic will be effective only to the extent that bacteria are sensitive at drug doses which are not toxic to the patient, that is, at doses where side effects are not sufficiently severe that drug usage must be discontinued. A drug, in other words, is only useful so long as benefits outweigh costs.
A broad spectrum of activity can be useful because it makes it possible to treat presumptively. For example, this would be antibiotic use by patients before the antibiotic-susceptibility of a pathogen has been determined, or before even the type of pathogen has been precisely identified.
While a broad spectrum of activity along with presumptive prescription can be highly convenient, it comes at the cost of antibacterial activity against more than just target species. Such "collateral damage" can result in what are known as antibiotic-associated superinfections such as Clostridium difficile-associated colitis and Candida albicans yeast infections.