Most clinicians would probably define "broad-spectrum" as an antimicrobial agent's ability to kill all clinically important bacteria.

However, such a proposed definition may differ depending upon the vantage point from which it is viewed.

From the perspective of the microbe, "narrow" may refer to a given class of organisms (eg, gram-positive cocci, gram-negative bacilli, etc.) or it may refer to the genus or species of a given bacterium (eg, Staphylococcus aureus).

One may also view the definition of broad-spectrum versus narrow-spectrum by indication for antimicrobial use, such as for treatment of urinary tract infections or respiratory tract infections.

Dr. Moellering then made reference to a United States Supreme Court definition of pornography: "...can't describe it, but we know it when we see it."

Typically, antimicrobial agents are classified as being either narrow-spectrum or broad-spectrum depending upon the range of organisms for which they have activity.

Examples of narrow-spectrum agents include penicillin G, glycopeptides, macrolides, nitrofurantoin, metronidazole, aztreonam, nalidixic acid, and phosphomycin. Broad-spectrum agents include the carbapenems, the extended-spectrum cephalosporins, beta-lactam/beta-lactam inhibitor combinations, and the newer fluoroquinolones.

The distinction between broad versus narrow is not always clear, however. For example, tetracycline and chloramphenicol are both traditionally classified as broad-spectrum agents, but the development of resistance to these drugs has imposed significant limitations on their spectrum of activity, thereby altering their precise classification.

Determinants of a given antibiotic's spectrum of activity include the characteristics of the target at which it is directed, its ability to reach the bacteria, its concentration at the target site, and the effect of resistance development.