The terms broad- and narrow-spectrum antibiotics describe the extent of microorganisms affected by a particular antimicrobial agent.
Broad-spectrum antibiotics, such as extended-spectrum β-lactams (including aminopenicillins, third- and fourth-generation cephalosporins, and carbapenems), tetracyclines, and fluoroquinolones, exhibit activity against a wide variety of Gram-positive and Gram-negative bacteria, and in some cases anaerobic or atypical organisms. They are useful when the cause of infection is unknown or mixed infections are suspected, but their extensive activity may disrupt the normal microbiota and may increase the risk of resistance selection.
In contrast, narrow-spectrum antibiotics, including glycopeptides and lincosamides, are effective against a more limited subset of bacteria, often targeting specific Gram-positive organisms or defined anaerobic groups. When the pathogen is known, narrow-spectrum agents are generally preferred because they provide targeted antimicrobial activity while minimizing collateral damage to the microbiome and potentially reducing the selective pressure for antimicrobial resistance.
Developing broad- and narrow-spectrum antibiotics comes with distinct challenges:
- Broad-spectrum antibiotics must overcome multiple different permeability barriers and resistance mechanisms across diverse bacterial species, which complicates achieving sufficient antimicrobial activity without inducing toxicity. Additionally, their wide range of activity increases the likelihood of selecting for antimicrobial resistance during use.
- Narrow-spectrum antibiotics, while more targeted, require highly specific mechanisms of action and depend on rapid, accurate diagnostics for appropriate use. Their narrow specificity can lead to resistance emerging rapidly if the bacterial target is easily altered; therefore, the frequency of mutation and resistance development should be explored during antibiotic development. Additionally, the limited clinical utility can reduce commercial incentives for clinical development