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Spectrum of Activity


Depending on the range of bacterial species susceptible to these agents, antibacterials are classified as broad-spectrum, intermediate-spectrum, or narrow- spectrum. Note that the spectra of activity may change with acquisition of resistance genes, as will be discussed in the next module.


  1. Broad spectrum antibacterials are active against both Gram-positive and Gram-negative organisms.  Examples include: tetracyclines, phenicols, fluoroquinolones, “third-generation” and “fourth-generation” cephalosporins.   
  2. Narrow spectrum antibacterials have limited activity and are primarily only useful against particular species of microorganisms.  For example, glycopeptides and bacitracin are only effective against Gram-positive bacteria, whereas polymixins are usually only effective against Gram negative bacteria.  Aminoglycosides and sulfonamides are only effective against aerobic organisms, while nitroimidazoles are generally only effective for anaerobes. 


2-17 CLINICAL ICON.jpgClinical Implications: Intrinsic Resistance


Knowledge of the intrinsic resistance of a pathogen of concern is important in practice to avoid inappropriate and ineffective therapies.

For bacterial pathogens which are naturally insensitive to a large number of classes of antimicrobials, such as Mycobacterium tuber-culosis and Pseudomonas aeruginosa, this consideration can pose a limitation in the range of options for treatment and thus consequently further increase the risk for emergence of acquired resistance.



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