Discuss the cellular factors that might make a drug's spectrum of action narrow

When a drug is labeled "narrow-spectrum," meaning that it has activity against only a limited number of microbes, generally the reason is that many microbes possess some form of natural resistance against the drug. For example, many drugs work better against Gram-positive organisms than Gram-negative ones because Gram-negative cells possess an outer membrane that does not allow these drugs to enter the cell as readily as they enter Gram-positive cells. Other cellular factors that can have an effect on the spectrum of action of a particular drug include the presence of R-plasmids in the cell that carries resistance genes, the presence of altered cytoplasmic membrane or cell wall proteins that prevent the passage of some types of drugs, and alterations in a cell's metabolic pathways, which can make the cell more resistant to certain types of drugs.

Other antimicrobials have a narrow spectrum of action because they target a metabolic or structural feature unique to a single pathogen or small group of pathogens. One example of this type of narrow-spectrum antimicrobial is antisense RNA, which complements a specific nucleotide sequence that may be present in only one pathogen.