Why do antibiotics have different antibacterial spectra?

Broad-vs-Narrow-Spectrum-Antibiotics.jpg. Broad spectrum antibacterials are active against both Gram-positive and Gram-negative organisms. Examples include: tetracyclines, phenicols, fluoroquinolones, “third-generation” and “fourth-generation” cephalosporins. read more

Every antibiotic has its own site of action and it is specific to it, this is the same reason why an antibiotic which is effective against bacteria is ineffective against a human cell. When we talk about the specific sites of action then we have to talk about interaction between antibiotic and it's target site. read more

A broad spectrum antibiotic acts against both Gram-positive and Gram-negative bacteria, in contrast to a narrow spectrum antibiotic, which is effective against specific families of bacteria. An example of a commonly used broad-spectrum antibiotic is ampicillin. read more

CLASSIFICATION ACCORDING TO 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. read more