4.2 Activity against Gram-positive and Gram-negative bacteria
Narrow-spectrum antibiotics are effective against either Gram-positive or Gram-negative bacteria, whereas broad-spectrum antibiotics are effective against both types.
Not all Gram-positive and/or Gram-negative bacteria are affected by a single antibiotic. Why is this?
This is because of the specificity of the antibiotic/bacterial target interaction, whether the bacterial species has the target in question and whether the bacteria are resistant to the antibiotic.