A major reason is the cell wall: An antibiotic may be able to penetrate one type but not the other.
Finding new ways to sneak antibiotics past these cell wall barriers is one way that microbiologists are fighting antibiotic resistance.
Now, researchers have shown that a synthetic "double Trojan horse" drug can fool bacteria into willingly accepting a toxic antibiotic, essentially causing them to commit suicide.
Iron ions are not easy to come by, so some bacteria have evolved an elaborate mechanism to gather them.
Now, when the enzyme wielding bacterium destroys the first antibiotic, it unwittingly unleashes the second antibiotic, for which it has no defense.
The diagram below (which has been modified from the original) shows the mechanism by which the double Trojan horse drug works.
The bacterium, in this case the Gram-negative Acinetobacter, actively transports the double Trojan horse molecule across its first membrane. »