Both ‘gram positive’ and ‘gram negative’ (named for each type of bacteria’s ability to hold the gram stain due to thickness of its peptidoglycan cell wall) bacteria are susceptible to different types of antibiotics based on the drug’s ability to inhibit peptidoglycan synthesis of the cell wall, integral to the bacteria’s physical structure.
Gram negative bacteria has an unique outer membrane which acts as an effective protection and barrier against many detergents and antibiotics.
Gram negative bacteria therefore would not be susceptible to drugs like penicillin and must be treated with other types of antibiotics specifically inhibiting its unique outer membrane.
Gram negative bacteria:
The reason why it’s important to understand both the replication mechanisms and the structural differences in different types of bacteria is because any “rational” design of drugs that would serve as an effective antibiotic would target mechanisms specific to procaryotic replication without affecting eucaryotic replication (that would affect mammalian cells for instance, i.e. our own cells), and furthermore, target any specific traits of the bacteria we want to kill without affect the regular microbial flora in our bodies.