Classification of antibiotics according to mechanism of action

β-Lactam antibiotics are bactericidal agents that interrupt bacterial cell-wall formation as a result of covalent binding to essential penicillin-binding proteins (PBPs), enzymes that are involved in the terminal steps of peptidoglycan cross-linking in both Gram-negative and Gram-positive bacteria.

It binds to the small 16S rRNA of the 30S ribosomal subunit irreversibly, interfering with the binding of formyl-methionyl-tRNA to the 30S subunit. This causes codon misreading, inhibition of protein synthesis, and ultimately death of the cell through mechanisms that are not well understood.

Amoxicillin is similar to penicillin in its bactericidal action against susceptible bacteria during the stage of active multiplication. It acts through the inhibition of cell wall biosynthesis that leads to the death of the bacteria.

Metronidazole diffuses into the organism, inhibits protein synthesis by interacting with DNA, and causes a loss of helical DNA structure and strand breakage. Therefore, it causes cell death in susceptible organisms.

The bacteriostatic action of tetracyclines, like doxycycline hyclate, is intended to stop the growth of bacteria by allosterically binding to the 30S prokaryotic ribosomal unit during protein synthesis.

It inhibits DNA replication by inhibiting bacterial DNA topoisomerase and DNA-gyrase. Of the fluoroquinolone class, ciprofloxacin is the most potent against gram-negative bacilli bacteria (notably, the Enterobacteriaceae such as Escherichia coli, Salmonella spp., Shigella spp., and Neisseria).