Name of antibiotic

Gram +ve/ Gram –ve or both

                      Mechanism of Action

Cephalothin

Both. (mainly positive)

It shows bactericidal action by interfering primarily with the synthesis of peptidoglycan layer of bacterial cell walls. It binds to Penicillin Binding Proteins (PBP) and inactivates them. Thus, inhibiting bacterial cell wall synthesis.

Ciprofloxacin

Both. (Mainly Gram-ve)

It inhibits DNA replication and transcription by inhibiting bacterial DNA gyrase or topoisomerase IV enzyme.

Gentamicin

Both. (only aerobic bacteria)

It shows its bactericidal action by binding 30s subunit of bacterial ribosome, interrupting protein synthesis. Process includes:-                    

1. Inhibition of initiation of protein synthesis.
2. Misreading of codon resulting in defective protein synthesis.
3. Promotion of premature termination and formation of incompletely synthesized protein.

Penicillin

Mainly Gram+ve. Very few Gram-ve

It binds to Penicillin Binding Protein (PBP) on the cell wall and inhibits transpeptidation which prevents the synthesis of peptidoglycan layer of cell wall. Release of autolytic enzyme causes autolysis of bacterial cell.

Amoxicillin

Both. (Mainly Gram+ve)

It binds to Penicillin Binding Protein (PBP) on the cell wall and inhibits transpeptidation which prevents the synthesis of peptidoglycan layer of cell wall. Release of autolytic enzyme causes autolysis of bacterial cell.

Bacitracin

Mainly Gram+ve. (Neisseria spp. As Gram-ve)

It prevents the dephosphorylation of lipid compound (P-P phospholipid) which carries peptidoglycan out of cell membrane to growing cell wall. Thus, inhibiting the cell wall synthesis of bacteria. This eventually leads to bacterial cell lysis.

Sulfamethoxazole

Both.

• It inhibits dihydropteroate synthase enzyme which results in inhibition of folic acid synthesis by bacteria.
• Due to its structural similarities with PABA, it may itself get incorporated to form defective folate that is metabolically harmful to bacteria.

Vancomycin

Gram+ve

It works by inhibiting cell wal synthesis. It gets attached between D-ala-D-ala terminase of NAM/NAG unit of peptidoglycans. Thus it inhibits action of transpeptidase disturbing cell wall synthesis.