Vancomycin is not a macrolide antibiotic. This medication belongs to the glycopeptide class of antimicrobials. It is reserved for treating severe bacterial infections, especially those resistant to more common treatments.
Separating Glycopeptides from Macrolides
The difference between these two classes lies in their complex chemical architecture. Vancomycin is a glycopeptide; its structure is a large, complex cyclic peptide linked to carbohydrate (glyco) molecules.
Macrolides, in contrast, are characterized by a large macrocyclic lactone ring, which is a ring structure containing an ester linkage. This distinct ring structure often contains 14, 15, or 16 members and is decorated with deoxy sugars.
Vancomycin’s Mechanism of Action
Vancomycin prevents the bacterial cell wall from being properly constructed. It is classified as a cell wall synthesis inhibitor, an action that is generally bactericidal. The target is the peptidoglycan layer, which provides strength and rigidity to the bacterial cell wall, particularly in Gram-positive organisms.
The antibiotic works by physically binding to the D-alanyl-D-alanine (D-Ala-D-Ala) precursor units of the peptidoglycan structure. This binding prevents the transpeptidation and transglycosylation steps necessary for linking the peptidoglycan strands into a strong, cross-linked network. The result is a weakened cell wall that cannot withstand internal pressure, leading to cell lysis and bacterial death.
How Macrolides Target Bacteria
Macrolides, such as Erythromycin, Azithromycin, and Clarithromycin, target the machinery responsible for building bacterial proteins. They are classified as protein synthesis inhibitors, an action that is typically bacteriostatic, meaning they inhibit bacterial growth and reproduction.
Macrolides achieve their effect by binding to the 50S ribosomal subunit inside the bacterial cell. They bind specifically to the 23S ribosomal RNA, located near the entrance of the nascent peptide exit tunnel. This binding partially obstructs the tunnel, preventing the newly synthesized protein chain from exiting the ribosome. By blocking the translocation of peptides, macrolides stop the bacteria from manufacturing the proteins necessary for survival.
When Vancomycin is Necessary
Vancomycin is typically reserved for treating serious infections caused by Gram-positive bacteria. It is often used when a patient has an infection resistant to first-line agents. The most frequent indication is for infections caused by Methicillin-resistant Staphylococcus aureus (MRSA).
For systemic infections, such as pneumonia, bloodstream infections, or complicated skin infections caused by MRSA, Vancomycin is administered intravenously (IV). The IV route ensures the drug reaches high concentrations in the bloodstream.
Vancomycin is poorly absorbed through the gut wall. This poor systemic absorption is utilized when treating the intestinal infection caused by Clostridioides difficile (C. difficile). For this condition, Vancomycin is given orally, allowing it to act directly on the bacteria in the gastrointestinal tract. Oral administration will not treat a systemic infection.