Bacterial Resistance to Chloramphenicol

1. Enzymatic Inactivation - Chloramphenicol Acetyltransferase (CAT)

• CAT enzymes (encoded by plasmid-carried cat genes) catalyze the acetylation of the 3-hydroxy group of chloramphenicol using acetyl-CoA as a cofactor.
• The acetylated drug product cannot bind to the 50S ribosomal subunit, rendering it inactive.
• CAT genes are found in both gram-positive and gram-negative bacteria and their expression can be induced by drug exposure.
• Because this resistance is plasmid-mediated, it spreads readily between bacterial species via horizontal gene transfer.

"Resistance to chloramphenicol usually is caused by a plasmid-encoded acetyltransferase that inactivates the drug... Acetylated derivatives of chloramphenicol fail to bind to bacterial ribosomes." - Goodman & Gilman's Pharmacological Basis of Therapeutics

2. Efflux Pumps

• Many bacteria possess active efflux transporters that pump chloramphenicol out of the cell before it can inhibit ribosomes.
• This prevents intracellular drug accumulation.
• Broad-spectrum efflux pumps (e.g., chromosomally encoded ones in gram-negative bacteria) can include chloramphenicol among their substrates.

"Other mechanisms of resistance include efflux pumps and decreased membrane permeability." - Jawetz, Melnick & Adelberg's Medical Microbiology, 28th Ed.

3. Decreased Membrane Permeability

• Low-level resistance can emerge by selection of mutants that are less permeable to the drug.
• In gram-negative rods (e.g., E. coli, Enterobacter), chromosomal mutations alter outer membrane porin proteins, reducing drug entry into the cell.
• This mechanism tends to give low-level resistance rather than high-level clinical resistance.

"Less commonly, chromosomal mutations alter the outer membrane porin proteins, causing gram-negative rods to be less permeable." - Medical Microbiology 9e

4. Ribosomal Modification (Less Common)

• Ribosomal mutations causing resistance to chloramphenicol are uncommon in most bacteria because they have multiple copies of rRNA genes (requiring simultaneous mutations in all copies to achieve resistance).
• However, in some staphylococci, a plasmid-encoded ribosomal methylase has been found that confers cross-resistance to chloramphenicol, clindamycin, and oxazolidinones by modifying the drug-binding site on the 50S subunit.
• This mechanism is more likely in organisms with few rRNA gene copies (e.g., mycobacteria, Helicobacter pylori, Treponema), where fewer mutations are needed.

"Among staphylococci, some resistant strains have been found to have a plasmid-encoded ribosomal methylase that confers resistance to chloramphenicol, clindamycin, and oxazolidinones." - Harrison's Principles of Internal Medicine 22E

Summary Table