What is Chloramphenicol?

Chloramphenicol is a broad-spectrum antibiotic that belongs to the class of compounds known as the nitroaromatics. It was first isolated from the bacterium Streptomyces venezuelae in the late 1940s. Chloramphenicol is a kind of bactericide and an efficient broad-spectrum antibiotic, which has a good inhibitory effect on both gram-positive and gram-negative bacteria. It has a good therapeutic effect on infections caused by typhoid, dysentery, Escherichia coli, influenza, Brucella and pneumococcus.

Chloramphenicol at BOC Sciences

Chloramphenicol structure

Chloramphenicol is one of the rare natural compounds which carry a nitro group. Nevertheless, the molecule has a structure which is one of the simplest of the known antibiotics. Subsequently, a large number of chloramphenicol derivatives was prepared, which exceeds more than 500 compounds. The molecule of chloramphenicol is composed of three parts: (I) a p-nitrobenzene moiety, (II) a dichloracetyl moiety, and (III) a 2-amino-propanediol moiety. In more general terms, part I represents an aromatic ring system and part II an aliphatic halo acetyl side-chain. The propanediol moiety possesses two asymmetric carbon atoms. Accordingly, four stereoisomers of chloramphenicol theoretically exist. Two of these, the D-threo and the L-threo enantiomers, carry the amide sidechain (part II) and the hydroxyl on carbon 1 on opposite sides of the plane of the two asymmetric centers. The other two stereoisomers, the D-erythro and L-erythro enantiomers, carry the two substituents on the same side of the plane of the two asymmetric centers.

Chloramphenicol itself is the D-threo isomer, which is the only isomer with strong bacteriostatic properties. The absolute stereospecificity of chloramphenicol's action also applies to active derivatives of the antibiotic. The aromatic ring system and the acyl side-chain of chloramphenicol allow a wide variety of substitutions without loss of bacteriostatic potency of the drug.

Chloramphenicol mechanism of action (MOA)

Ribosomal binding

Chloramphenicol targets the bacterial ribosome, specifically the 50S ribosomal subunit. The binding site of chloramphenicol is located near the peptidyl transferase center (PTC), a crucial region for peptide bond formation between amino acids during protein synthesis. Chloramphenicol binds reversibly to the 23S rRNA of the 50S subunit, interfering with the proper positioning of the aminoacyl-tRNA necessary for peptide bond formation.

Inhibition of peptidyl transferase activity

The primary action of chloramphenicol is the inhibition of peptidyl transferase activity. Peptidyl transferase is an enzymatic function of the ribosome responsible for catalyzing the transfer of the growing polypeptide chain from the peptidyl-tRNA in the P site to the aminoacyl-tRNA in the A site. By binding to the ribosome, chloramphenicol obstructs this catalytic process, leading to the cessation of protein elongation and ultimately resulting in the inhibition of bacterial growth.

Effects on protein synthesis

The inhibition of peptidyl transferase activity by chloramphenicol results in the stalling of ribosomal function and a halt in protein synthesis. This inhibition is bacteriostatic rather than bactericidal, meaning it prevents bacterial proliferation but does not directly kill the bacterial cells. Consequently, chloramphenicol is particularly effective in controlling bacterial infections by allowing the host's immune system to clear the infection.

Chloramphenicol uses

Typhoid fever

Chloramphenicol was historically the first-line treatment for typhoid fever caused by Salmonella typhi. Its ability to penetrate tissues and cells effectively made it particularly useful in eradicating the intracellular bacteria responsible for typhoid fever. Despite the emergence of antibiotic resistance and the development of newer antibiotics, chloramphenicol remains a valuable option in regions where resistance to other antibiotics is prevalent or in patients who cannot tolerate other treatments.

Bacterial meningitis

Chloramphenicol's ability to cross the blood-brain barrier makes it an essential drug for treating bacterial meningitis, especially in cases caused by Haemophilus influenzae, Neisseria meningitidis, and Streptococcus pneumoniae. In developing countries or resource-limited settings, chloramphenicol is often used as a cost-effective alternative to more expensive antibiotics like third-generation cephalosporins.

Rickettsial infections

Rickettsial infections, including Rocky Mountain spotted fever and typhus, respond well to chloramphenicol. Its effectiveness in treating these infections is due to its intracellular activity, which targets the obligate intracellular pathogens responsible for rickettsial diseases. Chloramphenicol remains a recommended treatment option, particularly in areas where tetracyclines are less available or contraindicated.

Eye infections

Chloramphenicol is widely used in ophthalmology for treating bacterial conjunctivitis and other superficial eye infections. Its broad-spectrum activity makes it effective against common ocular pathogens, including Staphylococcus aureus, Streptococcus pneumoniae, and Haemophilus influenzae. Chloramphenicol eye drops and ointments are frequently prescribed due to their efficacy and low systemic absorption, minimizing the risk of systemic side effects.

Anaerobic infections

Chloramphenicol is effective against anaerobic bacteria, such as Bacteroides fragilis. Its use is particularly relevant in mixed infections involving anaerobes, such as intra-abdominal infections, abscesses, and severe dental infections. The drug's ability to penetrate abscesses and its broad-spectrum activity make it a valuable option in these complex clinical scenarios.

Antibiotic resistance studies

Chloramphenicol is extensively used in research to study mechanisms of antibiotic resistance. Its use in laboratory settings helps elucidate the genetic and biochemical pathways that bacteria employ to resist antibiotics. Research on chloramphenicol resistance often focuses on the role of chloramphenicol acetyltransferase (CAT), an enzyme that inactivates the antibiotic through acetylation.

Molecular biology

In molecular biology, chloramphenicol is used as a selective agent in bacterial culture. It is particularly useful in plasmid maintenance where chloramphenicol resistance genes are incorporated into plasmid vectors. This allows for the selection of bacteria that have successfully taken up the plasmid, facilitating genetic manipulation and cloning experiments.

Chloramphenicol as an antibiotic

Chloramphenicol is classified as a broad-spectrum antibiotic. Its ability to inhibit protein synthesis makes it effective against a wide array of bacteria, including:

• Gram-positive Bacteria: Streptococcus pneumoniae, Staphylococcus aureus.
• Gram-negative Bacteria: Haemophilus influenzae, Escherichia coli, Salmonella spp.
• Anaerobic Bacteria: Bacteroides fragilis.
• Other Pathogens: Effective against Chlamydia, Mycoplasma, and Rickettsia.

Chloramphenicol derivatives and analogues

The development of chloramphenicol derivatives, such as thiamphenicol and florfenicol, has expanded the utility of this antibiotic class. These derivatives are designed to retain the efficacy of chloramphenicol while addressing issues related to resistance and toxicity. Thiamphenicol, for instance, has a methylsulfonyl group replacing the nitro group, which enhances its safety profile. Florfenicol, used extensively in veterinary medicine, is resistant to degradation by chloramphenicol acetyltransferase, making it effective against resistant strains.

References

1. Schmeing, T. M.; Ramakrishnan, V. What recent ribosome structures have revealed about the mechanism of translation. Nature. 2009, 461(7268): 1234-1242.
2. Pongs, O. Chloramphenicol. In Mechanism of action of antibacterial agents. Berlin, Heidelberg: Springer Berlin Heidelberg. 1979: 26-42.