Macrolides -- Erythromycin, Clarithromycin, Azithromycin

Mechanism of Action -- These agents are bacteriostatic.  They act by binding to the 50S subunit of bacterial ribosomes (refer to figure in previous section).  Specifically, they bind to the site at which tRNA transferase acts, thus inhibiting the transfer of an amino acid to the forming protein.  Therefore, bacterial protein synthesis is inhibited. Refer to the diagram in the AG section.

Spectrum of Activity -- The macrolides are most effective against Gram positive cocci and bacilli including strep.  They are also effective against Clostridium perfringens (gas gangrene) and Clostridium tetani and the Gram negative N. gonorrhœa.  They exhibit modest activity against some Gram negative organisms such as H. flu.  They are also effective against Pasteurella spp., Bordetella pertussis, Borrelia spp., Legionella spp., and Corynebacterium spp.  Clarithromycin exhibits better activity against erythromycin-resistant strains and azithromycin has more Gram negative bacteria including Borrelia, Chlamydia, and Legionella spp.  The two newer agents are also effective against Toxoplasma gondii.

Therapeutics -- The macrolides are used for many infections including legionnaire's disease; diphtheria; pertussis; staph and strep OM, RTI; tetanus, gangrene, toxoplasmosis, and acne.

Adverse Effects -- The primary side effect of the macrolides is GI upset.  They may also cause hypersensitivity reactions and in toxic effects, may produce cholestatic jaundice.

Drug Interactions -- The macrolides alone may cause a lengthening of the QT interval with subsequent torsade de pointes.  This action is magnified with the co-administration of an non-sedating antihistaminic.  They also inhibit the cytochrome P₄₅₀ system, which will enhance the elimination of any drug metabolised by that system.

Mechanism of Action -- Although structurally unrelated, these agents act at the same site as the macrolide antibiotics.

Spectrum -- Similar to the macrolides plus Staphylococcus (NOT methicillin resistant staph, however).  They are more active against anærobic organisms including Bacillus spp., and Clostridium perfringens (NOTE that they are NOT effective against Cl. difficile and actually may cause pseudomembranous colitis quite readily).  Additionally, they are effective against Pneumocystic carnii and Toxoplasma gondii.

Therapeutics -- Gangrene, acne, toxoplasmosis, pneumocystis pneuomonia.

Adverse Effects -- Diarrhea, pseudomembranous colitis, and rashes, including Stevens-Johnson type.

Mechanism of Action -- Again, structurally unrelated to the macrolides, but acts at the same site in a similar manner.

Spectrum -- Relatively broad spectrum including H. flu, Neisseria, Salmonella, Bacteroides, Brucella, Bordetella, Clostridium, Streptococcus, E. coli, Vibrio, Shigella, and less activity against Klebsiella and Enterobacter.

Therapeutics -- Typhoid fever, bacterial meningitis, anaerobic infections, rickettsia (less effective that TCNs) and brucellosis (less effective that TCNs).

Adverse Effects

Most Common -- Nausea, vomiting, diarrhœa, unpleasant taste, perianal irritation

Relatively Common -- Dose dependent bone marrow suppression causing anæmia, thrombocytopænia, and leukopænia and an idiosyncratic aplastic anæmia.  These effects often limit the use of chloramphenicol to "must" situations (when no other antibiotic is effective).

Gray Baby Syndrome -- Overdose, especially in infants, has caused vomiting, decreased suckling reflex, increase respirations, abdominal distension, cyanosis, and loose green stools.  This is due to the inability of the neonate to conjugate the antibiotic and to excrete un-conjugated antibiotic.  The condition can be lethal.  Overdose may cause a similar reaction in adults, although this is rare.

Rare -- Visual blurring, digital paræstesias.

Mechanism of Action -- Muporicin decreases bacterial protein synthesis by inhibition of isoleucyl-tRNA synthetase.

Spectrum -- Primarily Gram positive, especially staph and strep.

Therapeutics -- It is only used topically for the treatment of superficial infections such as impetigo.