Linezolid - Spectrum of Activity

Spectrum of Activity

Scanning electron micrographs of vancomycin-resistant Enterococcus (top) and methicillin-resistant Staphylococcus aureus (bottom; false colors)

Linezolid is effective against all clinically important Gram-positive bacteria—those whose cell wall contains a thick layer of peptidoglycan and no outer membrane—notably Enterococcus faecium and Enterococcus faecalis (including vancomycin-resistant enterococci), Staphylococcus aureus (including methicillin-resistant Staphylococcus aureus, MRSA), Streptococcus agalactiae, Streptococcus pneumoniae, Streptococcus pyogenes, the viridans group streptococci, Listeria monocytogenes, and Corynebacterium species (the latter being among the most susceptible to linezolid, with minimum inhibitory concentrations routinely below 0.5 mg/L). Linezolid is also highly active in vitro against several mycobacteria. It appears to be very effective against Nocardia, but because of high cost and potentially serious adverse effects, authors have recommended that it be combined with other antibiotics or reserved for cases that have failed traditional treatment.

Linezolid is considered bacteriostatic against most organisms—that is, it stops their growth and reproduction without actually killing them—but has some bactericidal (killing) activity against streptococci. Some authors have noted that, despite its bacteriostatic effect in vitro, linezolid "behaves" as a bactericidal antibiotic in vivo because it inhibits the production of toxins by staphylococci and streptococci. It also has a post-antibiotic effect lasting one to four hours for most bacteria, meaning that bacterial growth is temporarily suppressed even after the drug is discontinued.