What organisms is Linezolid (oxazolidinone antibiotic) effective against?

Linezolid Antimicrobial Coverage

Linezolid is a broad-spectrum oxazolidinone antibiotic that covers aerobic Gram-positive bacteria, including multidrug-resistant strains such as MRSA, vancomycin-resistant enterococci (VRE), and penicillin-resistant Streptococcus pneumoniae, with additional activity against select anaerobic bacteria. 1

Primary Gram-Positive Coverage

Staphylococcus Species

• Methicillin-resistant Staphylococcus aureus (MRSA): Linezolid demonstrates excellent activity and is FDA-approved for MRSA infections, including both nosocomial and community-acquired strains 2, 1
• Methicillin-susceptible S. aureus (MSSA): Full coverage with proven clinical efficacy 1, 3
• Vancomycin-intermediate S. aureus (VISA): Maintains in vitro activity against these resistant strains 2, 3
• Vancomycin-resistant S. aureus (VRSA): Active against these highly resistant organisms 2

Enterococcus Species

• Vancomycin-resistant Enterococcus faecium (VRE): FDA-approved indication with demonstrated clinical efficacy 1, 3
• Enterococcus faecalis: Effective coverage including vancomycin-resistant strains 3, 4

Streptococcus Species

• Streptococcus pneumoniae: Covers all strains including multidrug-resistant isolates (MDRSP) resistant to penicillin, second-generation cephalosporins, macrolides, tetracycline, and trimethoprim/sulfamethoxazole 2, 1
• Streptococcus pyogenes (Group A Streptococcus): Full coverage with FDA approval for uncomplicated skin infections 1
• Streptococcus agalactiae (Group B Streptococcus): Documented activity 1
• Linezolid demonstrates bactericidal activity against most streptococci, unlike its bacteriostatic effect against staphylococci and enterococci 1

Anaerobic Coverage

Linezolid has activity against select anaerobic organisms, though this is not its primary indication 1, 3:

• Clostridium perfringens: Active 3
• Clostridium difficile: Active 3
• Peptostreptococcus species: Active 3
• Bacteroides fragilis: Some activity documented 3

Important Limitations

Organisms NOT Covered

• Gram-negative bacteria: Linezolid lacks clinically useful activity against aerobic Gram-negative organisms including Pseudomonas aeruginosa, Enterobacteriaceae, and Haemophilus influenzae 1
• Atypical pathogens: No established activity against Mycoplasma pneumoniae, Chlamydia pneumoniae, or Legionella species 2

Resistance Considerations

• Resistance develops through point mutations in 23S ribosomal RNA at position 2576 1
• Resistance has been documented in Enterococcus faecium (particularly in patients with unremoved prosthetic devices or undrained abscesses) and rarely in S. aureus during clinical use 2, 1
• Cross-resistance with other antibiotic classes is unlikely due to linezolid's unique mechanism of action (inhibition of 70S initiation complex formation) 1, 4
• No resistance has been reported in Streptococcus species to date 1

Clinical Pearls

The Infectious Diseases Society of America recommends linezolid as first-line therapy for MRSA skin and soft-tissue infections (Grade 1A recommendation) and as an alternative to vancomycin for nosocomial pneumonia caused by MRSA. 2

• Linezolid achieves excellent tissue penetration, including into biofilms and poorly vascularized tissues such as the prostate 2, 5
• The oral formulation has 100% bioavailability, allowing seamless IV-to-oral transition when patients can tolerate oral medications 2, 1
• Reserve linezolid for documented serious infections caused by resistant Gram-positive organisms to minimize resistance development 4, 6