Gentamicin Coverage Spectrum
Gentamicin is primarily effective against Gram-negative bacteria including Pseudomonas aeruginosa, Enterobacteriaceae species (Escherichia coli, Klebsiella, Enterobacter, Serratia, Citrobacter), and Proteus species, as well as some Gram-positive bacteria like Staphylococcus species. 1
Gram-Negative Coverage
Gentamicin demonstrates excellent activity against:
- Pseudomonas aeruginosa 1
- Escherichia coli 1
- Klebsiella species 1
- Enterobacter species 1
- Serratia species 1
- Citrobacter species 1
- Proteus species (both indole-positive and indole-negative) 1
Gram-Positive Coverage
Gentamicin has more limited activity against Gram-positive organisms:
- Staphylococcus species (both coagulase-positive and coagulase-negative) 1
- When combined with penicillin or ampicillin, gentamicin demonstrates synergistic activity against certain strains of Enterococcus faecalis, E. faecium, and E. durans 1
Important Limitations
Gentamicin is generally NOT effective against:
- Most streptococcal species (including Streptococcus pneumoniae) 1
- Most enterococcal species when used as monotherapy 1
- Group D streptococci (unless combined with penicillin) 1, 2
- Anaerobic organisms (Bacteroides species and Clostridium species) 1
- Salmonella and Shigella species (despite appearing active in vitro) 1
Clinical Applications
Gentamicin is indicated for serious infections caused by susceptible organisms including:
- Respiratory tract infections 1, 3
- Urinary tract infections 1, 3
- Bacteremia and septicemia 1, 3
- Skin and soft tissue infections 1, 2
- Bone and joint infections 1, 3
- Intra-abdominal infections including peritonitis 1
- Bacterial meningitis 1
- Infective endocarditis (in combination with other antibiotics) 2, 1
Resistance Patterns
- Resistance to gentamicin generally develops slowly but varies by region 1
- In low and lower-middle-income countries, gentamicin resistance rates range from 42% to 70% among common Gram-negative pathogens 2
- Resistance is particularly concerning in neonatal sepsis caused by Gram-negative bacteria 2
Synergistic Effects
Gentamicin demonstrates synergistic effects when combined with:
- Penicillins against enterococcal strains 1
- Carbenicillin against Pseudomonas aeruginosa 1
- Beta-lactams against many Gram-negative organisms 1
- Penicillin for treatment of infective endocarditis caused by group D streptococci 1
Pharmacodynamic Considerations
- Gentamicin exhibits concentration-dependent killing against Gram-negative bacteria like Pseudomonas aeruginosa 4
- Against Staphylococcus aureus, gentamicin demonstrates partially concentration-dependent killing, with concentrations beyond 4-8 times the MIC not resulting in significantly faster bacterial killing 4
- The antibacterial activity of gentamicin can be inhibited in acidic, concentrated urine, potentially requiring higher doses for urinary tract infections 5
Monitoring Considerations
- Therapeutic drug monitoring (TDM) can optimize dosing regimens, improve treatment efficacy, and reduce nephrotoxicity 2
- TDM-guided gentamicin treatment has been associated with shorter hospital stays, lower mortality rates, and lower incidence of nephrotoxicity compared to non-TDM-guided treatment 2
Understanding gentamicin's spectrum of activity is essential for appropriate clinical use, particularly given concerns about antimicrobial resistance and potential toxicity.