Gentamicin Coverage in Bacterial Keratitis
Gentamicin primarily covers gram-negative bacteria, particularly Pseudomonas aeruginosa, along with some gram-positive organisms including Staphylococcus species, making it a critical component of empiric therapy for contact lens-related bacterial keratitis. 1
Spectrum of Coverage
Gram-Negative Organisms (Primary Coverage)
Gentamicin demonstrates robust activity against the following gram-negative bacteria that commonly cause bacterial keratitis in contact lens wearers:
- Pseudomonas aeruginosa - the most common gram-negative pathogen in contact lens-associated keratitis 2, 1
- Escherichia coli 1
- Klebsiella pneumoniae 1
- Enterobacter aerogenes 1
- Haemophilus influenzae 1
- Serratia marcescens 1
- Neisseria gonorrhoeae 1
Gram-Positive Organisms (Secondary Coverage)
Gentamicin also covers several gram-positive bacteria, though resistance patterns are increasingly problematic:
- Staphylococcus aureus 1
- Staphylococcus epidermidis 1
- Streptococcus pyogenes 1
- Streptococcus pneumoniae 1
Critical Clinical Context for Contact Lens Wearers
Why Pseudomonas Coverage Matters
In contact lens-related bacterial keratitis, Pseudomonas aeruginosa is the most common causative organism, particularly in cases involving water exposure (swimming, hot tubs, tap water contamination of lens cases) 2, 3. Gentamicin's reliable activity against Pseudomonas makes it a cornerstone of fortified antibiotic regimens 3.
Important Resistance Caveat
Staphylococcal resistance to gentamicin is increasingly common and can be severe, with some strains showing minimum inhibitory concentrations exceeding levels achievable with topical application 4. When staphylococcal keratitis is suspected, a cephalosporin or vancomycin should be included alongside gentamicin to ensure adequate gram-positive coverage 4.
Practical Treatment Implications
When Gentamicin Is Used
Gentamicin is typically employed as part of fortified antibiotic therapy (often combined with a cephalosporin or vancomycin) for:
- Large or central corneal infiltrates 5
- Presence of hypopyon 5
- Severe bacterial keratitis requiring broad-spectrum coverage 3
Efficacy Data
Research demonstrates that gentamicin remains highly effective against Pseudomonas in contact lens-related keratitis, with most isolates showing sensitivity 3. However, geographic variation exists, particularly in the Middle East where resistance patterns may differ 3.
Combination Therapy Rationale
Because bacterial keratitis in contact lens wearers can involve both gram-negative organisms (especially Pseudomonas) and gram-positive organisms (especially Staphylococcus species), and because 43% of positive cultures yield multiple organisms 2, gentamicin is typically combined with an agent providing robust gram-positive coverage to ensure comprehensive empiric treatment while awaiting culture results 4.