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Gentamicin-resistant pseudomonal infection. Rationale for a redefinition of ophthalmic antimicrobial sensitivities
L D Ormerod1, P N Heseltine, E Alfonso
1Massachusetts Eye and Ear Infirmary, Department of Ophthalmology, Harvard Medical School, Boston.
Abstract:
Eight pseudomonal species were involved in 106 invasive infections of the eye; all were community acquired. Eighteen percent of the total and 9% of the Pseudomonas aeruginosa strains were gentamicin resistant, as defined using conventional criteria. All 10 cases of "resistant" pseudomonal (nine P. aeruginosa) keratitis responded satisfactorily to treatment with gentamicin. The resistance breakpoint (defined by safe serum levels in parenteral therapy) for most P. aeruginosa is much lower than ocular gentamicin levels achievable by optimal local application. We argue for a specific ophthalmologic definition of antibiotic resistance in infections of the cornea and external eye. MIC quantitative determinations of ocular isolates would provide more useful information to ophthalmologists than conventional qualitative disc sensitivity testing.
Insights
Gentamicin resistance in eye infections is common, but conventional criteria may overestimate it. Ocular gentamicin therapy remains effective, suggesting a need for specific ophthalmologic resistance definitions.
Area of Science:
- Ophthalmology
- Microbiology
- Infectious Diseases
Background:
- Pseudomonas species are common causes of community-acquired invasive eye infections.
- Gentamicin resistance is a growing concern in bacterial infections.
Observation:
- Eight pseudomonal species caused 106 invasive eye infections, all community-acquired.
- Eighteen percent of all pseudomonal strains and 9% of Pseudomonas aeruginosa strains showed gentamicin resistance by conventional criteria.
- All 10 cases of resistant pseudomonal keratitis, predominantly P. aeruginosa, responded well to gentamicin treatment.
Findings:
- Conventional gentamicin resistance breakpoints, based on systemic therapy, do not accurately reflect efficacy in ocular infections.
- Achievable gentamicin concentrations in the eye via local application exceed systemic levels, potentially overcoming conventional resistance.
- Microbiological Minimum Inhibitory Concentration (MIC) testing offers more relevant data for ophthalmologists than standard disc diffusion tests.
Implications:
- A distinct definition of antibiotic resistance for ophthalmologic infections is warranted.
- Current resistance criteria may lead to unnecessary avoidance of effective topical gentamicin therapy.
- Ophthalmologists should consider MIC values for a more accurate assessment of gentamicin efficacy in eye infections.