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Empirical antibacterial therapy in keratitis is based on the likely pathogen, the available drugs, and the severity of the condition.1 Ongoing treatment is modified by clinical response and the result of initial microbial investigations.2
There is a large degree of variation in organism type and their resistance patterns from centre to centre, and so local contemporaneous data are essential to make a rational choice of initial antibiotic therapy. Reference centres specialising in corneal disease with good microbiological backup have a key role in analysing local trends and disseminating their results. Ongoing audit is also required, as patterns of resistance are invariably changing.3
Overall, there are some common pathogens that cause acute bacterial keratitis. Streptococcal species, staphylococcal species, pseudomonas and enterobacteriaceae make up the four most common classes of infective agents and any empirical therapy has to cover all these groups.
STREPTOCOCCAL SPECIES
There have been some concerns about the lack of in vitro efficacy by fluoroquinolone in the streptococcal species.4,5 In practical terms, this has not proved to be a common problem to date in the United Kingdom and Australia.6 In vitro resistance is based on the MIC90 and assumes drug levels found following intravenous infusion. However, topical administration can produce far higher tissue levels in the cornea than those obtained by intravenous administration and can be effective in local control of streptococcal keratitis. That being said, there will be occasions where the MIC90 will be at such a level that topical administration cannot control the disease and we have certainly had clinical experience of resistant streptococci. Newer methods to assess in vitro sensitivity other than the Kirby-Bauer test that can provide a more clinically relevant definition of resistance need to be introduced.
Penicillin or cephalosporins have been …