Microbial Keratitis: microbiology results of infective corneal ulcers at a tertiary Hospital in South Africa

Koetsie, Karen Monica
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Purpose: To describe the microbiology results of corneal scrapings and morphology results of corneal ulcers over a one year period at the St John Eye Hospital with the following objectives: (i) to describe the positive culture results (ii) to describe the commonest causative organisms (iii) to describe resistance patterns to antibiotics (iv) to correlate the positive culture results with the clinical characteristics of the ulcer. Methods: A retrospective cross sectional review of patient medical records and microbiology reports of patients who presented with corneal ulcers at the St John Eye Hospital between October 2007 and October 2008. One hundred and fifty one (151) corneal scrapings submitted to the National Health Laboratory Services (NHLS) for microbiology, culture and sensitivity testing were analyzed. The following information was extracted from the microbiology reports and patient medical records: patient demographics, microbial isolations, antibiotic sensitivity and resistance, and corneal ulcer morphology (central versus peripheral). Results: Of the 151 patients who had corneal scrapings, 63(42%) were female and 88(58%) were male. The median age was 39.6(range 1-95; SD 19.3). An organism was identified in 78(52%) of the samples. Of the 93 pathogens isolated, 78(83.9%) were gram positive, 10(10.8%) were gram negative, and 5(5.4%) were fungi. Mixed isolates were found in 15 of the 151 corneal scrapings. The most common gram positive isolates were Staphylococcus aureus 23(29.5%), coagulase negative Staphylococcus 18(23.1%), and Streptococcus pneumoniae 16(20.5%). The two most commonly isolated gram negative organisms were Pseudomonas aeruginosa 3(30%) and Haemophilus influenza 3(30%). A total of 5 fungi were isolated from the 151 corneal scrapings with Fusarium 3(60%) being the most common fungus isolated. Antibiotic resistance patterns were as follows: Gram positive isolates (73) consistently showed 100% sensitivity to vancomycin. A small number of gram positive organisms showed in vitro resistance to the second generation fluoroquinolone ciprofloxacin. This was, however only a small number of gram positive isolates and therefore the P value (P<0.001) remained significant. Overall the gram positives isolates showed a 95.3% sensitivity to ciprofloxacin. Both second and fourth generation fluoroquinolones, ciprofloxacin and moxifloxacin respectively, showed equivalent (100%) in vitro activity against the gram negative isolates. All gram negative isolates showed 100% laboratory susceptibility to the aminoglycosides, gentamicin and amikacin. Inpatient medical records were available for 56 of the 151 corneal ulcer scrapings. Of the 56 inpatient records reviewed 42(75%) were central ulcers. Streptococcus pneumoniae 10(23.8%) was the most common organism isolated in central corneal ulcers, while staphylococcus aureus 4(28.6%) was the most common organism isolated in peripheral corneal ulcers. Conclusion: Compared with previous reports from the St John Eye Hospital, the spectrum of causative organisms has remained unchanged over the past 25 years. The organisms commonly responsible for microbial keratitis at the hospital are significantly susceptibility to the antibiotics currently being used as therapy.