Risk factors and causative organisms in microbial keratitis.

PURPOSE: To establish the risk factors, causative organisms, levels of antibiotic resistance, patient demographics, clinical presentations, and clinical outcomes of microbial keratitis at a tertiary hospital in Australia.
METHODS: Patients who had a corneal scraping for culture over a 5-year period were identified through the local microbiology database, and a retrospective audit of their medical records was carried out. Clinical information was gathered from medical records, and smear, culture, and antibiotic resistance results were from the microbiology database. An index of disease severity was calculated for each patient from scores for the magnitude of the epithelial defect and anterior-chamber reaction and the location of the lesion. Associations between risk factors for keratitis and variables such as patient demographics, causative organism and antibiotic resistance, disease severity, and outcome were analyzed by using analysis of variance and chi tests with appropriate correction for multiple comparisons.
RESULTS: Two hundred fifty-three cases of microbial keratitis in 231 patients were included. Sixty percent of patients were men, and there was a bimodal distribution in the age of presentation. Common risk factors for keratitis were contact lens wear (53; 22%), ocular surface disease (45; 18%), ocular trauma (41; 16%), and prior ocular surgery (28; 11%). Gram stains were positive in 33%, with a sensitivity of 53% and specificity of 89%. Cultures of corneal scrapings were positive in 65% of cases, and Pseudomonas aeruginosa (44; 17%), coagulase-negative staphylococci (22; 9%), Staphylococcus aureus (19; 8%), and fungi (7; 3%) were commonly recovered. P. aeruginosa was more common than other culture results in contact lens-related cases (55% vs. 0%-23%; P < 0.001), and S. aureus was more common than other culture results in ocular surgery-related cases (29% vs. 0%-21%; P < 0.001). Patients with keratitis related to prior ocular surface disease had more severe keratitis at the time of scraping (P = 0.037). Cultures positive for Fusarium, P. aeruginosa, and other Gram-negative organisms had statistically significantly more severe keratitis at the time of scraping, whereas patients with negative cultures had milder keratitis (P = 0.030). Only 2% of all bacterial isolates were resistant to ciprofloxacin, 20% of Gram-positive isolates were resistant to cephalothin, and no Gram-negative isolates were resistant to gentamicin.
CONCLUSIONS: In this series, the most common risk factor for keratitis was contact lens wear and the most commonly isolated organism was P. aeruginosa.