Antimicrobial resistance trends in bacterial keratitis over 5 years in Sydney, Australia

Importance Antimicrobial resistance (AMR) patterns in bacterial keratitis may fluctuate in a geographic location over time. Background To investigate any change in AMR patterns in Sydney, Australia. Design Retrospective case series. Participants All patients with microbial keratitis who underwent a corneal scrape and culture from 2012 to 2016 at the Sydney Eye Hospital. Methods Matrix‐assisted laser desorption ionization‐time of flight (MALDI‐TOF) mass spectrometry identified organisms. The Calibrated Dichotomous Susceptibility method determined antibiotic susceptibilities. Main Outcome Measures Isolated organisms and antibiotic susceptibilities. Results There were 1084 corneal scrapes from 957 patients. The mean age was 54 years (range 18‐100) and 52% were male. Cultures were positive in 711 of 1084 scrapes (66%), with 884 organisms identified. Of the bacteria isolated, 685 of 884 (78%) were Gram‐positive and 199 of 884 (22%) were Gram‐negative. Overall, the most common bacteria were coagulase‐negative Staphylococci (CoNS) (405/884, 46%). Methicillin‐resistance was detected in 7% of Staphylococcus aureus isolates (7/103). Methicillin‐resistance in CoNS (ie, also cefalotin resistance) was reported in 19% of isolates and ciprofloxacin 8%. For methicillin‐sensitive S aureus (MSSA), 5% of isolates were resistant to ciprofloxacin. For Corynebacterium spp., 34% of isolates were resistant to chloramphenicol and 9% to ciprofloxacin. The most common Gram‐negative bacteria was Pseudomonas aeruginosa (109/199, 55%). One case was resistant to ciprofloxacin. Conclusions and Relevance Coagulase‐negative staphylococcal species were the most frequently suspected of causing bacterial keratitis. Increased resistance to cefalotin was identified for CoNS and to ciprofloxacin for Corynebacterium spp., MSSA and P aeruginosa compared to a previous study in Sydney in 2002 to 2003.