A 20-year Retrospective Analysis of Methicillin and Moxifloxacin Susceptibility among Staphylococcus aureus Keratitis Isolates
Eric G. Romanowski, Alex Mammen, Deepinder K. Dhaliwal, Vishal Jhanji, Hazel Q. Shanks, Robert M. Q. Shanks
The Charles T. Campbell Ophthalmic Microbiology Laboratory, Department of Ophthalmology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
Purpose: Staphylococcus aureus (SA) is one of the most common causes of bacterial keratitis. There are numerous reports of increasing antibiotic resistance among bacterial pathogens. The goal of the current study was to determine the methicillin and moxifloxacin (MOX) susceptibility patterns of SA keratitis isolates over a 20-year period.
Methods: The antibiotic susceptibility reports of total of 426 SA keratitis isolates from 7/1/2002 – 6/30/2022 were retrospectively reviewed for methicillin and MOX resistance. Susceptibilities were determined using Kirby-Bauer disk diffusion testing to oxacillin (methicillin) and MOX up to 6/30/2017. From 7/1/2017 – 6/30/2022, susceptibilities were determined using MIC testing with cefoxitin (methicillin) and MOX using MIC Test Strips (Liofilchem®).
Results: For each of the four 5-year periods, methicillin-resistant SA (MRSA) isolates comprised 36.2%, 35.7%, 41.4%, and 52.7% respectively. The most recent 5-year period saw a significant increase in the percentage of MRSA isolates compared to the first two 5-year periods (p≤0.022, Chi-square). This was the first 5-year period that MRSA isolates outnumbered methicillin-susceptible SA (MSSA) isolates. MOX-resistant (MOX-R) isolates demonstrated no significant increases over the four 5-year periods comprising 35.1%, 32.9%, 40.6%, and 32.6% of all SA isolates respectively (p=NS, Chi-square). Breaking down this data, there were no differences in the percentages of MOX-R isolates among MSSA isolates over the 5-year time periods (18.3%, 13.3%, 15.4%, 11.5%) (P=NS, Chi-square). However, among MRSA isolates there was a significant decrease in the percentage of MOX-R isolates over the last 5 years (51.5%) compared to the previous 5-year period (76.4%) (p=0.004, Chi-square). Over the entire 20-year period, MOX resistance among SA keratitis isolates is more prevalent among MRSA isolates (63.7%) than MSSA isolates (14.8%) (p<0.001, Chi-square). Among the data over the past 5 years, a disturbing trend was discovered among the MOX-resistant MRSA isolates with 46% having MOX MICs >32 µg/ml.
Conclusions: It appears the percentage of MRSA isolates has increased among SA keratitis isolates over the last 5 years while the overall percentages of MOX-R SA isolates have remained steady over 20 years. However, the percentage of highly MOX-R isolates among MRSA keratitis isolates over the past 5 years is cause for concern and may have implications for empiric therapy of suspected bacterial keratitis.
Disclosure: N, S (The Campbell Lab)