Abstract
Purpose :
Systemic doxycycline has been prescribed to enhance corneal healing in the setting of bacterial keratitis. Topical autologous serum drops containing doxycycline via oral supplementation may additionally confer an anti-bacterial effect. The potential of this treatment supplementation was evaluated by determining the in vitro susceptibility of bacterial keratitis isolates to doxycycline.
Methods :
The minimum inhibitory concentrations (MICs) to doxycycline of 100 bacterial keratitis isolates submitted to the Charles T. Campbell Ophthalmic Microbiology Laboratory at the University of Pittsburgh Medical Center were determined using Etests. Twenty-seven (27%) Staphylococcus aureus, 10 (10%) coagulase-negative Staphylococci, 6 (6%) Streptococcus pneumoniae, 7 (7%) Streptococcus viridans group, 7 (7%) other Gram-positive bacteria, 19 (19%) Pseudomonas aeruginosa, 8 (8%) Serratia marcescens, 4 (4%) Moraxella spp., 2 (2%) Haemophilus spp., and 10 (10%) other Gram-negative bacteria isolates were tested. MICs to doxycycline were compared to a serum standard concentration of doxycycline (SD) of 4 µg/ml as well as concentrations of doxycycline in serum drops at 50% SD (2 µg/ml) and 20% SD (0.8 µg/ml) as would be found in clinical preparations of autologous serum 50% and 20% drops, respectively. MICs equal or less than these values were deemed susceptible.
Results :
For Gram-positive bacteria, susceptibilities to SD, 50% SD, and 20% SD were respectively, 86%, 65%, and 60%. For Staphylococcus aureus, susceptibilities to SD, 50% SD, and 20% SD were respectively, 92.6%, 85.2%, and 77.8%. For Gram-negative bacteria, susceptibilities to SD, 50% SD, and 20% SD were respectively, 37.2%, 23.3%, and 11.6%. Pseudomonas aeruginosa tested resistant to doxycycline at all concentrations. For the composite dataset of bacterial keratitis isolates, susceptibilities to SD, 50% SD, and 20% SD were respectively, 65%, 47%, and 39%. Chi-squared analyses comparing Gram-positive and Gram-negative susceptibilities showed significantly greater susceptibility of Gram-positive bacteria at all three tested MICs (<.0001, <.0001, <.0001).
Conclusions :
Our data suggests that serum drops containing doxycycline may have a select advantage for treating Gram-positive bacterial keratitis, especially Staphylococcus aureus, over Gram-negative bacterial keratitis.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.