Ocular Microbiology and Immunology Group
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POSTER PRESENTATION

Vancomycin is Poorly Active Against Staphylococcus Aureus Biofilms Formed by Endophthalmitis Isolates

Lucas Sejournet, Laurent Kodjikian, Paulo J.M. Bispo

Department of Ophthalmology, Massachusetts Eye and Ear, Harvard Medical School, Boston, Massachusetts

Purpose: The aim of this study was to evaluate the biofilm formation capabilities of Staphylococcus aureus isolates from endophthalmitis, and to determine the efficacy of vancomycin in eradicating S. aureus biofilms grown on polystyrene plates and on hydrophobic acrylic intraocular lenses (IOL).

Methods: 44 clinical S. aureus isolates were obtained from patients with endophthalmitis and screened for their ability to form biofilms in vitro using 96-well polystyrene plates precoated with porcine vitreous at 4°C overnight. Biofilms were grown for 24 hours in TSB without supplementation and bacterial biomass measured using crystal violet staining. The minimum inhibitory concentration (MIC) of vancomycin was determined by broth microdilution. The minimum biofilm eradication concentration (MBEC) was determined for mature biofilms (grown for 24 hours) and was defined as the concentration at which no bacteria survived following 24 hours exposure to vancomycin. Four strains shown to have different degrees of biofilm formation were selected for IOL experiments. IOLs were pre-soaked in porcine vitreous overnight at 4°C before biofilms were grown for 24 hours in TSB. Bacterial biomass on IOLs was assessed by quantitative culture.

Results: According to the biomass measured by crystal violet staining, we divided the population of S. aureus isolates into four groups: low (22.7%), moderate (31.8%), strong (27.2%) and very strong (18.3%) biofilm formers. ATCC 25923 was used as a positive control for group classification. Both methicillin-resistant and methicillin-susceptible isolates were susceptible to vancomycin (MIC <2 µg/mL) when growing planktonically. However, the vancomycin MBECs (>2,048 µg/mL) were above the therapeutic dose for all isolates. Vancomycin failed to completely eradicate these biofilms when grown on IOLs, with a log reduction ranging from 0 to 1.05 following 24h exposure at a concentration close to that used to treat endophthalmitis (1 mg/0.5 mL).

Conclusions: S. aureus isolated from endophthalmitis are highly capable of forming biofilms on abiotic surfaces coated with vitreous proteins, with nearly half of them forming strong to very strong biofilms. Despite the overall S. aureus susceptibility to vancomycin when growing as planktonic cells, therapeutic concentrations were insufficient to eradicate biofilms grown on IOLs. These findings highlight the potential need for an alternative or adjunctive therapy for biofilm-associated S. aureus endophthalmitis.