Toward improving therapeutic regimens for Bacillus endophthalmitis

Abstract

Purpose: Bacillus cereus causes the most virulent and refractory form of endophthalmitis. The authors analyzed the effectiveness of early treatment with vancomycin or gatifloxacin, with or without dexamethasone, for experimental B. cereus endophthalmitis.

Methods: Rabbit eyes were injected intravitreally with 100 colony-forming units of B. cereus. At 2, 4, or 6 hours after infection, eyes were injected intravitreally with 0.1 mL gatifloxacin (0.3%), vancomycin (1.0%), either antibiotic plus dexamethasone, dexamethasone alone (1.0%), or PBS. Eyes were analyzed by electroretinography, bacterial quantitation, and antibiotic penetration analysis. Drug toxicity toward Müller cells, retinal pigment epithelium, and cones was also analyzed.

Results: Eyes treated at 2 hours with vancomycin or gatifloxacin, with or without dexamethasone, maintained higher ERG amplitudes than the dexamethasone alone and PBS control groups. Eyes treated with antibiotic plus dexamethasone at 6 hours had reduced retinal function compared to antibiotic treatment alone. With the exception of vancomycin with or without dexamethasone at 6 hours, all antibiotic treatments sterilized eyes. Only gatifloxacin reached aqueous concentrations greater than the minimal inhibitory concentration for B. cereus when measured at 8 hours. Neither gatifloxacin nor vancomycin was toxic to retinal cells in vitro.

Conclusions: Early intravitreal injection of vancomycin or gatifloxacin improved the therapeutic outcome of B. cereus endophthalmitis. The addition of dexamethasone to antibiotic treatment did not provide a therapeutic benefit over antibiotics alone and appeared to reduce the antibiotic efficacy of vancomycin 6 hours after infection. In this model, delay in treatment past 6 hours significantly reduced the potential for salvaging useful vision.

Figure 1. Quantitation of CFU in the vitreous following treatment of experimental B. cereus endophthalmitis

Rabbit eyes were injected with 100 CFU B. cereus. Eyes were injected intravitreally with gatifloxacin, vancomycin, those antibiotics with dexamethasone, PBS, or were left untreated. Vitreal CFU counts were determined at 8 h postinfection following intravitreal treatment at either 2, 4, or 6 h postinfection. Eyes treated with dexamethasone or PBS were analyzed at 8 h postinfection following intravitreal treatment at 2 h only. Values represent the mean ± SEM of N≥4 eyes per group, *P>0.05.

Figure 2. Retinal function analysis following treatment of experimental B. cereus endophthalmitis

Rabbit eyes were injected with 100 CFU B. cereus. Eyes were injected intravitreally with gatifloxacin, vancomycin, those antibiotics with dexamethasone, PBS, or were left untreated. Scotopic ERGs were performed and A- and B-wave amplitudes recorded at 8 h postinfection following treatment at either 2, 4, or 6 h postinfection. Eyes treated with dexamethasone or PBS were analyzed at 8 h postinfection following intravitreal treatment at 2 h only. Values represent the mean ± SEM of N≥4 eyes per group, *P<0.05.

Figure 3. Histology following treatment of experimental B. cereus endophthalmitis

Rabbit eyes were injected with 100 CFU B. cereus. Eyes were injected intravitreally with gatifloxacin (Gati), vancomycin (Vanc), those antibiotics with dexamethasone (Dex), PBS, or were left untreated (Uninj). Eyes treated with dexamethasone or PBS were analyzed at 8 h postinfection following intravitreal treatment at 2 h only. Eyes were enucleated and fixed in 10% formalin for 24 h. The eyes were sectioned and stained with hematoxylin and eosin by standard procedures.

Figure 4. Intraocular antibiotic penetration following treatment of B. cereus endophthalmitis

Rabbit eyes were injected with 100 CFU B. cereus. Eyes were injected intravitreally with gatifloxacin (Gati), vancomycin (Vanc), those antibiotics with dexamethasone (Dex), PBS, or were left untreated. [A] For eyes treated with gatifloxacin or vancomycin alone at 2, 4, or 6 h postinfection, antibiotic concentrations were analyzed at 4, 6, or 8 h postinfection. *P<0.05 compared with the initial time point in each treatment group, **P<0.05 compared with initial time point of treatment at 2 h postinfection. [B] Eyes were treated with antibiotic/dexamethasone combinations at 2 h, and antibiotic concentrations in the vitreous and aqueous humor were quantified at 8 h postinfection. *P<0.05 compared with vancomycin treatment alone at 2 h postinfection. Values represent the mean ± SEM of N≥4 eyes per group.

Figure 5. Retinal cell drug toxicity

The effect of gatifloxacin, vancomycin, and dexamethasone on Müller cells, cone photoreceptors, and RPE cells was analyzed in vitro. Antibiotics or dexamethasone was added to confluent cell monolayers at the identical final concentrations used for intravitreal injection. LDH was measured in culture supernatants at 0, 2, 4, 6 and 8 h. Percent LDH release was calculated based on 100% lysis (freeze-thaw) controls. Values represent the mean ± SEM of N=6 per group.