Bacitracin Inhibits the Oyster Pathogen Perkinsus marinus in Vitro and in Vivo

Abstract

The in vitro growth rates of two isolates of Perkinsus marinus were significantly reduced by bacitracin. Upon coincubation with 1 mg bacitracin/mL, the doubling times rose from 27 ± 2.1 h to 34 ± 2.9 h for the LMTX-1 isolate (P < 0.001) and from 15 ± 1.9 h to 22.2 ± 2.4 h for the Perkinsus-1 isolate (P < 0.001). At 10 mg bacitracin/mL, viability of both isolates was much reduced (P < 0.0001). The sensitivity of P. marinus to bacitracin was examined in vivo in two clinical trials. In the first, individual eastern oysters Crassostrea virginica were injected with 10⁷ Perkinsus-1 cells, then fed bacitracin at a concentration of 5 or 50 mg/mL encapsulated in lipid vesicles daily for 6 weeks. Parasite body burden was significantly reduced in oysters administered 5 mg bacitracin/mL (3.3 × 10⁴ ± 2.5 × 10⁴ hypnospores/g wet tissue) or 50 mg/mL (5.3 × 10⁴ ± 6.4 × 10⁴ hypnospores/g) as compared with control oysters (3.2 × 10⁵ ± 4.7 × 10⁵ hypnospores/g, P < 0.05) that received encapsulated seawater only. In the second experiment, naturally infected oysters (average, 10.9 × 10⁶ ± 30.7 × 10⁶ hypnospores/g) received encapsulated bacitracin at 10 mg/mL for 10 weeks. Treated oysters had significantly lower levels of infection (2.5 × 10⁶ ± 3 × 10⁶ hypnospores/g) than did control oysters (67.4 × 10⁶ ± 144 × 10⁶ hypnospores/g, P < 0.05). Despite the sharp decrease in infection intensity in the bacitracin-treated oysters, survival rate improved by only 10%. It is possible that damage to the vital organs of infected oysters was too advanced and widespread to be reversed. The in vitro and in vivo findings of this study suggest that bacitracin has promise for use in P. marinus chemotherapy.