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 ...