Bacitracin inhibits the oyster pathogen Perkinsus marinus in vitro and in vivo

The in vitro growth rates of two isolates of Perkinsus marinus were signifi cantly reduced by bacitracin. Upon coincubation with I mg bacitracin/mL, th e doubling times rose from 27 +/- 2.1 h to 34 +/- 2.9 h for the LMTX-1 isol ate (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 wa s much reduced (P < 0.0001). The sensitivity of P. marinus to bacitracin wa s examined in vivo in two clinical trials. In the first, individual eastern oysters Crassostrea virginica were injected with 10(7) Perkinsus-1 cells, then fed bacitracin at a concentration of 5 or 50 mg/mL encapsulated in lip id vesicles daily for 6 weeks. Parasite body burden was significantly reduc ed in oysters administered 5 mg bacitracin/mL (3.3 x 10(4) +/- 2.5 x 10(4) hypnospores/g wet tissue) or 50 mg/ mt (5.3 x 10(4) +/- 6.4 x 10(4) hypnosp ores/g) as compared with control oysters (3.2 x 10(5) +/- 4.7 x 10(5) hypno spores/g, P < 0.05) that received encapsulated seawater only. In the second experiment, naturally infected oysters(average, 10.9 x 10(6) +/- 30.7 x 10 6 hypnospores/g) received encapsulated bacitracin at 10 mg/mL for LO seeks. Treated oysters had significantly lower levels of infection (2.5 x 10(6) /- 3 x 10(6) hypnospores/g) than did control oysters (67.4 x 10(6) +/- 144 x 10(6) 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 wa s too advanced and widespread to be reversed. The in vitro and in vivo find ings of this study suggest that bacitracin has promise for use in P. marinu s chemotherapy.