A B S T R A C T
Diseases of fishes caused by Aeromonas spp. are common, have broad host ranges and may
cause high mortality. Treatments of captive-reared populations using antimicrobials are limited
with concerns for bacterial resistance development and environmental dissemination. This study
was done to determine whether selected plant-derived essential oils were bactericidal to Aeromonas
spp. Initially, twelve essential oils were evaluated using a disk diffusion assay to an isolate of
A. salmonicida subsp. salmonicida, cause of fish furunculosis. The greatest zones of inhibition
were obtained with oils of cinnamon Cinnamomum cassia, oregano Origanum vulgare, lemongrass
Cymbopogon citratus and thyme Thymus vulgaris. Minimum bactericidal concentrations
(MBC’s) were determined for these four oils, Allimed (garlic extract, Allium sativum) and colloidal
silver to sixty-nine isolates representing nine Aeromonas spp. The lowest mean MBCs
(0.02–0.04%) were obtained with three different sources of cinnamon oil. MBCs for three
sources of oregano and lemongrass oils ranged from 0.14% to 0.30% and 0.10% to 0.65%,
respectively, and for two thyme oils were 2.11% and 2.22%. The highest concentration (5%)
of Allimed tested resulted in MBCs to twelve isolates. A concentration of silver greater than
15 mg/L would be required to determine MBCs for all but one isolate.