Effect of mono- and bicyclic monoterpenoids upon Dictyostelium minutum and Dictyostelium macrocephalum

We have studied the effect of four monoterpenes on the growth of two cellular slime molds, Dictyostelium minutum Raper and D. macrocephalum Hagiwara, Yeh et Chien. (4S)-(+)-carvone was the monocyclic monoterpene we tested, while (1S)-(-)-verbenone, (1R)-(-)-fenchone, and (1R,5R)-(+)-beta-pinene were the bicyclic monoterpenes we employed. One mu L of four different concentrations (0.006M, 0.06M, 0.6M and 6M) of each monoterpene were applied to both species, through the use of the Disk Volatilization Technique. (4S)-(+)-carvone possessed a superior inhibitory activity against both D. minutum and D. macrocephalum, at 0.6M concentration (p <= 0.001), while growth rate enhancement occurred after application of (4S)-(+)-carvone at concentrations of 0.006M and 0.06M on D. minutum and D. macrocephalum, (p <= 0.05). The inhibitory effects of the monoterpenes on the growth of D. macrocephalum increased proportionally to the applied compound concentrations in most cases. When the bicyclic monoterpenes (1R)-(-)-fenchone and (1S)-(-)verbenone were administered to D. macrocephalum, however, it was greatly inhibited even at low concentrations (p <= 0.05 and p <= 0.001). The bicyclic monoterpene (1R,5R)-(+)-beta-pinene inhibited the growth rate of D. minutum and D. macrocephalum at most concentrations, but at 0.006M concentration (1R,5R)-(+)-beta-pinene caused slight enhancement. Microscopic examination of changes in the shapes of the fruiting bodies matched the effects observed on the growth rates. These results suggest that the inhibitory or enhancing effects of the selected monoterpenes on both D. minutum, and D. macrocephalum depend on the number of cyclic skeletons as well as the concentration of the individual compounds.