Evaluation of antibacterial properties and biochemical effects of monoterpenes on plant pathogenic bacteria

The antibacterial activity of twelve monoterpenes, namely camphene, (R)-camphor, (R)-carvone, 1,8-cineole, cuminaldehyde, (S)-fenchone, geraniol, (S)-limonene, (R)-linalool, (1R,2S,5R)-menthol, myrcene and thymol was tested against two plant pathogenic bacteria Agrobacterium tumefaciens and Erwinia carotovora var. carotovora using agar dilution method. For a better understanding of monoterpenes mechanisms of action, the inhibitory effect of three monoterpenes (R)-linalool, myrcene and thymol was assessed on dehydrogenases and polyglacturonase activities. Among the tested monoterpenes, thymol, (S)-limonene and myrcene were the most potent antibacterial compounds against A. tumefaciens with minimum inhibitory concentration (MIC) of 1000 mg/L. Thymol was also the most effective compounds against E. carotovora var. carotovora, while camphene, cunimaldhyde and 1,8-cineole were the less effective compounds against both bacteria. In biochemical studies, myrcene caused the highest inhibitory effect on dehydrogenases activity of the two tested bacteria, followed by thymol. However, thymol showed the highest inhibitory effect on polygalacturonase activity of both tested bacteria, followed by (R)-linalool. In general, there was a positive correlation between the antibacterial activity of monoterpenes and their inhibitory effects on both enzymes. This is the first report for the determination of MIC and enzymes inhibitory effects of tested monoterpenes on plant pathogenic bacteria.