When the long incubation time of the original method was applied, the dipped TLC plate turned extremely humid resulting in soaking of the carrier silica layer of the plates, which prevented any further devel- opment. For this reason we reduced the original overnight (~16 h) incubation to 1 h. This short co-incubation of the silica layer with the bacterium suspension, however, was not enough to achieve a proper detection, so we applied an increased bacterium concentration. Based on our pilot experiments 10-fold increase in concentration (OD600 = 1 instead of OD600 = 0.1) compensated for the shorter incubation time for all six strains tested. Due to these two essential modifications the TLC plates remained intact, and the consequent evaluation processes could be carried out successfully. Turning of the bright grey bioautographic layer after the MMT incubation to bluish-violet colour is the direct indicator of dehydrogenase activity of attached living bacte- ria on the surface of the silica layer. In contrast the colourless, white spots indicated a lack of dehydrogenase activity, due to the antibacterial effect of certain separated EO components.
The applicability of the modified method for clove and thyme EO components could be verified on the C. perfringens and C. jejuni strains. The inhibition zones resulting from the antibacterial activity of the var- ious components of clove EO against the two species showed similar patterns: eugenol and four further components could be detected and shown to have antibacterial effect against both C. perfringens and C. jejuni (Figs. 1, 2). The main component eugenol elicited the largest in- hibition areas followed by β-caryophyllene and 3 other components. Significant differences were found in the sizes of the inhibition zones of eugenol for C. perfringens and C. jejuni strains measured by ImageJ software and C. jejuni strains showed significantly higher sensitivity to eugenol than to thymol and carvacrol.
The same components of thyme EO have antimicrobial activity against C. perfringens and C. jejuni strains, but the inhibiting effect of lin- alool were more marked against C. perfringens than C. jejuni (Figs. 3, 4). This was also observed for the main components: C. perfringens showed significantly greater sensitivity to thymol and carvacrol than C. jejuni; however, no significant differences were revealed between the effec- tiveness of eugenol, thymol and carvacrol on C. perfringens. In general,