Antilisterial activity of Euclea natalensis and its naphthoquinones

The roots of Euclea natalensis DC (Ebenaceae) are used by the indigenous people of Southern Africa against various bacterial infections [1]. Napthoquinones (NQs) isolated from its roots (diospyrin, neodisopyrin, isodiospyrin, 7-methyljuglone), a few synthesized napthoquinones and its root extracts were investigated against the foodborne pathogen, Listeria monocytogenes (ATCC 7644).

Extract and compounds were tested at concentrations ranging from 500.0 to 0.12µg/ml for the determination of the minimum inhibitory concentration (MIC) against L. monocytogenes. The optical density of the culture was adjusted to 0.1 using fresh broth to give a standard inoculum of ca. 10⁶ colony forming units. The minimum bactericidal concentration (MBC), defined as the concentration producing a 99.9% reduction in CFU in the initial inoculum was also determined.

The chloroform extract of the roots of E. natalensis exhibited an MIC of 250.0µg/ml. A complete inhibition of the bacteria was observed at 500µg/ml indicating its bactericidal effect. Among naphthoquinones tested, 7- methyljuglone was found to be the most potent naphthoquinone, exhibiting an MIC and MBC of 125µg/ml followed by its dimer, diospyrin. The MIC's of the naphthoquinones indicates that the keto groups at C1 and C4 are important for antibacterial activity. This can be seen by comparing the activity of 7-methyljuglone (MIC=125.0µg/ml) with shinanolone (MIC=>1000.0µg/ml). The keto group at carbon 1 of shinanolone is reduced to the corresponding hydroxyl group. The MICs of other naphthoquinones, neodiospyrin, isodiospyrin, chloro-methyljuglone, ranged from 125–500µg/ml. In conclusion, the most potent antilisterial compound, 7-methyljuglone, merits further studies to determine its pharmacological properties in vivo for antilisterial therapy.

Acknowledgements: National Research Foundation, University of Pretoria, CNRS, Orleans France.

Reference: [1] Van Wyk, B et al (1997). Field Guide to Trees of Southern Africa, McKenzie, Cape Town.