Abstract:
Background: Traditional medicinal plants have been used as an alternative medicine in many parts of the world,
including Ethiopia. There are many documented scientific reports on antimicrobial activities of the same. To our
knowledge, however, there is no report on the anti-Quorum Sensing (Quorum Quenching, QQ) potential of traditional
Ethiopian medicinal plants. As many of the opportunistic pathogenic bacteria depend on Quorum Sensing (QS) systems
to coordinate their virulence expression, interference with QS could be a novel approach to control bacterial infections.
Thus, the aim of this study was to evaluate selected medicinal plants from Ethiopia for their antimicrobial activities against
bacterial and fungal pathogens; and to assess the interference of these plant extracts with QS of bacteria.
Methods: Antimicrobial activities of plant extracts (oil, resins and crude extracts) were evaluated following standard agar
diffusion technique. The minimum inhibitory concentrations (MIC) of potent extracts were determined using 96 well
micro-titer plates and optical densities were measured using an ELISA Microplate reader. Interference with Quorum
Sensing activities of extracts was determined using the recently established E. coli based reporter strain AI1-QQ.1 and
signaling molecule N-(ß-ketocaproyl)-L-homoserine lactone (3-oxo-C6-HSL).
Results: Petroleum ether extract of seed of Nigella sativa exhibited the highest activity against both the laboratory
isolated Bacillus cereus [inhibition zone (IZ), 44 ± 0.31 mm] and B. cereus ATCC 10987 (IZ, 40 ± 2.33 mm). Similarly, oil
extract from mature ripe fruit husk of Aframomum corrorima and mature unripe fruit of A. corrorima revealed promising
activities against Candida albicans ATCC 90028 (IZ, 35 ± 1.52 mm) and Staphylococcus aureus DSM 346 (IZ, 25 ± 1.32 mm),
respectively. Antimicrobial activities of oil extract from husk of A. corrorima and petroleum ether extract of seed of N.
sativa were significantly higher than that of the control antibiotic [Gentamycin sulfate, (IZ, 25–30 mm)]. The lowest MIC
value (12.5 mg/mL) was recorded for oil from husk of A. corrorima against Pseudomonas aeruginosa. Of the total eighteen
extracts evaluated, two of the extracts [Methanol extract of root of Albiza schimperiana (ASRM) and petroleum ether
extract of seed of Justica schimperiana (JSSP)] interfered with cell-cell communication most likely by interacting with the
signaling molecules.
(Continued on next page)
