Evaluation of the anti-Listeria potentials of some plant-derived triterpenes

Background: Listeriosis is a fatal disease caused by pathogenic Listeria bacteria and it is most prevalent in immune-compromised individuals. The increase in numbers of immune-compromised individuals against a background of Listeria antibiotic resistance, limits listeriosis treatment options. This therefore calls for research into substitute treatments, of which, medicinal plants derived compounds offer a viable alternative. Methods: The broth microdilution assay was used to determine the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of three plant triterpenes namely 3 beta-hydroxylanosta-9,24-dien-21-oic acid, methyl-3 beta-hydroxylanosta-9,24-dien-21-oate and 3 beta-acetylursolic acid, against Listeria monocytogenes, Listeria ivanovii and Listeria grayi species. The chequerboard method was used to assess the interactions between the triterpenes and conventional antibiotics: ampicillin, neomycin, gentamicin and penicillin G. The lactate dehydrogenase membrane damage method was used to assess the triterpenes' membrane damaging potentials against the Listeria bacteria. Results: The triterpenes' MIC values were found to range from 0.185 to 1.67 mg/ml while, the MBC determination assay results revealed that the test triterpenes were bacteriostatic against the Listeria bacteria. The interactions involving 3 beta-hydroxylanosta-9,24-dien-21-oic acid were mainly additive with ampicillin and synergistic with neomycin, gentamicin and penicillin G. The interactions involving methyl-3 beta-hydroxylanosta-9, 24-dien-21-oate were mainly antagonistic with ampicillin, indifferent with neomycin, ranging from synergistic to indifference with gentamicin and synergistic with penicillin G. The interactions involving 3 beta-acetylursolic acid were mainly indifferent with ampicillin, synergistic with neomycin and gentamicin while ranging between synergistic and additive with penicillin G. The low levels of cytosolic lactate dehydrogenase released from the cells treated with 4x MIC concentration of the triterpenes in comparison to that of cells treated with 3% Triton X-100 proved that membrane damage was not the mode of action of the triterpenes. Conclusion: This study therefore shows the potential that these plant triterpenes have in listeriosis chemotherapy especially as shown by the favourable interactions they had with penicillin G, one of the antibiotics of choice in listeriosis treatment.