Comparative study on antibacterial activity of synthetic analogues of biologically active compounds and their combination with rhamnolipid-biosurfactant

Correspondence: Danka Galabova Department of General Microbiology, The Stephan Angeloff Institute of Microbiology, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Block 26, 1113 Sofia, Bulgaria e-mail: [email protected]; ABSTRACT The resistance of bacteria to antibiotics is a serious public problem with increasing significance. The development of novel pharmaceuticals with wide spectrum of activity for application in human and veterinary medicine or for plant protection is necessary. The biological activity of two groups of synthetic analogues of naturally occurring compounds with antimicrobial properties was studied. The esters of thiosulfonic acid are similar to allicin the antibiotic substance of Allium sativa. Compounds with 1,4-naphthoquinone structure are derivatives of lawsone, contained in the leaves of henna (Lawsonia inermis). The antimicrobial potential of these substances was assessed against model strains from different genera, able to cause damages to industrial manufacture, agriculture and human health. The results revealed that the naphthoquinone derivatives demonstrated relatively lower antibacterial activity compared to thiosulfonate esters and had not bactericidal effect even at the highest tested concentrations. The possibility for potentiating of the antibacterial activity of thiosulfonates and 1,4 naphthoquinones by rhamnolipid – biosurfactant was also studied. It was suggested that the biosurfactant provoked changes in the cell membrane organization that facilitate the access of antimicrobials into bacterial cell. The presence of rhamnolipid biosurfactant decreased significantly the minimal bactericidal concentration of thiosulfonates, but affects only the minimal inhibitory concentration of naphthoquinones and bactericidal effect was not registered again. The enhancement of antibacterial activity of inhibitors in the presence of biosurfactant significantly increased the therapeutic potential of these compounds. The development of combined preparations enables to modify the effect of the antimicrobial agents thus contributing to overcome the drug resistance.