In vitro synergistic effect of doxycycline & ofloxacin in combination with ethanolic leaf extract of Vangueria spinosa against four pathogenic bacteria.

ABSTRACT

Background & objectives: The in vitro antibacterial activity of ethanolic leaf extract of Vangueria spinosa Roxb. (Rubiaceae) alone and in combination with antibiotics (doxycycline and ofloxacin) by means of fractional inhibitory concentration indices (FICI) as well as by the use of time-kill assays against one Gram-positive bacterium (Staphylococcus aureus) and three Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa) was studied. Methods: Antibacterial activity was assayed by using the microdilution method. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined for the ethanolic leaf extract of V. spinosa alone and also in combination with antibiotics using the fractional inhibitory concentration (FIC) and time-kill assay method. Synergism was also tested using checker board dilution method. Results: MIC/MBC values for ethanolic leaf extract of V. spinosa against all the tested bacteria ranged between 25.5 - 52.6/22.4 - 60.5 μg/ml, for doxycycline 4.0/4.0 - 4.5 μg/ml and for ofloxacin 0.625 - 2.5/1. 25 - 5.0 μg/ml respectively. The average log reduction in viable cell count in time-kill assay ranged between 2.4 log10 - 4.5 log10 cfu/ml after 1 h of interaction and between 3.9 log10 -5.0 log10 cfu/ml after 3 h interaction in 1 × MIC to 4 × MIC. When leaf extract and antibiotics were combined, the average log reduction in viable cell count for doxycycline from 1.5 log10 - 5.18 log10 cfu/ml and for ofloxacin 3.06 log10- 5.39 log10 cfu/ml. Synergistic actions were observed in all the cases except against P. aeruginosa which showed an additive effect for ofloxacin and plant extract combination. Interpretation & conclusions: This study provides a preliminary report of synergistic activity of V. spinosa Roxb, ethanolic leaf extract with antibiotics.