Fig. 3

Evaluation of the synergism of doxycycline in combination with essential oil (EO) of A. conyzoides. Data sets in blue, red and green correspond to doxycycline, EO, combinations, respectively. a Dose–response curves of each individual treatment. Doxycycline was evaluated twice, for each of the combinations with EO. The antibacterial effect was determined by measurement of foci size. b Median-effect representation of the dose–response curves for each individual compound, using Eq. (3). fa is the “fraction affected”, or (% effect) ÷ 100. c Normalised isobologram that represents, for each combination, the normalised dose of each treatment individually required to reach the observed effect in combination (Eq. 2). d CI-effect plot representing the combination index CI, calculated using Eq. (1), of each combination as a function of their associated antibacterial effect. e Log (DRI)-effect plot representing the drug reduction index (DRI) of compounds as a function of their antibacterial effect in combination. The DRI is calculated for each drug in each combination according to Eq. (4) and represents the dilution factor required for a drug to reach the same level of inhibition individually compared with it when in combination. The results are representative of 3 independent experiments