Application of the combination index integrated with confidence intervals to study the toxicological interactions of antibiotics and pesticides in Vibrio qinghaiensis sp.-Q67

• A novel procedure integrating a combination index with the observed confidence intervals.
• The effective identification of toxicity interactions in mixtures at various effect levels.
• IMI–PIR rays of higher PIR ratios being synergistic at high concentration in long-term test.
• The effects of all binary mixture rays but IMI–PIR rays to V. qinghaiensis being additive.

It is necessary to explore the effect of confidence intervals on the combination index (CI) so that rationally evaluate the toxicological interaction (synergism or antagonism) which is dependent on the concentration ratio, the mixture concentration and the exposure time. To effectively detect the toxicological interaction taking place in mixtures, we combined the CI with the observation-based confidence intervals (OCI) which can characterize the uncertainty in toxicity test and in data fitting. In time scale, the short-term (15 min) and long-term (12 h) toxicities of three chemicals (imidacloprid (IMI), pirimicarb (PIR) and streptomycin sulfate (STR)) and their binary mixtures on Vibrio qinghaiensis sp.-Q67 were determined by the microplate toxicity analysis (MTA). The mixtures of IMI, PIR and STR have additive actions all but four IMI–PIR rays (R2–R5) at the effect levels above about 30–40% whose long-term toxicological interaction are synergism.