Abstract: The aim of this study was to evaluate the combined toxicity of enrofloxacin with each of three antimicrobials (ciprofloxacin,florfenicol and sulfamethazine).MRC-5 cells were used as a cell model to simulate the damage of lung cells caused by mixed contamination of antimicrobials.Multiple concentration gradients and mixing ratios were set.CCK-8 method was used to determine the inhibition rate of cell growth caused by four antimicrobials and the inhibition rate of cell growth caused by enofloxacin mixed with three antimicrobials,respectively.Then Chou-Talalay method was used to fit the median effect plot and to calculate the combination index (CI) value.The results showed that the growth inhibition rates of MCR-5 cells caused by four single drugs went up in a step-like manner with the increase of drug concentration in the tested concentration range,among them,the growth inhibitory rate of MCR-5 cells by florfenicol was low (<4.5%).The combined toxicity of the three binary combinations showed a concentration-dependent and mixing-ratio dependence.Mixing enrofloxacin and ciprofloxacin showed synergistic toxicity (CI<1) on MRC-5 cells at high and middle concentration groups,and antagonistic toxicity (CI>1) at the very-low-concentration groups.Mixing ciprofloxacin and florfenicol mainly showed an inhibited toxicity (CI>1).Binary combination enrofloxacin and sulfamethazine might present either a synergistic joint toxicity (CI<1) or an antagonistic joint toxicity (CI>1) as the concentration and mixing ratio changing.This study showed that it was necessary to assess the combined toxicity of antimicrobials in the toxicity evaluation of antimicrobials.Using Chou-Talalay method,the joint toxicity of multiple antimicrobials could be quickly and efficiently determined at cellular level.

Key words: enrofloxacin; joint toxicity; Chou-Talalay method