The flotation reagents absorbed on the mineral surfaces may have some positive or negative effects on the bacteria involving in the bioleaching process; however a few researches have paid attention to these effects. In this research, the effects of the galena and sphalerite flotation reagents on the activity of mesophilic bacteria were investigated. The bioleaching experiments were carried out using three microbial species including Acidithiobacillus Ferrooxidans (AF), Acidithiobacillus Thiooxidans (AT), Leptosprillum Ferrooxidans (LF), and also the mixed culture of these mesophilic microorganisms. The flotation reagents were used exactly in similar concentrations and in the same way as applied in the industrial lead-zinc flotation plants. Moreover, the effects of two xanthate collectors of potassium ethyl xanthate (REX) and potassium amyl xanthate (KAX) were separately studied on the bioleaching of zinc sulphide by AF bacteria. The zinc dissolution rate under bioleaching process was increased due to the cumulative effects of the flotation reagents at the concentration ranges needed to float both galena and sphalerite minerals. However, such a positive effects of the reagents was more pronounced on the mixed-culture of bacteria compared to that of single species. The maximum zinc extraction was 97% after 30 days using the mixed-culture bacteria in the presence of chemical flotation reagents while it was only 74% in the absence of the flotation reagents. It should be noted that the corresponding percentages of zinc extraction for AF bacteria were 86% and 64% and for the LF bacteria were 70% and 61%, respectively. An increase in the bioleaching rate was also observed when the KAX collector was added into the solid sample instead of adding to the solution of cell culture media. The zinc extraction was increased from 64% to 74% when the solid sample was conditioned by adding 350 g/t of KAX to the solid sample before bioleaching. (C) 2015 Elsevier B.V. All rights reserved.