Effect of QDs on viability and antibiotic susceptibility of E. coli wt and mutant strains. (A) CFUs of E. coli wt, ∆zntA, ∆trxC and ΔsoxS strains after 30 min exposure to green or red QDs. Numbers indicate the viability decrease percentage between control and QDs-treated cells. Values are mean ± SD. Experiments were repeated three times and asterisks represent statistical differences between control and treatments (*p < 0.05, ***p < 0.001). (B) MICs to tetracycline and polymyxin B for E. coli wt and ∆zntA strains pre-treated 15 min with cadmium, green or red QDs (for details see Methods).