Antibacterial efficacy interference of the photocatalytic TiO2 nanoparticle and the lytic bacteriophage vb_EcoS_bov25_1D on the Enterohaemorragic Escherichia coli strain SAKAI
Post-antibiotic era requires the use of alternative pesticides against bacterial infections. One potential application field is agriculture, where pesticides are routinely applied in combinations. In this study we tested the interference of antibacterial effects of two alternative antimicrobials with basically different mode of actions if applied together in vitro by using the Enterohemorrhagic E. coli strain SAKAI as a model organism, one strain of a pathotype that is frequently associated with meat and plant derived infections. TiO2 is a photocatalytically active nanomaterial, which can generate reactive oxygen species (ROS), exerting destructive effects on macromolecules, while the vb_EcoS_bov25_1D bacteriophage has a specific lytic action. Both, bacteriophages and SAKAI were sensitive against ROS if tested separately, during that PFUs of bacteriophages dropped from 5x105 to 0 in 4 hours, while in case of SAKAI CFUs decreased with 5 and 2 logs of magnitude in the presence of 0,05 % and 0,025 % of TiO2 respectively. In SAKAI by the sixth minute of ROS exposition the expressions of superoxide dismutases and catalases were boosted, as revealed by whole transcriptomic analyses, but the elevated levels rclC and bshA support some roles of these genes under this stress situation. Combined application of phages and TiO2 under UV-A exposure have revealed that beside the inner enzymatic defence mechanisms presenting phage particles served as shields and spoiled the antimicrobial effect of TiO2 (0,0125 %). As a consequence phages became sacrificed as during exposition a 3-log drop (5x105?5x102) in their PFUs was revealed. Survived bacteriophages however in the system remained active and under the subsequent dark phase the 3-log drop in the PFU was compensated in 24 hours. Our results show that joint application of the two alternative antimicrobial agents TiO2 and a bacteriophage can have two consequences depending on the circumstances they were used. From one side they complement each other's effects in that TiO2 can exert its effect on UV-A or sunlight exposed areas, whereas the bacteriophage on non-exposed surfaces. On the other hand they also can spoil each others effect as phages can bind ROS and by that protect target bacteria, but bacteria themselves can protect phages from the destroying effect of ROS by leaving them to exert their antibacterial effects on a longer timerange.