2.1. Growth Conditions

S. aureus (25923) was purchased from ATCC (Manassas, VA, USA) and P. putida F1 (6899) from DSMZ (Braunschweig, Germany). The bacteria were grown in BHI agar for 24 h at 37 °C. A few isolated colonies were suspended overnight in 12.5 mL mineral medium (MM) supplied with 1% glucose as the carbon source [38], followed by incubation at 25 °C and low agitation at 70 rpm, in order to reach the log phase at the beginning of the experiment. The culture was centrifuged (Avanti J-E centrifuge, Beckman Coulter, Brea, CA, USA) at 4000 g for 10 min at 4 °C, and the sediment was washed with ultrapure (UP) water with a resistance of 18.4 MΩ-cm at 25 °C (Synergy UV water purification system, Merck, Darmstadt, Germany). The bacterial sediment was suspended in different concentrations of PBS and UP water, to a final optical density of 0.01 at 600 nm, using a spectrophotometer (Genesys 10S UV-VIS, Thermo Scientific, Waltham, MA, USA).