2.8.3. Postantibiotic Effect

This experiment was performed in accordance with our previous published studies; thus, the overnight culture of microorganisms was diluted in fresh TSB (1:20), and then the bacterial suspensions were mixed with MIC of antibiotic. All samples were then covered with aluminium foil and incubated for 2 h at 37 °C in an orbital shaker Innova40 (Brunswick, Hessen, Germany). Immediately after incubation, samples were centrifuged (3.5 min, 4500 rpm) and washed with a fresh TSB medium. After this step, cells were transferred in the amount of 100 μL to a 96-well plate and exposed to ½ MIC dose of blue light for aBL. In the next step, the optical density (λ 600 nm) of samples was measured for 15 h in multiplate reader Envision (PerkinElmer, Waltham, MA, USA) every 30 min. Obtained data were normalised and the postantibiotic effect (PAE) was determined based on the following equation: PAE = T − C (T, the time required to reach the optical density to value 0.5 (OD₆₀₀) after removal of an agent; C, the time required to achieve the optical density (OD₆₀₀) of untreated control samples). A postantibiotic effect value ≥ 3 h indicates synergy, whereas the 1.5 h ≤ PAE < 3 h confirms the partial synergistic effect.