Figure 3: Intracellular viability of propionibacteria in THP-1 cells. THP-1 cells were infected with P. acnes strains P6, KPA and 266, and P. freudenreichii at an MOI of 3. Extracellular bacteria were killed after 24 h by 3 h incubation with 300 μg/mL gentamicin. 100 μg/mL gentamicin was kept in the medium at all times to prevent reinfection with extracellular bacteria. Cells were lysed with 0.5% saponin, and remains were scraped off and mixed by vortexing. A dilution series was plated out on Brucella agar plates to determine viable intracellular bacteria by determining CFUs after 5 days of incubation at 37°C under anaerobic conditions. Data were evaluated using Microsoft Excel and GraphPad Prism. Values represent mean of n = 2 ± SEM. (a) Presence of intracellular bacteria at 24 h p.i., compared to original inoculum (100%). (b) CFU counts of intracellular bacteria over a time course of 3 days.