Pre-macrophagic J774 cells from murine lymphomas were cultivated in Dulbecco’s modified Eagle’s essential medium (DMEM, Sigma-Aldrich, USA) supplemented with 5% foetal bovine serum, 50 μg/mL gentamicin and 2.5 μg/mL fungizone at 37°C in a 5% CO2 atmosphere. The CP09 mutant strain and T1 wild-type strain were grown for 48 hours at 37°C and washed three times with PBS, resuspended in DMEM to a concentration of 106 CFU/mL, and used to infect J774 cells (multiplicity of infection [MOI]: 10 bacteria: 1 cell) grown to approximately 95% confluence in 24-well tissue culture plates.
For determination of intracellular viable bacteria, after 1, 3 and 6 hours of incubation, infected J774 monolayers were washed six times with PBS and treated with 150 μg/mL gentamicin sulphate (Sigma-Aldrich, USA) diluted in DMEM for 1 h. The number of intracellular bacteria was determined by viable counts after lysis of monolayers with 0.5 mL of 0.1% TritonX-100 (Sigma-Aldrich, USA) in PBS.