Cultivation and Infection of C. jejuni Strains

The C. jejuni wild-type (wt) strain 81-176 and its isogenic knockout mutant C. jejuni ΔhtrA, the complemented mutant ΔhtrA/htrA and protease-inactive S197A point mutation in the htrA gene were used throughout this study (Boehm et al., 2012 (link); Boehm et al., 2015 (link)). Bacterial cells were cultured using Campylobacter blood-free selective agar base including Campylobacter growth supplement provided by Oxoid (Wesel, Germany). Alternatively, the bacteria were grown on Mueller-Hinton (MH) agar supplemented with chloramphenicol (20 μg/ml) or kanamycin (30 μg/ml), respectively. Incubation was for 48 h at 37°C in jars using microaerobic conditions provided by the CampyGen™ system from Oxoid. All C. jejuni strains were harvested using sterile cotton swabs and resuspended in liquid BHI medium. The optical density (OD) was measured at 600 nm in an Eppendorf spectrophotometer to calculate the number of bacterial cells followed by host cell infection of C. jejuni using a multiplicity of infection (MOI) of 100.