Heat killed (HK) Escherichia coli
E. coli MG1655 were grown on Luria-Bertani (LB) agar and incubated at 37°C overnight. One colony was inoculated into 10 ml LB broth and incubated on a shaker (200 rpm) at 37°C overnight. The bacteria were centrifuged for 10 min at 3000 × g, washed with 3 ml phosphate buffered saline (PBS; 8 g NaCl, 1.16 g Na2HPO4, 0.2 g KH2PO4, 0.2 g KCl, pH7) and resuspended in 50 μl PBS. The bacteria were killed by heating to 70°C for 1 h. To ensure that the bacteria were killed; 10 μl of the heat-killed suspension was spread on a LB plate and incubated overnight at 37°C.