Invasion and cytotoxicity of Salmonella Typhimurium for porcine alveolar macrophages. Panel A, invasion of the wild-type Salmonella Typhimurium (at specified MOI) and isogenic ΔSPI-1 and ΔSPI-2 mutants into porcine alveolar macrophages 4 h post-infection. Panel B, cytotoxicity of Salmonella Typhimurium and its isogenic mutants for porcine alveolar macrophages 24 h post-infection. The data are presented as mean and SD of percentage of LDH released following particular stimulation relative to LDH released from PAMs lysed by freeze-thaw disruption (100%). Significantly different groups (P < 0.05) are marked by different letters (“a” differs from “bc”, but not “ab”, etc.). Data are presented as mean and SD.