Epidemiology and interaction of Salmonella enterica serovar Derby, Infantis and Typhimurium with porcine alveolar macrophages

In this study we were interested in the serovars which are frequently isolated from pigs, i.e. S. Typhimurium, S. Derby and S. Infantis. First we collected different isolates of S. Infantis and S. Derby and compared them by macrorestriction analysis. In the second part of the study we infected porcine alveolar macrophages (PAMs) with representative strains of these serovars and S. Typhimurium and determined intracellular survival, cytotoxicity and cytokine response. In S. Derby, 17 different profiles in 51 isolates have been identified and in S. Infantis, 12 different profiles in 37 isolates have been identified. Four hours post-addition of bacteria to PAMs, higher numbers of intracellular S. Typhimurium than S. Derby or S. Infantis were observed. However, within next 24 h, counts of S. Typhimurium did not change while S. Derby and S. Infantis increased their counts 10 and 5 times, respectively. The apparent inability of S. Typhimurium to multiply inside PAMs was caused by its higher cytotoxicity because PAMs infected with S. Typhimurium released LDH 24 h post-infection to a significantly higher level than when infected with the other two serovars. The IL-1β, IL-8, IL-12p40, IL-23p19 and TNFα response to S. Derby and S. Infantis was always higher than to S. Typhimurium and the differences among the serovars were more significant at 4 than 24 h post-infection. The lower cytokine signaling but higher cytotoxicity of S. Typhimurium for macrophages correlates with the higher virulence for pigs of this serotype when compared with S. Derby or S. Infantis.