Interaction of Salmonella serotypes with porcine macrophages in vitro does not correlate with virulence

The interaction between Salmonella serotypes and macrophages is potentially instrumental in determining the outcome of infection. The nature of this interaction was characterized with respect to virulence and serotype-host specificity using pigs as the infection model. Experimental infection with Salmonella typhimurium, Salmonella choleraesuis or Salmonella dublin resulted in enteric, systemic or asymptomatic infection, respectively, which correlates well with the association of S. choleraesuis with systemic disease in pigs in epidemiological studies. Persistence within porcine alveolar macrophages in vitro did not directly correlate with virulence since S. typhimurium persisted in the highest numbers, and S. choleraesuis in the lowest. Comparison to other studies revealed that the relatively high persistence of S. typhimurium in macrophages correlates with its virulence in a broad range of animals: this could be a virulence mechanism for broad-host-range serotypes. There were little or no significant differences in the induction of pro-inflammatory cytokines by macrophages infected with the three serotypes. S. typhimurium and S. dublin, but not S. choleraesuis, damaged porcine macrophages, and the mechanism of damage did not resemble apoptosis. In conclusion, the virulence of Salmonella serotypes in pigs did not directly correlate with their interaction with porcine macrophages in vitro. The interaction of Salmonella and macrophages in vitro may not accurately model their interaction in vivo, and this will form the basis of further study.