Purpose: The purpose of this study was to determine the uptake, survival and persistence of Salmonella strains from human clinical cases and bovine PLNs in an immortal bovine macrophage cell line, a cell type found in PLNs.
Methods: Intracellular growth and survival of 32 Salmonella isolates, including isolates commonly or sporadically isolated from bovine PLNs and strains responsible for the majority of human disease, were probed in the BOMAC immortalized bovine macrophage cell line. Briefly, each strain was inoculated into duplicate confluent monolayers of BOMAC cells at a multiplicity of infection of 1,000:1. BOMAC monolayers were treated with gentamicin, washed and harvested at 0, 5, 9, and 12h post-inoculation. Resultant intracellular bacterial populations were enumerated by spread-plating on non-selective media. All strains were assayed in at least three biologically independent experiments and data were analyzed in mixed model.
Results: Statistical analysis revealed that source (human or bovine PLN) and serotype were significantly different (P < 0.0001), while time (0 to 12h) was marginally different (P = 0.104). Interestingly, human clinical isolates were taken up at a higher level and showed increased intracellular growth in BOMAC cells as compared to bovine PLN isolates. At 12h post-inoculation the mean intracellular population of isolates from human clinical cases was 6.09 log CFU/ml, while the mean bovine PLN population was 5.21 log CFU/ml.
Significance: These data show that Salmonella enterica serotypes isolated from bovine PLNs do not grow and survive inside of bovine macrophages as well as the common human serotypes do. Lower numbers may allow the bacteria to persist inside of the lymph node by not eliciting a large immune response.