In this study, we characterized isolates (n=33) from septicemia outbreaks in swine herds as well as isolates (n=33) recovered from healthy animals at slaughter. At first, the rates of adhesion and invasion on intestinal epithelial cell lines were measured for all isolates. The isolates recovered from diseased animals have been shown to invade at a higher rate (P < 0.05) than isolates from healthy pigs. There was no significant difference for adhesion between 2 groups of isolates (P > 0.05). Some isolates were chosen for the remaining experiments according to invasion assay results. No difference between isolates from septicemic pigs and isolates from healthy pigs was detected for other phenotypic methods such as phagocytosis rates and survival in monocytes, apoptosis, adhesion to intestinal mucus and microbial surface properties. However, for the phagocytosis rates, a significant difference between the 2 groups of isolates was observed at 15 minutes (P < 0.05). These results suggest that early steps in the infection and spread within host phagocytes can be important in the outcome of the disease caused by these virulent isolates.