Interaction between mammalian cells and Pasteurella multocida B:2. Adherence, invasion and intracellular survival.

A Pasteurella multocida B:2 strain from a case of bovine haemorrhagic septicaemia (HS) and a derivative, JRMT12, that was attenuated by a deletion in the aroA gene, were shown to adhere to, invade and survive within cultured embryonic bovine lung (EBL) cells. By comparison, bovine strains of Mannheimia haemolytica serotype A1 and P. multocida serotype A:3, although able to adhere to EBL cells, were not found intracellularly. The B:2 strains were viable intracellularly over a 7 h period, although a steady decline in viability was noted with time. Entry into the mammalian cells was inhibited by cytochalasin D, indicating that cell uptake was by an actin-dependent process. Viability assessment of EBL cells by trypan blue staining indicated that none of the bacterial strains was toxic for the EBL cells. Transmission electron microscopy (TEM) showed that, after entry into the mammalian cells, the B:2 strain resided in a vacuolar compartment. However, only a low percentage of mammalian cells appeared to contain one or more P. multocida B:2, suggesting that only certain EBL cells in the population were capable of being invaded by, or of taking up, the bacteria. TEM showed that P. multocida A:3 and M. haemolytica A:1 were found loosely adhering to the cell surface of EBL cells and were not detected intracellularly. The cell-invasive capacity of P. multocida B:2 may be a virulence property related to its ability to translocate from the respiratory tract into the blood stream.