Pasteurella multocida A is a respiratory and systemic pathogen affecting a wide range of animal hosts, including rabbits. The lipopolysaccharide (LPS) is considered one of the main virulence factors of gram negative bacteria, generally inducing systemic responses. However, local effects by this molecule at the primary site of infection are less well understood. We propose that the LPS of P. multocida A could be an adhesion candidate to the respiratory epithelium of nasal septa of rabbits. Experimentally, by exposing cultured nasal septa of fetal rabbits to the LPS of P. multocida A and to the bacterium simultaneously or sequentially, we demonstrated that the LPS significantly increased the number of P. multocida A bacteria attached to the respiratory epithelium. Double labeling with an LPS-specific lectin by histochemistry and with bacterium-specific antibodies, showed that the LPS was located mainly within the cytoplasm of goblet cells and the apical border of the epithelial cells, while P. multocida adhered preferentially to the apical membrane of ciliated cells. The severity of the lesions in the respiratory epithelial cells was significantly higher (P<0.001) in the two treatments than in the control group or in cultures exposed only to P. multocida A.
In order to explore the possibility that P. mutocida A could spontaneously release its LPS during the first steps of infection, similar cultures were exposed to the bacterium and analyzed by transmission electron microscopy and by labeling of the LPS with specific-lectin conjugated to colloidal gold (5 nm). Thread-like structures positive for the lectin, and resembling LPS, were seen spanning from detritus in the lumen into the apical membrane of ciliated cells; similar structures were found within the cytoplasm of these cells.
In conclusion, the LPS of P. multocida seems to play an important role during the first steps of the infection with this microorganism..