Proximity-Dependent Inhibition of Growth of Mannheimia haemolytica by Pasteurella multocida

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Abstract
Mannheimia haemolytica,Pasteurella multocida, andBibersteinia trehalosihave been identified in the lungs of pneumonic bighorn sheep (BHS;Ovis canadensis). Of these pathogens,M. haemolyticahas been shown to consistently cause fatal pneumonia in BHS under experimental conditions. However,M. haemolyticahas been isolated by culture less frequently than the other bacteria. We hypothesized that the growth ofM. haemolyticais inhibited by other bacteria in the lungs of BHS. The objective of this study was to determine whetherP. multocidainhibits the growth ofM. haemolytica. Although in monoculture both bacteria exhibited similar growth characteristics, in coculture withP. multocidathere was a clear inhibition of growth ofM. haemolytica. The inhibition was detected at mid-log phase and continued through the stationary phase. When cultured in the same medium, the growth ofM. haemolyticawas inhibited when both bacteria were separated by a membrane that allowed contact (pore size, 8.0 μm) but not when they were separated by a membrane that limited contact (pore size, 0.4 μm). Lytic bacteriophages or bactericidal compounds could not be detected in the culture supernatant fluid from monocultures ofP. multocidaor fromP. multocida-M. haemolyticacocultures. These results indicate thatP. multocidainhibits the growth ofM. haemolyticaby a contact- or proximity-dependent mechanism. If the inhibition of growth ofM. haemolyticabyP. multocidaoccursin vivoas well, it could explain the inconsistent isolation ofM. haemolyticafrom the lungs of pneumonic BHS.

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