Growth of Mannheimia haemolytica: Inhibitory agents and putative mechanism of inhibition

•Growth inhibition of Mannheimia haemolytica.•Inhibitors: Bibersteinia trehalosi, Pasteurella multocida, Escherichia coli.•Mechanism: proximity-dependent inhibition.•No involvement of plasmids and phages.

Leukotoxin-producing Mannheimia haemolytica consistently causes fatal pneumonia in bighorn sheep (BHS) under experimental conditions. Surprisingly, by culture methods, it has been isolated from pneumonic BHS lungs less frequently than other bacteria. However, in one study PCR assays detected M. haemolytica from over 70% of the pneumonic lung samples that were negative for this organism by culture, suggesting that the growth of M. haemolytica is inhibited by other bacteria. Previously, we have shown that Bibersteinia trehalosi inhibits the growth of M. haemolytica. Herein we report that 100% of a diverse panel of B. trehalosi isolates (n = 55) tested in a bacterial competition assay inhibited the growth of M. haemolytica, suggesting that the inhibitory phenotype is conserved. Further, no plasmids were isolated from any of the 30 B. trehalosi isolates tested, suggesting that the effectors are chromosomally encoded. An earlier study by us showed that Pasteurella multocida also inhibits the growth of M. haemolytica. However, M. haemolytica has not been isolated even from pneumonic BHS lungs that did not carry B. trehalosi or P. multocida. Consequently, we tested Staphylococcus spp., Streptococcus spp., and Escherichia coli, the bacteria that have been detected frequently in pneumonic BHS lungs, for possible inhibition of M. haemolytica. Neither the Staphylococcus spp. nor the Streptococcus sp. strains inhibited the growth of M. haemolytica. E. coli inhibited the growth of M. haemolytica by a proximity-dependent mechanism. Growth inhibition of M. haemolytica by several bacterial species is likely to contribute to the infrequent detection of this bacterium from pneumonic BHS lungs by culture.