Mucosal colonization of <i>Mannheimia haemolytica</i> capsular and adhesin mutants in cattle

<i>Mannheimia haemolytica</i> (Mh) is a normal inhabitant of the upper respiratory tract of ruminants and is associated with bovine respiratory disease. Polysaccharide capsule and surface adhesins are suggested to function in adherence and colonization of <i>M. haemolytica</i> to the mucosa of the upper respiratory tract. <i>M. haemolytica</i> serotype 1 mutant strains containing deletions of either the capsule biosynthetic gene cluster (<i>∆cap</i>) or putative adhesin genes (<i>∆adh123</i>) were created using a temperature-sensitive plasmid and tested for colonization in a calf challenge model. Two treatment groups were used in the study: Sham-Mh-BHV-1 (SMB; intranasal administration of uninfected cell culture lysate/supernatant [sham; S] 4 days before intranasal <i>M. haemolytica</i> inoculation, and intranasal inoculation of bovine-herpesvirus-1 [BHV-1] 20 days post-Mh) and BHV-1-Mh-Sham (BMS; intranasal inoculation of BHV-1 4 days before intranasal Mh inoculation and intranasal sham administration 20 days post-Mh). A mixture of wild-type <i>M. haemolytica</i> parent strain, <i>∆cap</i>, and <i>∆adh123</i> mutants was included in the Mh inoculum. Animals were observed for clinical signs and nasal colonization for approximately 7 weeks. The <i>∆adh123</i> mutant and parent strain colonized the nasopharynx, whereas the <i>∆cap</i> mutant was not detected after 1 day post-inoculation. The <i>∆adh123</i> mutant colonized the nasopharynx at significantly higher levels (<i>P</i> < 0.0001) compared to wild type. Higher colonization of <i>∆adh123</i> was also found in palatine tonsils. These findings suggest a requirement of capsule in long-term colonization and an advantage for <i>∆adh123</i> in colonization over the parent strain.IMPORTANCEUnderstanding the colonization dynamics of <i>Mannheimia haemolytica</i> is crucial for developing effective prevention and treatment strategies for bovine respiratory disease (BRD), a significant cause of economic loss in the cattle industry. This study highlights the role of capsular polysaccharide and surface adhesins in nasopharyngeal colonization. These findings demonstrate that the deletion of putative surface adhesins leads to enhanced colonization compared to the wild-type strain, while mutants containing a deletion of the capsule biosynthetic gene cluster failed to establish long-term colonization. These results suggest that targeting bacterial adhesion mechanisms could influence bacterial persistence and immune response, offering potential avenues for controlling BRD.