<i>Mannheimia haemolytica</i> isogenic capsular and LPS-sialylation gene deletion mutants are attenuated in a calf lung challenge model

Bovine respiratory disease complex (BRDC) is a multifactorial syndrome that involves complex interactions between environment, bacterial/viral pathogens, and the host. <i>Mannheimia haemolytica</i> is the most significant bacterial pathogen associated with BRDC. This study investigated the virulence of a <i>M. haemolytica</i> serotype 1 capsular-deficient (Δ<i>cap</i>) mutant and a <i>M. haemolytica</i> lipopolysaccharide (LPS)-sialylation-deficient (Δ<i>neuA</i>, cytidine monophosphate-sialic acid synthetase) mutant in a calf lung challenge model. Twelve colostrum-deprived calves were divided into three groups (four calves per group) and intratracheally administered inoculum of <i>M. haemolytica</i> wild-type (WT), <i>M. haemolytica</i> Δ<i>cap</i>, or <i>M. haemolytica</i> Δ<i>neuA</i> strains (~5 × 10⁸ CFU per animal). Animals were observed for signs of pneumonia and were humanely euthanized 2 to 3 days post-bacterial challenge. Lungs were examined for gross pulmonary lesions, histopathological changes, and bacterial culture. Calves administered WT <i>M. haemolytica</i> exhibited severe lung lesions characterized by extensive consolidation and hemorrhage. In contrast, calves administered <i>M. haemolytica</i> Δ<i>cap</i> or <i>M. haemolytica</i> Δ<i>neuA</i> mutants displayed significantly reduced lung lesions (<i>P <</i> 0.05). The most severely affected lung lobes were the right cranial and right middle lobes, with ~50% consolidation. The WT group exhibited significantly higher lung tissue bacterial loads than either of the groups receiving the mutant strains (<i>P <</i> 0.05). The reduced clinical signs, pneumonic lung lesions, and bacterial recovery in the lungs of the calves challenged with either the Δ<i>cap</i> or the Δ<i>neuA M. haemolytica</i> mutant strains indicated that these mutations were significantly less virulent than the parent strain.IMPORTANCEWhile <i>Mannheimia haemolytica</i> leukotoxin is well recognized as a major virulence factor, the roles of other possible virulence factors, such as capsule and sialic acid (sialylation of LPS) in <i>M. haemolytica</i>, have not been investigated in animal models. This study revealed that the abolishment of capsule (Δ<i>cap</i>) or LPS sialylation (Δ<i>neuA</i>) significantly reduced the virulence of each mutant in calf lung challenges. These results demonstrate that both <i>M. haemolytica</i> capsule and sialylated LPS are important virulence factors that play a key role in the evasion of host defenses.