Non-haemolytic Mannheimia haemolytica as a cause of pleuropneumonia and septicemia in a calf

Pure cultures of non-haemolytic Mannheimia haemolytica, were cultivated from pleural effusion fluid and blood from a 1-month old Belgian Blue bull calf that was presented with apathy and anorexia. The isolates were identified as M. haemolytica by 16S rRNA gene sequencing and MALDI-TOF-MS. Since haemolysis on blood agar plates is considered a hallmark of M. haemolytica we wanted to elucidate the unusual phenotype of the isolated strain. Therefore the leukotoxin operon (lktCABD), responsible for the haemolytic phenotype of M. haemolytica and regarded as the most important virulence factor, was completely sequenced. The leukotoxin operon of the isolated strain showed a deletion in the lktA gene, resulting in a truncated LktA protein. The absence of a complete LktA protein is responsible for the non-haemolytic phenotype of the strain. To the best of our knowledge, this is the first report of a well-characterized non-haemolytic M. haemolytica isolate causing disease in cattle.