Isolation, phenotypical and genotypical characterization of Mannheimia haemolytica serotype A5 strain associated with pneumonic Pasteurellosis

Background Mannheimia haemolytica ( M. haemolytica ) is the primary pathogen responsible for respiratory diseases in ruminants. As an opportunistic pathogen, it often co-infects with other bacteria and viruses, leading to severe pneumonia. In this study, a suspected M. haemolytica pathogen was isolated from the lungs of sheep on a farm in Luoyang that exhibited respiratory symptoms and died acutely. The species classification, biological characteristics, and genome sequence analysis of the pathogen were determined. Results Morphological observations, biochemical tests, and phylogenetic analysis confirmed that the isolate was closely related to serotype A1 M. haemolytica GCA-900474405.1. Furthermore, sequence comparison of the capsular gene region revealed that the bacterium belonged to serotype A5 M. haemolytica and was named MH-1. Antibiotic sensitivity tests showed that MH-1 was resistant to tetracycline, erythromycin, spectinomycin and penicillin-G, and sensitive to other selected antibiotics. In animal experiments conducted on mice via intraperitoneal inoculation with MH-1, depression symptoms and dishevelled hair were observed in all mice in the highest-dose group, leading to death. The LD ₅₀ value for mice was determined to be 1.27×10 ⁹ CFU. Whole-genome sequence analysis revealed that MH-1 had a total of 20 open reading frames (ORFs) encoded genes related to pathogenicity, including proteins involved in adhesion, invasion, iron uptake, and antiphagocytosis. While 8 ORFs were responsible for drug resistance genes, such as the macrolide resistance gene macB, the tetracycline resistance gene tet(35), the aminoglycoside resistance gene APH(3')-Ia, and the β-lactam resistance gene CRP. These findings were consistent with the results obtained from the antibiotic susceptibility test. Conclusion In conclusion, we successfully isolated and identified a strain of M. haemolytica serotype A5 from sheep. Through whole-genome sequencing and biological characterization analysis, we have enriched the understanding of the pathogenic properties of M. haemolytica in sheep. This information provides valuable insights for prevention and treatment strategies against M. haemolytica infections in sheep.