Comprehensive capsular, lipopolysaccharide, virulence, and antimicrobial resistance profiling of Pasteurella multocida isolated from buffaloes in Vietnam: First report of capsular type D and predominant L2 genotype

Background and Aim: Pasteurella multocida is a primary cause of hemorrhagic septicemia (HS) in buffaloes across tropical regions, leading to rapid deaths and significant economic losses. In Vietnam, recurrent HS outbreaks have been reported, yet most studies have solely focused on bacterial isolation or species-level identification. Comprehensive molecular data, including capsular typing, lipopolysaccharide (LPS) genotyping, virulence–associated genes (VAGs), and antimicrobial resistance genes (ARGs), remain limited for buffalo-derived strains. This molecular gap hampers understanding of strain diversity, epidemiology, and vaccine development. Notably, no previous Vietnamese study has concurrently characterized capsule, LPS, VAG, and ARG profiles or reported atypical serogroups in buffaloes. Therefore, integrated molecular surveillance is crucial to detect emerging lineages and guide One Health–oriented disease management. This study aimed to provide the first comprehensive molecular characterization of P. multocida strains isolated from buffaloes in Vietnam’s Central Highlands, incorporating capsular typing, LPS genotyping, virulence gene profiling, and antimicrobial resistance detection. Materials and Methods: Sixty-seven P. multocida isolates were recovered from lungs, bone marrow, and nasal swabs of clinically affected buffaloes (2022–2025). Species confirmation and molecular screening for capsular types (A, B, D), LPS genotypes (L1–L8), 12 VAGs, and seven ARGs were performed using polymerase chain reaction-based assays. Three representative isolates underwent 16S rRNA sequencing for phylogenetic analysis. Prevalence estimates, along with their 95% confidence intervals, were calculated, and chi-square tests were performed. Results: Capsular type B was the most common (62.7%), followed by type A (31.3%). Importantly, capsular type D (5.9%) was identified for the first time in Vietnamese buffaloes. LPS genotyping showed L2 as the predominant genotype (56.7%), with L6 (19.4%), L1 (16.4%), and L3 (7.5%) also present. All isolates contained eight conserved VAGs, while pfhA (58.2%) and hgbB (34.3%) showed variable presence. ToxA and nanH were not detected. Four ARGs, floR (22.3%), tetB (11.9%), blaROB1 (10.4%), and tetH (4.4%), were observed. Phylogenetic analysis clustered all isolates within the P. multocida group, with the type D isolate forming a minor diverging sub-branch. Conclusion: This study establishes the first multilocus molecular profile of P. multocida in Vietnamese buffaloes, highlighting the emergence of capsular type D and the dominance of L2 genotypes. These findings support better diagnostics, vaccine antigen selection, and antimicrobial stewardship. Future work using multilocus sequence typing/whole–genome sequencing across livestock species will improve understanding of regional transmission dynamics within a One Health framework. Keywords: antimicrobial resistance genes, buffalo, capsular typing, hemorrhagic septicemia, lipopolysaccharide genotype, molecular epidemiology, One Health, Pasteurella multocida, Vietnam, virulence–associated genes.