Characterization and Genomic Analysis of Pasteurella multocida NQ01 Isolated from Yak in China

Hemorrhagic septicemia (HS) is a fulminant bovine disease across Asia and Africa, yet <i>Pasteurella multocida</i> (<i>P. multocida</i>) isolated from yak is poorly reported. We isolated strain NQ01 from a fatal HS case in Xizang, China and identified it as <i>P. multocida</i> B:2 by morphology, Gram stain, and PCR (<i>kmt1</i>⁺, <i>bcbD</i>⁺, LPS L2). NQO1 formed smooth, non-hemolytic colonies. After Gram staining, the cells appeared as red rods with bipolar staining. Antimicrobial testing showed broad susceptibility to β-lactams, aminoglycosides, tetracyclines, fluoroquinolones, midecamycin, florfenicol, polymyxin, and vancomycin, with resistance to metronidazole, trimethoprim sulfamethoxazole, and clindamycin. Streptomycin and ofloxacin had intermediate activity. In mice, the intraperitoneal and intranasal LD₅₀ values were 40.64 CFU/mL and 9.53 × 10⁶ CFU/mL, respectively. The intranasal fatal cases were characterized by bacteremia with multifocal disseminated intravascular coagulation involving lung, liver, and spleen. The complete genome comprises a single 2.33 Mb chromosome (40.47% GC, 2115 CDS, no plasmids) with only one resistance gene (<i>Eco_EFTu_PLV</i>) and 28 virulence genes spanning adhesion (<i>tadA</i>, <i>rcpA</i>, <i>ppdD</i>, <i>pilB</i>, <i>tuf/tufA</i>, <i>htpB</i>, <i>PM_RS00430</i>, <i>PM_RS00425</i>, <i>PM_RS08640</i>), immune modulation (<i>lpxB/C/D</i>, <i>msbB</i>, <i>manB</i>, <i>rfaE/F</i>, <i>gmhA/lpcA</i>, <i>kdsA</i>, <i>pgi</i>, <i>wecA</i>, <i>galE</i>, <i>bexD'</i>, <i>ABZJ_RS06285</i>, <i>ABD1_RS00310</i>), and nutritional/metabolic factor (<i>hgbA</i>, <i>hemR</i>, <i>hemN</i>), plus a YadA-like factor. Phylogenetically, NQ01 clusters with regional B:2 bovine/yak isolates. Collectively, these data define NQ01 as a highly virulent, low-resistance yak isolate and a practical model for natural-route HS pathogenesis and targeted control in high-altitude pastoral settings yaks.