Pasteurella multocida (P. multocida) is a Gram-negative bacterium which causes diseases in poultry, livestock, and humans, resulting in huge economic losses. P. multocida serovar A CQ6 (PmCQ6) is a naturally occurring attenuated strain with a thin capsule. Thus, we aimed to explore why this strain is less virulent and produces less capsule compared with P. multocida serovar A strain CQ2 (PmCQ2). Analysis of capsular polysaccharide synthesis genes in PmCQ6 revealed that, compared with PmCQ2, there was only a single point mutation in the initiation codon sequence of the hyaC gene. To test whether this point mutation caused capsular deficiency and reduced virulence, we rescued this hyaC mutation and observed a restoration of capsule production and higher virulence. Transcriptome analysis showed that the hyaC point mutation led to a downregulation of capsule synthesis and/or iron utilization related-genes. Taken together, the results indicate that the start codon mutation of hyaC is an important factor affecting the capsule synthesis and virulence of PmCQ6.

多杀巴氏杆菌（P. multocida）是一种革兰氏阴性菌，可引起家禽、牲畜和人类疾病，造成巨大的经济损失。多杀假单胞菌血清型 A CQ6 (PmCQ6) 是一种天然存在的减毒菌株，具有薄荚膜。因此，我们旨在探索为什么该菌株与多杀性假单胞菌血清型 A 菌株 CQ2 (PmCQ2) 相比毒性较低且产生较少的荚膜。对PmCQ6荚膜多糖合成基因的分析表明，与PmCQ2相比，hyaC基因的起始密码子序列仅存在单点突变。为了测试这个点突变是否会导致荚膜缺乏和毒力降低，我们拯救了这个hyaC突变并观察到荚膜产生的恢复和更高的毒力。转录组分析表明，hyaC点突变导致胶囊合成和/或铁利用相关基因的下调。综上所述，结果表明hyaC的起始密码子突变是影响PmCQ6荚膜合成和毒力的重要因素。