肺炎克雷伯菌是奶牛乳房炎的重要诱因，常引起奶牛的产奶量降低和死亡率增高。目前尚缺乏奶牛乳房炎源肺炎克雷伯菌分子特征的研究数据，限制了对其在食品生产链中传播风险的评估。本研究于2018—2019 年从我国北方地区患临床型乳房炎奶牛中采集了6301 份奶样，共分离到183 株肺炎克雷伯菌，两年的平均分离率分别为3.03%和2.80%。与人类临床肺炎克雷伯菌分离株相似，奶牛临床型乳房炎源肺炎克雷伯菌分离株可分为肺炎克雷伯菌(KpI，143 株）、准肺炎克雷伯菌（KpII-B，37 株）和变栖肺炎克雷伯菌（KpIII，3 株）三个种群。在KpI 分离株中检测到超广谱β-内酰胺酶（ESBL）的编码基因bla_SHV-2a、bla_CTX-M-14和bla_CTX-M-15，以及可能与奶牛致病性和宿主适应性有关的编码基因clpC、lpfA、lacI、lacZ、lacY和fecABDEIR。KpI 菌株比KpII-B 和KpIII 菌株表现出更高的流行率以及抗生素耐药基因和毒力基因携带率，表明它可能对奶牛的危害更大。此外，还发现奶牛KpI菌株与人医临床分离的KpI菌株在种群结构上存在明显差异，且奶牛源KpI菌株中很少见到与人类侵袭性感染相关的基因，表明奶牛源KpI菌株对人类健康的威胁较小；奶牛KpII-B 分离株与人类感染分离株具有较高的核苷酸序列一致性，且它们携带固氮基因nif的比例都很高，表明奶牛和人类KpII-B 分离株与植物源性KpII-B 菌株之间存在一定关联。

Klebsiella pneumoniae (K. pneumonia, KpI) is a predominate inducement of bovine mastitis, which is associated with high mortality and milk yield reduction. However, data is lacking on the molecular characteristics of bovine K. pneumoniae, limiting the risk assessment of its transmission through the food chain. Herein, we investigated the prevalence of K. pneumoniae in 6301 clinical mastitis (CM) milk samples from dairy cattle in northern area of China. In total, 183 K. pneumoniae isolates were recovered, with detection rates of 3.0% and 2.8% in 2018 and 2019, respectively. Like human clinical K. pneumoniae, all CM K. pneumoniae isolates belonged to one of three phylogroups: KpI (n = 143), Klebsiella quasipneumoniae subsp. similipneumoniae (KpII-B) (n = 37), and Klebsiella variicola (KpIII) (n = 3). We detected the extendedspectrum β-lactamase-encoding genes blaSHV-2a, blaCTX-M-14, and blaCTX-M-15, as well as clpC, lpfA, lacI, lacZ, lacY, and the fecABDEIR operon in the KpI isolates, which may contribute to their pathogenicity and host adaptability in cows. The high prevalence of KpI in dairy farms may be problematic, as it showed relatively higher rates of antibiotic resistance and virulence gene carriage than the KpII-B and KpIII isolates. Furthermore, we observed distinct differences in population structure between CM- and human infection-associated KpI isolates, with the genes associated with invasive infection in humans rarely being observed in bovine isolates, indicating that few CM-associated K. pneumoniae isolates pose a threat to human health. Nevertheless, bovine KpII-B isolates shared a high level of nucleotide sequence identity with isolates from human infections and frequently carried the nitrogen-fixation gene nif, suggesting an association between KpII-B isolates from cattle and humans, and plant-derived bacteria.