McGarry Naoise, Smith Stephen G J
Department of Clinical Microbiology, School of Medicine, Trinity College Dublin, D08 RX0X, Dublin, Ireland.
FEMS Microbes. 2025 Jul 2;6:xtaf009. doi: 10.1093/femsmc/xtaf009. eCollection 2025.
Extraintestinal pathogenic (ExPEC) is a major cause of urinary tract infections, bacteraemia, and sepsis. CFT073 is a prototypic, urosepsis isolate of sequence type (ST) 73. ST73 isolates are associated with higher virulence scores than other pandemic clonal groups, such as ST131. This laboratory, among others, has previously shown that strain CFT073 is serum-resistant, with virulence factors such as the exopolysaccharide capsule and other extracellular polysaccharides imparting resistance to the complement system. In this study, it was shown that culture supernatants were protective in standardized serum killing assays, when compared to cultures standardized in fresh medium. Diluting cultures in fresh medium in place of conditioned medium significantly increased sensitivity of CFT073 to serum, indicating that a secreted factor may provide resistance to serum. Haemolysin, a pore-forming toxin, is secreted by CFT073 in a calcium-dependent manner. This study found that a CFT073 mutant is significantly more sensitive to 50% serum than the wild-type, implicating haemolysin in the response of CFT073 to serum. In addition to acting as a toxin upon secretion, it has been shown previously that HlyA forms a complex with lipopolysaccharide (LPS), which permits modulation of host immune responses by HlyA whilst cell-associated. The effect of HlyA on capsule expression and serum resistance was examined and characterized in this study, with results indicating that perhaps the HlyA-LPS complex interacts with surface capsule. This study is the first to identify haemolysin as a virulence factor promoting resistance to serum in CFT073, acting whilst associated with the cell.
肠外致病性大肠杆菌（ExPEC）是尿路感染、菌血症和败血症的主要病因。CFT073是序列型（ST）73的原型泌尿道败血症分离株。与其他大流行克隆群（如ST131）相比，ST73分离株具有更高的毒力评分。包括本实验室在内的其他研究先前已表明，CFT073菌株具有血清抗性，其毒力因子如胞外多糖荚膜和其他胞外多糖可赋予对补体系统的抗性。在本研究中，结果表明，与在新鲜培养基中标准化培养的菌液相比，培养上清液在标准化血清杀伤试验中具有保护作用。用新鲜培养基代替条件培养基稀释菌液显著增加了CFT073对血清的敏感性，表明一种分泌因子可能提供对血清的抗性。溶血素是一种形成孔道的毒素，由CFT073以钙依赖的方式分泌。本研究发现，CFT073突变体对50%血清的敏感性明显高于野生型，这表明溶血素与CFT073对血清的反应有关。除了在分泌时作为毒素起作用外，先前已表明HlyA与脂多糖（LPS）形成复合物，这使得HlyA在与细胞相关时能够调节宿主免疫反应。本研究检测并表征了HlyA对荚膜表达和血清抗性的影响，结果表明，HlyA-LPS复合物可能与表面荚膜相互作用。本研究首次确定溶血素是促进CFT073对血清抗性的毒力因子，在与细胞相关时起作用。
