Magda Michal, Boschloo Wendy, Bettoni Serena, Fairley Derek, Russo Thomas A, Giske Christian G, Tellapragada Chaitanya, Rooijakkers Suzan H M, Riesbeck Kristian, Blom Anna M
Department of Translational Medicine, Lund University, Malmö, Sweden.
Department of Microbiology, Belfast Health and Social Care Trust, Belfast, UK.
J Innate Immun. 2025;17(1):112-125. doi: 10.1159/000543664. Epub 2025 Jan 22.
Acinetobacter baumannii is a gram-negative opportunistic bacterium that causes life-threatening infections in immunocompromised hosts. The complement system is a critical mechanism of innate immunity that protects the human body from bacterial infections. Complement activation leads to the deposition of the membrane attack complex (MAC), which can directly lyse gram-negative bacteria. However, A. baumannii has developed evasion mechanisms to protect itself from complement.
Complement deposition was investigated by flow cytometry and Western blotting. Soluble MAC formation was assessed by ELISA. Bacterial serum resistance was determined by the SYTOX Green Assay. Galleria mellonella was used as an infection model. Genome sequencing revealed virulence genes carried by isolates.
We examined clinical isolates of A. baumannii and found 11 isolates with MAC deposition and 5 isolates without deposition. Trypsinization of MAC-positive isolates significantly reduced MAC, indicating incorrect insertion, consistent with a lack of lysis of these strains. MAC-negative isolates inhibited alternative pathway activation and were significantly more serum-resistant. These strains were also more virulent in a G. mellonella infection model. Whole genome sequencing revealed that MAC-negative isolates carried more virulence genes, and both MAC-negative and MAC-positive A. baumannii significantly differed in capsule type. Importantly, a correlation was observed between complement inhibition and capsule type (e.g., capsule locus KL171) of MAC-negative bacteria, while the capsule type (e.g., KL230) of MAC-positive A. baumannii was associated with increased sensitivity to MAC-mediated lysis.
Our findings suggest a relationship between capsule type, complement resistance, and host virulence in A. baumannii.
Acinetobacter baumannii is a gram-negative opportunistic bacterium that causes life-threatening infections in immunocompromised hosts. The complement system is a critical mechanism of innate immunity that protects the human body from bacterial infections. Complement activation leads to the deposition of the membrane attack complex (MAC), which can directly lyse gram-negative bacteria. However, A. baumannii has developed evasion mechanisms to protect itself from complement.
Complement deposition was investigated by flow cytometry and Western blotting. Soluble MAC formation was assessed by ELISA. Bacterial serum resistance was determined by the SYTOX Green Assay. Galleria mellonella was used as an infection model. Genome sequencing revealed virulence genes carried by isolates.
We examined clinical isolates of A. baumannii and found 11 isolates with MAC deposition and 5 isolates without deposition. Trypsinization of MAC-positive isolates significantly reduced MAC, indicating incorrect insertion, consistent with a lack of lysis of these strains. MAC-negative isolates inhibited alternative pathway activation and were significantly more serum-resistant. These strains were also more virulent in a G. mellonella infection model. Whole genome sequencing revealed that MAC-negative isolates carried more virulence genes, and both MAC-negative and MAC-positive A. baumannii significantly differed in capsule type. Importantly, a correlation was observed between complement inhibition and capsule type (e.g., capsule locus KL171) of MAC-negative bacteria, while the capsule type (e.g., KL230) of MAC-positive A. baumannii was associated with increased sensitivity to MAC-mediated lysis.
Our findings suggest a relationship between capsule type, complement resistance, and host virulence in A. baumannii.
鲍曼不动杆菌是一种革兰氏阴性机会致病菌，可在免疫功能低下的宿主中引起危及生命的感染。补体系统是先天性免疫的关键机制，可保护人体免受细菌感染。补体激活导致膜攻击复合物（MAC）的沉积，其可直接裂解革兰氏阴性细菌。然而，鲍曼不动杆菌已形成逃避机制以保护自身免受补体攻击。
通过流式细胞术和蛋白质免疫印迹法研究补体沉积。通过酶联免疫吸附测定法评估可溶性MAC的形成。通过SYTOX Green检测法测定细菌的血清抗性。使用黄粉虫作为感染模型。基因组测序揭示了分离株携带的毒力基因。
我们检测了鲍曼不动杆菌的临床分离株，发现11株有MAC沉积，5株无MAC沉积。对MAC阳性分离株进行胰蛋白酶处理可显著减少MAC，表明插入不正确，这与这些菌株缺乏裂解现象一致。MAC阴性分离株可抑制替代途径激活，且血清抗性显著更强。在黄粉虫感染模型中，这些菌株的毒性也更强。全基因组测序显示，MAC阴性分离株携带更多毒力基因，并且MAC阴性和MAC阳性的鲍曼不动杆菌在荚膜类型上存在显著差异。重要的是，观察到MAC阴性细菌的补体抑制与荚膜类型（如荚膜位点KL171）之间存在相关性，而MAC阳性鲍曼不动杆菌的荚膜类型（如KL230）与对MAC介导的裂解敏感性增加有关。
我们的研究结果表明鲍曼不动杆菌的荚膜类型、补体抗性和宿主毒力之间存在关联。
鲍曼不动杆菌是一种革兰氏阴性机会致病菌，可在免疫功能低下的宿主中引起危及生命的感染。补体系统是先天性免疫的关键机制，可保护人体免受细菌感染。补体激活导致膜攻击复合物（MAC）的沉积，其可直接裂解革兰氏阴性细菌。然而，鲍曼不动杆菌已形成逃避机制以保护自身免受补体攻击。
通过流式细胞术和蛋白质免疫印迹法研究补体沉积。通过酶联免疫吸附测定法评估可溶性MAC的形成。通过SYTOX Green检测法测定细菌的血清抗性。使用黄粉虫作为感染模型。基因组测序揭示了分离株携带的毒力基因。
我们检测了鲍曼不动杆菌的临床分离株，发现11株有MAC沉积，5株无MAC沉积。对MAC阳性分离株进行胰蛋白酶处理可显著减少MAC，表明插入不正确，这与这些菌株缺乏裂解现象一致。MAC阴性分离株可抑制替代途径激活，且血清抗性显著更强。在黄粉虫感染模型中，这些菌株的毒性也更强。全基因组测序显示，MAC阴性分离株携带更多毒力基因，并且MAC阴性和MAC阳性的鲍曼不动杆菌在荚膜类型上存在显著差异。重要的是，观察到MAC阴性细菌的补体抑制与荚膜类型（如荚膜位点KL171）之间存在相关性，而MAC阳性鲍曼不动杆菌的荚膜类型（如KL230）与对MAC介导的裂解敏感性增加有关。
我们的研究结果表明鲍曼不动杆菌的荚膜类型、补体抗性和宿主毒力之间存在关联。
