NlpI of Escherichia coli K1 contributes to the recruitment of C4b binding protein to evade serum killing mediated by the classical complement pathway

• Main Authors: Yu-TingTseng, 曾渝婷
• Other Authors: Ching-Hao Teng
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/06404257045472897347

碩士 === 國立成功大學 === 分子醫學研究所 === 98 === E. coli K1 is the most common organism associated with neonatal Gram-negative meningitis. Neonatal E. coli K1 meningitis usually develops as a result of hematogenous spread. Therefore, survival and multiplication in the blood is a prerequisite for E. coli K1 to further infect the central nervous system. Complement system plays a pivotal role in host defense against infection. To survive in the blood, bacteria need to develop a strategy of complement evasion. C4b binding protein (C4bp), an inhibitor of the classical and the lectin complement pathways, has been reported to be recruited to the surface of pathogenic bacteria to interfere activation of the complement system. Our preliminary data have shown that deletion of the nlpI gene of E. coli K1 decreased the bacterium’s ability to cause high-degree bacteremia in 5-day-old rats, suggesting that NlpI is involved in E. coli K1 survival in the blood stream. This study aims to investigate how NlpI contributes to E. coli K1 survival in the blood stream. We found that deletion of nlpI decreased the survival of E. coli K1 in the normal human serum but did not affect the bacterial survival in the heat-inactivated serum in which the complement system was inactivated. In addition, after incubation with serum, the nlpI mutant exhibited significantly higher levels of C3 and MAC (membrane attack complex) deposition than did the wild-type strain. These results demonstrated that NlpI is involved in E. coli K1 evasion of complement attack. The nlpI mutant showed similar survival with the wild-type strain in the modified human serum in which the classical complement pathway was blocked, suggesting that NlpI is involved in E. coli K1 evasion of attack by the classical complement pathway. In addition, deletion of nlpI decreased the ability of E. coli K1 to recruit C4bp to the bacterial surface. By comparing the survival of the wild-type and the nlpI mutant strains in the normal and C4bp-depleted sera, we demonstrated that the decreased C4bp binding on the nlpI mutant is responsible for its decreased resistance to serum-mediated killing. In conclusion, we found that NlpI, a novel bacterial factor, is involved in E. coli K1 evasion of serum-mediated attack. This bacterial factor contributes to E. coli K1 serum survival through facilitating C4bp deposition on bacterial surface to block the attack by the classical complement pathway.