NlpI contributes to Escherichia coli K1 evasion of serum mediated-killing by
recruiting C4bp to inactivate the classical complement pathway
NlpI 蛋白幫助 K1 大腸桿菌與 C4b 結合蛋白交互作用而躲避血清中典型路徑
所調控的補體系統之擊殺
Advisor: Dr. Ching-Hao Teng
Student: Yu-Ting Tseng
Abstract
E. coli K1 is the most common organism associated with neonatal Gram-negative bacteremia and
meningitis. E. coli meningitis develops as a result of hematogenous spread. Therefore, survival and
multiplication in the blood is a prerequisite for E. coli K1 to cross the blood-brain barrier (BBB) and
penetrate into the central nervous system. However, the microbial determinants contributing to E. coli K1
survival in the blood remain incompletely understood
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. To further investigate the mechanism how NlpI contributes to the
survival of E. coli K1 in the blood stream, we examined whether NlpI is involved in evasion of
complement-mediated attack by comparing the survival of the wild-type E. coli K1 strain RS218 and its
nlpI mutant in the human serum. The two strains showed similar ability to survive in the heat-inactivated
serum, while the nlpI mutant exhibited significantly lower ability to survive in the normal human serum
compared with that of the wild-type strain. The results demonstrate that NlpI is likely to be involved in E.
coli K1 evasion of complement-mediated attack.
We next determined which complement pathways were involved in the NlpI-mediated E. coli K1
evasion of serum killing. The wild-type and the mutant strains exhibited similar survival rates in the serum
in which the classical complement pathway has been blocked, while the mutant still exhibited significantly
lower ability than the wild-type strain to survive in the serum in which the alternative complement pathway
has been blocked. The results suggest that NlpI is able to assist E. coli K1 in evading the attack of classical
complement pathway in the blood stream.
C4bp is regulatory factor which is able to inactivate classical complement pathways. It has been
reported that E. coli K1 are able to recruit C4bp to their surface to inactivate the attack mediated by the
classical complement pathway in the blood. We examined whether NlpI was involved in recruitment of
C4bp to the surface of E. coli K1. After incubation with normal human serum, the wild-type and nlpI
mutant strains were subjected to flow cytometry analysis with antibodies against C4bp to determine the
levels of C4bp deposition on the bacteria. The results showed that the nlpI mutant exhibited significantly
lower ability to recruit C4bp than that the wild-type strain did. In addition, the nlpI mutant exhibited
similar survival rates in the normal and C4bp-depleted serum, while the wild-type strain exhibited
significantly lower ability to survive in the C4bp-depleted serum than that in the normal serum. The results
suggested that the decreased ability of the nlpI mutant to recruit C4bp contributed to the decreased ability
of the mutant to survive in the human serum.