Abstract
Evasion of complement-mediated killing is a common phenotype for many different types of pathogens, but the mechanism is still poorly understood. Most of the clinic isolates of Edwardsiella tarda, an important pathogen infecting both of human and fish, are commonly found serum-resistant. To explore the potential mechanisms, we applied gas chromatography-mass spectrometry (GC-MS)-based metabolomics approaches to profile the metabolomes of E. tarda EIB202 in the presence or absence of serum stress. We found that tricarboxylic acid (TCA) cycle was greatly enhanced in the presence of serum. The quantitative real-time PCR (qRT-PCR) and enzyme activity assays validated this result. Furthermore, exogenous succinate that promotes the TCA cycle increased serum resistance, while TCA cycle inhibitors (bromopyruvate and propanedioic acid) that inhibit TCA cycle, attenuated serum resistance. Moreover, the enhanced TCA cycle increased membrane potential, thus decreased the formation of membrane attack complex at cell surface, resulting serum resistance. These evidences suggested a previously unknown membrane potential-dependent mechanism of serum resistance. Therefore, our findings reveal that pathogen mounts a metabolic trick to cope with the serum complement-mediated killing.
Highlights
Edwardsiella tarda is a Gram-negative intracellular pathogen that belongs to the Enterobacteriaceae family with a broad host range that includes mammals, reptiles, and fish [1–3]
To investigate whether E. tarda EIB202 is resistant to serum complement-mediated killing, percent survival of EIB202 was detected in the presence or absence of crucian carp plasma, where Escherichia coli K12 was treated with the same amount of plasma as control
Three mechanisms have been reported for serum resistance in bacteria: cleavage of complement components with protease; inhibition of complement activation through recruitment of factors such as factor H and C4BP to the bacterial cell surface; and lipopolysaccharide- and capsular polysaccharide-mediated suppression of complement activation [38, 39], all of which leads to the failure of membrane attack complex (MAC) formation
Summary
Edwardsiella tarda is a Gram-negative intracellular pathogen that belongs to the Enterobacteriaceae family with a broad host range that includes mammals, reptiles, and fish [1–3]. Further study indicates that E. tarda evades the serum complement-mediated killing by preventing complement activation via the alternative pathway, and that heat-labile surface structures likely play an essential role in the complement evasion of E. tarda [23]. These data provide the basis for further revealing of the detailed mechanisms of complement evasion in E. tarda. The TCA cycle positively regulates the membrane potential through generating NADH, which is used as electron carrier to increase proton motive force or membrane potential This metabolic flow represents an unknown mechanism for serum resistance in Gram-negative bacteria. The development of inhibitors for TCA cycle or screening metabolites that attenuate TCA cycle activation could possibly facilitate the clearance of serum-resistant pathogens
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