Edwardsiella tarda evades serum killing by preventing complement activation via the alternative pathway

• Edwardsiella tarda exhibited a strong resistance against the bactericidal effect of fish serum.
• The presence of Mg2+ decreased the serum survival of E. tarda.
• E. tarda-incubated serum (EIS) possessed strong hemolytic and bactericidal activity.
• EIS exhibited significantly less chemotactic activity than Escherichia coli-incubated serum.
• Serum incubated with heat-inactivated E. tarda had no apparent hemolytic activity.

Edwardsiella tarda is a Gram-negative bacterium with a broad host range that includes a wide variety of farmed fish as well as humans. E. tarda has long been known to be able to survive in host serum, but the relevant mechanism is unclear. In this study, we investigated the fundamental question, i.e. whether E. tarda activated serum complement or not. We found that (i) when incubated with flounder serum, E. tarda exhibited a high survival rate (87.6%), which was slightly but significantly reduced in the presence of Mg2+; (ii) E. tarda-incubated serum possessed strong hemolytic activity and bactericidal activity, (iii) compared to the serum incubated with a complement-sensitive laboratory Escherichia coli strain, E. tarda-incubated serum exhibited much less chemotactic activity, (iv) in contrast to the serum incubated with live E. tarda, the serum incubated with heat-inactivated E. tarda exhibited no apparent hemolytic capacity. Taken together, these results indicate for the first time that E. tarda circumvents serum attack by preventing, to a large extent, complement activation via the alternative pathway, and that heat-labile surface structures likely play an essential role in the complement evasion of E. tarda.