Characterization of the hemolytic activity of Riemerella anatipestifer

Riemerella anatipestifer infection causes serious economic losses in the duck industry worldwide. Acute septicemia and high blood bacterial loading in R. anatipestifer infected ducks indicate that R. anatipestifer may be able to obtain iron and other nutrients by lysing duck erythrocytes to support its rapid growth and proliferation in the blood. However, so far, little is known about the hemolytic activity of R. anatipestifer to duck erythrocytes. In this study, 29 of 52 R. anatipestifer strains showed hemolytic activity on duck blood agar, whereas all the tested dba⁺ (with hemolytic activity on duck blood agar) and dba^- strains created pores in the duck red blood cells, with 4.35-9.03% hemolytic activity in a liquid hemolysis assay after incubation for 24 h. The concentrated culture supernatants of all the tested R. anatipestifer strains and the extracted outer membrane proteins (OMPs) from dba⁺R. anatipestifer strains showed hemolytic activity on duck blood agar. These results, together with the median lethal dose (LD50) of some dba⁺ and dba^-R. anatipestifer strains in ducklings, suggested that there was no direct relationship between the hemolytic capacity of R. anatipestifer on duck blood agar and its virulence.