Deletion of the <i>Riemerella anatipestifer</i> type IX secretion system gene <i>sprA</i> results in differential expression of outer membrane proteins and virulence

<i>Riemerella anatipestifer</i> (RA), the causative agent of infectious serositis that targets ducklings and other poultry, secretes protein via the type IX secretion system (T9SS). The proteins transported by T9SS are located on the bacterial cell surface or secreted into the extracellular milieu. In this study, a <i>sprA</i> deletion mutant was constructed encoding a core protein of T9SS to investigate its influence on outer membrane protein expression and its role in virulence. Compared with the wild-type RA-YM strain, the deletion mutant Δ<i>sprA</i> failed to digest gelatin, showed the same growth rate in the logarithmic phase and exhibited greater sensitivity to the bactericidal activity of duck sera, whereas the complemented strain restored these phenotypes. The outer membrane proteome of RA-YM and the Δ<i>sprA</i> mutant were analyzed by Tandem Mass Tags, which revealed 198 proteins with predicted localization to the cell envelope. Sixty-three of these proteins were differentially expressed in the outer membrane, with 43 up-regulated and 20 down-regulated. Among the twelve outer membrane proteins which were secreted by T9SS, four proteins were up-regulated and one protein was down-regulated. Animal experiments demonstrated that the median lethal dose of the mutant strain Δ<i>sprA</i> was about 500 times higher than that of the wild-type RA-YM strain, and bacterial loads in blood, brain, heart, liver and spleen of the Δ<i>sprA</i>-infected ducks were significantly reduced. Our results indicate that the SprA is a virulence-associated factor of RA, and its absence results in altered abundance of outer membrane proteins, and secretion disorders associated with some of the T9SS effector proteins.