Binding of haemin by the fish pathogen Photobacterium damselae subsp. piscicida

Whole cells of virulent (DI 21 and B 51) and avirulent (ATCC 29690 and EPOY 8803-II) strains of Photobacterium damselae subsp. piscicida, grown under iron-supplemented or iron-restricted conditions, were able to bind haemin. Iron limitation resulted in an increased binding of haemin by DI 21, B 51 and ATCC 29690 cells but did not affect the haemin-binding ability of the EPOY 8803-II cells. Proteinase K treatment of whole cells markedly reduced the binding of haemin, indicating that protein receptors located at the cell surface are involved in the binding. This was confirmed by the observation that isolated total as well as outer membrane proteins from all the strains, regardless of the iron levels of the media, were able to bind haemin, with the outer membranes showing the strongest binding. Haemin binding by membrane protein extracts was not affected by heat treatment but was almost completely abolished by Proteinase K treatment, suggesting the presence of thermostable protein receptors for haemin. The capsular polysaccharide also appears to play a minor role in binding of haemin. It was concluded that constitutive as well as inducible mechanisms of haemin binding occur in P. damselae subsp. piscicida. These mechanisms would rely mainly upon the direct interaction between the haemin molecules and surface-exposed outer membrane protein receptors.