UTILIZATION OF HEME COMPOUNDS BY AEROMONAS-SALMONICIDA

The ability of A-layer-positive (A+) and A-layer-negative (A-) strains of Aeromonas salmonicida to utilize haem sources of iron under conditions of iron-restriction was evaluated. In a plate bioassay, only A+ strains of A. salmonicida were able to utilize haem from a variety of sources including haem, haemin, myoglobin, haemoglobin, haemoglobin-haptoglobin and haem-albumin complexes. Trypsin-digestion of whole cells abolished haembinding, indicating that binding was cell-surface associated, involving a protein binding site or receptor. Competitive binding studies indicated that all haem compounds were bound by a common receptor which was not iron-regulated and was associated with the presence of the 49-kDa A-layer protein. The ability of both typical A+ (siderophore-positive) and atypical A+ (siderophore-negative) strains to utilize haem indicated that the mechanism of haem utilization was not siderophore-mediated and that A. salmonicida possesses both siderophore-dependent and siderophore-independent mechanisms to overcome iron-restricted conditions encountered in vivo.