Utilization of haem compounds by Aeromonas salmonicida

Abstract

Abstract. The ability of A‐layer‐positi ve (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 haem‐ binding, 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.