Siderophore-Mediated Utilization of Transferrin- and Lactoferrin-Bound Iron by Acinetobacter baumannii

Abstract

Acinetobacter baumannii produces the siderophore called acinetobactin (AB) in response to iron starvation. In this study, in vitro growth experiments were conducted to evaluate the ability of AB to sequester iron bound to human transferrin and lactoferrin and mediate bacterial utilization of the iron. Strain ATCC 19606 producing AB was able to grow in the presence of either 30% iron-saturated human transferrin (30% Fe-TF) or 15% iron-saturated human lactoferrin (15% Fe-LF) as a sole source of iron even when they were separated by a dialysis membrane. The radiolabel after equilibrium dialysis between ⁵⁵Fe-TF and AB was accumulated by cells grown under iron-deficient conditions, but not by those grown under iron-sufficient conditions. Addition of AB to the medium caused great enhancement in the growth of a poor producer strain of AB in the presence of 30% TF or 15% LF. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the outer membrane protein fractions revealed the presence of 77- and 81-kDa proteins only in the cells grown under iron-deficient conditions, suggesting that either of them may function as the receptor for Fe³⁺-AB complex. No strain tested was able to utilize hemin and hemoglobin as a sole source of iron. These results indicate that A. baumannii can utilize iron bound to TF and LF as host iron sources through the action of AB. This system is probably involved in survival and proliferation in the host.