Hemoglobin and the hemoglobin-haptoglobin, heme-hemopexin,
and heme-albumin complexes as well as catalase and myoglobin-haptoglobin can all be utilized by H. influenzae as heme sources in vitro [12–14]. The mechanisms underlying the utilization of these protein heme sources have been extensively studied [6, 15–19]. In addition to its ability to utilize these multiple heme sources, H. influenzae can also grow when supplied with PPIX in the presence of an iron source in vitro. Iron sources that can be utilized under such circumstances include various iron salts as well as iron bound to the human iron-binding protein transferrin [20–24]. Utilization of iron bound to transferrin by H. influenzae is mediated by specific selleck compound outer membrane binding proteins [25, 26]. In many microbial species utilization of iron is mediated by small secreted iron binding molecules termed siderophores (generally < 1 kDa) [27, 28]. Siderophores have high affinity and specificity for ferric iron, which they bind in the extracellular milieu. The siderophore-iron complex then binds to the corresponding membrane protein receptor on the
cell surface as the first step PF-573228 datasheet in the utilization of the bound iron [27, 28]. It generally has been assumed that H. influenzae neither produces nor utilizes siderophores as a means of iron acquisition. Evidence to support this conclusion includes the following: 1) using the universal Thiamet G siderophore assay of Schwyn
and Neilands [29], modified to permit growth of Haemophilus species [30], the H. influenzae type b strain Eagan did not produce detectable siderophore(s) [21, 30]; 2) strain Eagan was unable to utilize the exogenously supplied siderophores enterobactin, aerobactin or ABT-263 deferroxamine as an iron source [24]; 3) utilization of iron bound transferrin by H. influenzae is dependent on direct contact between the bacterial cell and transferrin, indicating that there is no release of a small iron binding molecule(s) by the bacteria [25]; 4) outer membrane proteins from iron-restricted H. influenzae did not react to polyclonal antisera raised against various siderophore receptor proteins from E. coli, whereas similar outer membrane preparations from the closely related H. parainfluenzae did react [24]; 5) DNA probes based on the sequence of genes encoding E. coli siderophore receptor proteins did not hybridize to H. influenzae chromosomal DNA [24]. Although these data are essentially limited to examination of type b strains they have been generally interpreted to indicate that the species H. influenzae in general neither produces nor utilizes siderophores. Recently multiple genomic sequences from strains of H. influenzae have become available. One of these genomic sequences contains a gene cluster with significant homology to components of ferric hydroxamate uptake systems present in other bacteria.