Comparative study of iron acquisition by biotype 1 and biotype 2 strains of Actinobacillus pleuropneumoniae.

Four strains of the swine pathogen, Actinobacillus pleuropneumoniae, namely, the type strain (ATCC 27088; biotype 1), the 'reference' strain of biotype 2 (Bertschinger 2008/76), and two additional biotype 1 strains, strain BC181, which is less virulent than the type strain, and strain K17, which was isolated from a lamb, were investigated with respect to iron acquisition. All strains produced iron-repressible outer membrane proteins. However, only the type and biotype 2 strains could acquire iron from porcine transferrin and no organism could utilize human, bovine or ovine transferrin, or ovine or porcine lactoferrin; haemoglobin supported good growth of all strains except strain K17. In all cases, iron acquisition from transferrin and haemoglobin required direct contact between the organisms and the proteins indicating the existence of specific receptors. An affinity isolation technique, using biotinylated porcine transferrin plus streptavidin-agarose, allowed the isolation of the following polypeptides from total membranes of organisms grown under iron-restricted conditions: 99 kDa and 64 kDa from ATCC 27088; 93 kDa from Bertschinger 2008/76; 95 kDa (trace amounts) and 60 kDa from BC181; none from K17. These results indicate that the 93-99 kDa polypeptides are involved in the acquisition of iron from porcine transferrin and that the inability of strain K17 to use transferrin as an iron source is due, probably, to the lack of, or a defect in, an analogous component.