The low concentration of free iron in body fluids creates bacteriostatic conditions for many microorganisms and is therefore an important defense factor of the body against invading bacteria. Pathogenic bacteria have developed several mechanisms for acquiring iron from the host. Siderophore-mediated iron uptake involves the synthesis of low molecular weight iron chelators called siderophores which compete with the host iron-binding glycoproteins lactoferrin (LF) and transferrin (TF) for iron. Other ways to induce iron uptake, without the mediation of siderophores, are the possession of outer membrane protein receptors that actually recognize the complex of TF or LF with iron, resulting in the internalization of this metal, and the use of heme-compounds released into the circulation after lysis of erythrocytes. In this review, the nonsiderophore-mediated iron-uptake systems used by certain pathogenic bacteria are emphasized. The possible contribution of these iron-uptake systems to the virulence of pathogens is also discussed.

Summary. The importance of K antigen of Klebsiella as a virulence factor was studied in nine pairs of K + and K-strains, each pair isogenic apart from the.presence of K antigen. Loss of K antigen by nine K + strains resulted in the reduced virulence of their K-variants in a mouse-skin model. This reduced virulence of K-strains for mice may be explained in all strains by a higher degree of phagocytosis as measured by chemiluminescence response of human polymorphonuclear leukocytes (PMNL) and in most strains by enhanced killing by either human PMNL or human serum or both. Although the protective role of the K antigen in serum-induced killing and killing by PMNL was generally evident, our results also suggested that other virulence factors were sometimes involved.

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Under iron starvation, Bacteroides fragilis expresses various iron-regulated outer membrane proteins. In this study, a deferrated minimal medium was used in growth experiments, and the role of one of these iron-regulated outer membrane proteins (a 44-kDa protein) in an iron uptake mechanism which acquires iron from heme compounds was elucidated. When a specific 44-kDa protein antiserum was used in a medium with heme as the only iron source, growth inhibition was observed. These results demonstrate that the 44-kDa outer membrane protein plays an important role in the uptake of heme in B. fragilis.

Of the five Bacteriodes species of the ‘fragilis group’ only Bacteroides fragilis was able to grow in human plasma. Therefore the capacity of several iron sources to stimulate to growth of Bacteroides species under iron restricted conditions in vitro was tested. The iron chelator bipyridyl was used for the restriction of iron in the media. Ferrous sulphate, ferric ammonium sulphate and ferric citrate stimulated the growth of all five Bacteroides species tested to the same extent. B. fragilis, and to a lesser extent B.thetaiotaomicron and B. distasonis were better able than B. vulgatus and B. ovatus to use haem‐compounds as an iron source in the presence of the iron chelator bipyridyl. All five Bacteroides species tested could use 30% iron‐saturated transferrin. There was no correlation between the ability of the strains to grow in human plasma and the ability to use either haem‐compounds of transferrin as a source of iron.