The growth-promoting effect of bacterial iron for serum-exposed bacteria

Bacterial ability to obtain iron in bovine serum or in media containing transferrin (Tr) or conalbumin (Ca) was investigated by using serum-resistant (virulent) and serum-sensitive (avirulent) strains of Escherichia coli and Salmonella typhimurium. Bacteria growing in bovine serum enriched with radioactive iron-saturated Tr or with radioactive iron-saturated enterobactin (E) did not acquire radioactive iron. It has been found that the passage of siderophore (Si)-iron complexes into bacteria is blocked in serum by Tr and in Ca-containing medium by Ca. The investigation of bacterial ability to take iron in synthetic media showed that bacteria take in Si-bound but no Tr-bound radioactive iron. In the absence of free iron, the growth of serum-exposed virulent bacteria was supported by their stored iron. Virulent bacteria passaged in medium void of usable iron became depleted in stored iron and did not grow in animal sera unless sera were enriched by the addition of exogenous iron. Experiments with serum-exposed avirulent bacteria showed that their growth in Si-enriched serum should not be attributed to the iron-providing activity of Si but to the stimulating effect of Si which facilitates the use of stored iron. As distinct from avirulent bacteria, virulent bacteria used stored iron without the stimulating activity of extracellular Si.