THE ROLES OF IRON IN THE SURVIVAL, GROWTH, AND PATHOGENESIS OF LEGIONELLA PNEUMOPHILA (CHELATORS, SERUM, MEMBRANE, UPTAKE, BINDING

The essentiality of iron for living cells has long been recognized, and the availability of host-iron has been proposed as a contributing factor to virulence in bacterial, fungal, and protozoan infections. The mechanism by which Legionella pneumophila causes disease is unknown. Growth of fresh clinical or environmental isolates in pure culture requires 20 times more iron than is needed for most other bacteria. Thus, increased plasma iron levels may be needed for multiplication within human hosts. This study of the relationship between Legionella and iron was divided into 5 sections. We observed that: (i) this organism can be more readily deprived of iron by iron binding agents than all other bacteria studied, and this inhibition can be reversed by the addition of iron; (ii) normal human blood serum kills L. pneumophila and the bactericidal action is decreased when complement is inactivated or enough iron to saturate serum transferrin is added to the system; (iii) in assays with a radioactive isotope of iron (('55)Fe), no specific iron sequestering system was detected; (iv) in analyses of outer membrane proteins with ('55)Fe, SDS-polyacrylamide gel electrophoresis, and autoradiography, no specific outer membrane proteins responsible for iron acquisition were observed; and (v) in assays for protease, iron does not stimulate production of extracellular proteases. These observations indicate that L. pneumophila has no specific iron uptake mechanism, but instead relies on passive diffusion and/or non-specific mechanisms to obtain its iron. When no iron binding agents are present, L. pneumophila requires no more iron in its environments than do most other bacteria. Because of its tenuous method for obtaining iron, iron withholding in vitro is bactericidal against L. pneumophila. The importance of iron withholding against L. pneumophila in in vivo systems remains to be determined.