Insights into iron acquistion from mineral sources by pseudomonads

Almost all living organisms require iron (Fe) to live. Although iron is abundant on earth, aerobic conditions and circumneutral pH restrict it to the 3+ oxidation state, in which it is bound up as insoluble Fe(III) (hydr)oxide minerals (i.e. hematite, goethite), or, in a human host, tightly sequestered by proteins such as transferrin and ferritin. Hence, although a typical cell needs a concentration of ~10-6 M Fe to survive, readily available, soluble Fe is only ~10-18 M. Microbes have responded to this enormous discrepancy by evolving very efficient strategies for iron acquisition, primarily the production and secretion of Fe-chelating siderophores. However, the long-held assumption that siderophores alone deliver mineral Fe to microbes at a rate sufficient to support microbial growth has recently been called into question by abiotic measurements of siderophore-promoted dissolution. This dissertation presents an approach and bioinformatic analyses based on tools from molecular biology, such as knockout mutant and biosensor strains, in conjunction with classic biochemical and physiological assays, to investigate this process in a living system. The versatility of this approach allowed questions in both environmental and biomedical contexts to be addressed. In studies with the common environmental aerobe P. mendocina, it was found that the contribution of siderophores to Fe acquisition from hematite depended strongly on the presence and concentration of other organic ligands common in the soil. Mineral particle size was also shown to be an important parameter, as nanohematite proved to be far more bioavailable to P. mendocina, even supporting robust growth in the absence of siderophores. Of biomedical relevance, ferritin (a protein 'cage' with a core of nanomineral ferrihydrite) was shown to be an excellent source of Fe to the pathogen P. aeruginosa regardless of the presence of siderophores. Although close cell-mineral interaction was a requirement for siderophore-independent Fe acquisition from nanohematite by P. mendocina, this was not observed for ferritin and P. aeruginosa. Other possible mechanisms include secreted proteases and reductants. Overall, this work shows that microbial acquisition of Fe from mineral sources is complex, and reveals a variable role for siderophores.