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Pseudomonas fluorescens ATCC 13525 was found to grow in a minimal mineral medium supplemented with millimolar amounts of aluminum, a known environmental toxicant. During the stationary phase of growth, the trivalent metal was localized in a phosphatidylethanolamine (PE)-containing residue. The concentration of PE in pellets ranged from 1.7 to 13.9 mg ml of culture ؊1 in media supplemented with 1 to 30 mM aluminum. Although the gelatinous residue was observed during the stationary phase of growth, ultracentrifugation and dialysis experiments revealed that PE was produced from earlier stages of incubation and was associated with aluminum. A sharp diminution in the levels of PE and aluminum in the spent fluid was concomitant with the formation of the insoluble deposit. The aluminum content of the soluble cellular fraction increased during growth and reached an optimum of 1.85 mM of test metal at 45 h in cultures with 15 mM aluminum. Further incubation, however, led to a marked decrease in the cellular aluminum content, and during the stationary phase of growth, only trace amounts of the trivalent metal were detected in this fraction. When 45-h cells were incubated in fresh citrate medium, most of the intracellular aluminum was secreted in the spent fluid and citrate was rapidly consumed. Aluminum efflux was also observed in cultures in which D-glucose was substituted for citrate. However, no efflux of this trivalent metal was evident in media devoid of either citrate or D-glucose. Scanning electron microscopic studies and X-ray energy-dispersive analyses of the dialyzed supernatant aided in the visualization of nodule-like aluminum-and phosphorus-rich bodies associated with thread-like carbon-, oxygen-, and phosphorus-containing structures. Transmission electron microscopic and electron energy loss spectroscopic analyses revealed the presence of aluminum within bacteria after 45 h of incubation. Cells harvested after aluminum insolubilization did not shown aluminum inclusions. This aluminum-tolerant microbe may have potential application in bioremediation processes.

The presence of aluminum in a minimal mineral medium, with citrate as the sole source of carbon, elicited the production of an exocellular lipid‐rich residue in Pseudomonas fluorescens. Although aluminum affected the growth rate, no apparent diminution in cellular yield was observed in medium supplemented with up to 3 mM of the trivalent metal. While a 10% reduction in cellular yield was recorded in medium with 7.5 mM aluminum, a decrease of 69% was reported in medium supplemented with 50 mM aluminum at stationary phase of growth. The wet weight of the pellet ranged from 5.9 to 70.3 mg mL−1 of culture in medium supplemented with 1–30 mM aluminum. The trivalent metal was immobilized in this precipitate and associated with phosphate moieties. Both neutral lipids and phospholipids constituted the pellet. More than 90% of the weight of the wet pellet was lost following lyophilization. The percentage of aluminum immobilized increased with the concentration of trivalent metal in the medium. Preconditioned cells showed an improved growth rate, and the cellular yield was slightly higher than for unadapted cells.

Pseudomonas fluorescens appeared to circumvent the challenge imposed by millimolar amounts of metals (5 mM Al3+, 5 mM Fe3+, 2 mM Ca2+, 1 mM Ga3+ and 3 mM Zn2+) by the formation of phosphatidylethanolamine. This lipid moiety constituted an important organic component of an insoluble gelatinous residue in which most of the test metals were immobilized at stationary phase of growth. Ultracentrifugation and dialysis experiments showed that the metals were associated with phosphatidylethanolamine from early stages of growth. Transmission electron microscopy revealed metal rich bodies in the cytoplasm prior to their secretion in the spent fluid. These results demonstrate a role of phosphatidylethanolamine in multiple‐metal homeostasis.

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