Cellular response to a multiple-metal stress in Pseudomonas fluorescens

Abstract

Pseudomonas fluorescens multiplied in a minimal mineral medium supplemented with millimolar amounts of aluminum (5 mM), iron (5 mM), zinc (3 mM), calcium (2 mM) and gallium (1 mM). A slight decrease in growth rate and a 22% diminution in cellular yield were observed as compared to the control medium. Citrate, the sole source of carbon to which the test metals were complexed, was completely utilized. Although at stationary phase of growth most of the metals were immobilized in an exocellular lipid-rich residue, ultracentrifugation and dialysis studies revealed that metals were associated with phosphatidylethanolamine (PE) from early stages of growth. As growth progressed the metal content of the soluble cellular extract increased reaching an optimum at 35 h of incubation. However, no detectable amounts of metals in this cellular component were discerned at stationary phase of growth. There appeared to be no marked variation in exocellular protein and carbohydrate production in control and metal-stressed cultures. Transmission electron microscopic studies revealed metal rich bodies associated with the cytoplasm. Scanning electron microscopic analyses of the dialyzate aided in the identification of the metal-rich bodies associated with elongated structures comprised of carbon, oxygen and phosphorus. PE appeared to be an important organic constituent of the gelatinous residue.