David A Ams

Experimental Measurements of the Adsorption of Bacillus subtilis and Pseudomonas mendocina Onto Fe-Oxyhydroxide-Coated and Uncoated Quartz Grains

In this study, we compared the adsorption of the gram-positive bacterium Bacillus subtilis with adsorption of the gram-negative bacterium Pseudomonas mendocina onto Fe-oxyhydroxide-coated and uncoated quartz grains as a function of pH and bacteria: mineral mass ratio. We studied metabolically-inactive cells in order to focus on the initial bacterial attachment mechanisms. The data show that the presence of Fe-oxyhydroxide-coatings on quartz surfaces significantly enhances the adsorption of bacteria and that in general the extent of adsorption decreases with increasing pH and with decreasing bacteria: mineral mass ratio. B. subtilisadsorbs to a greater extent than does P. mendocina onto the surface of the Fe-coated quartz. The adsorption behavior appears to be controlled by the overall surface charge of both bacterial and mineral surfaces. We model the adsorption data using a semi-empirical chemical equilibrium model that accounts for the site speciation of the adsorbing surfaces. Models describing bacterial adsorption to Fe-oxyhydroxide-coated quartz can account for changes in pH and bacteria: mineral mass ratio using one set of equilibrium constants.

Siderophore production by an aerobic Pseudomonas mendocina bacterium in the presence of kaolinite

Abstract The purpose of this study was to quantify siderophore production by the aerobic bacterium, Pseudomonas mendocina , under Fe-limited conditions as a function of Fe source: supplied in dissolved form (as 30 μM Fe-EDTA), as natural Fe-containing kaolinite, and a no-Fe-added control. Siderophores are Fe(III)-specific organic ligands produced by microorganisms under conditions of Fe stress. Siderophore production was compared with previous experiments wherein Fe was supplied as hematite [Geomicrobiol. J. 17 (2000) 1]. Microbial growth increased in the order: no-added-Fe control kaolinite>hematite≥Fe-EDTA. Thus, the bacterium was less Fe stressed in the presence of kaolinite than in the no-added-Fe control, confirming that kaolinite serves as a source of Fe to the bacterium. Although more siderophore per cell was produced in the most stressed system (no-Fe-added), more total siderophore was produced in the least stressed system (Fe-EDTA). This is due to the presence of a larger total population size in the least stressed system. Hence, the ability of a microbial population to produce siderophores as a means to access mineral-bound Fe is a complex function of both Fe stress and population size.