Investigation of bacterial-mineral interactions using fluid tapping mode™ atomic force microscopy

Abstract

Bacterial adhesion to the porous media of subsurface environments and their mobility through these systems are of interest in biogeochemistry. Dissimilatory reducing bacteria can be major contributors to good or poor water quality due to their ability to catalyze sulfate and iron reduction as well as denitrification reactions. Advances in Fluid Cell Tapping Mode™ Atomic Force Microscopy (Fluid TMAFM) make it possible to better understand these natural phenomena through direct observations of live bacteria-mineral surface interactions. This study used Fluid TMAFM to investigate the effect of iron coatings on the interactions of Shewanella putrefaciens with silica glass surfaces (as analogues for quartz). S. putrefaciens are facultative anaerobic dissimilatory Fe-reducing bacteria that are closely related to Pseudomonas spp. These were seeded onto ferric iron oxyhydroxide coated (Fe-coated) and uncoated silica glass substrates in aqueous solutions of varying nutrient composition and incubated under aerobic conditions for 1-4 days. Seeded and control surfaces were examined using Air and Fluid TMAFM. Observations of live bacteria-surface interactions found that bacteria in nutrient-depleted solutions adhered to Fe-coated substrates more strongly than bacteria seeded in nutrient-rich solutions. Under nutrient-limited conditions, the adhered bacteria corrode the iron coating to form bacteria-shaped depresssions within 3 days of exposure. As much as 10% of the iron coatings were mobilized. Also, they adhered more strongly to Fe-coated than uncoated silica glass. Bacteria seeded in nutrient-enriched solutions were mostly flagellated suggesting that motility and adhesion are related. These findings suggest that adhesion, motility, and iron surface chemistry are interrelated in subsurface environments where Fe-reducing microorganisms are present.