Growth and Attachment-Facilitated Entry of Bacteria into Submicrometer Pores Can Enhance Bioremediation and Oil Recovery in Low-Permeability and Microporous Media

Bioremediation end points for biodegradable hydrophobic compounds in soil aggregates are regulated by bacterial accessibility to different pore sizes. We evaluated the accessibility of the nonmotile hydrocarbon-degrading bacterium Dietzia mans (d = 1 pm in the stationary phase) to 0.4 mu m pores. A significant fraction (22%) of the pore volume of the clayey soil from which the bacterium was isolated was associated with 0.4-1 mu m diameter pores. The entry of bacteria into the pores was observed by electron microscopy and by monitoring mineralization of [C-14]hexadecane placed well above membranes with fixed pore sizes (0.4 or 3 mu m), in a bioreactor. The membranes were used as a surrogate for soil pores of fixed diameters. When membranes were not wetted or were wetted with nonbiodegradable heptamethylnonane, bacteria did not penetrate pores even if they attached to the membrane. However, bacteria penetrated pores when membranes were wetted with hexadecane, as growth on hexadecane yielded a crowd of smaller rod-shaped cells (d(min) = 0.54 +/- 0.14 mu m) in biofilms formed on the membrane. A morphological progression with time from smaller, elongated cells in the early growth phase to cocci-shaped cells was observed. The results suggest proliferation accompanied by morphological changes as a mechanism of bacterial migration in submicrometer pores and low-permeability media.