Imaging a pseudomonad in mineral suspensions with scanning force and electron microscopy

Techniques for imaging bacteria in their physical environments yield information potentially relevant to better understanding the role of soil and sediment colloids in altering microbial activity. Although cell structure and morphology of bacteria have been previously investigated using light and electron microscopies, both approaches are not without deficiencies. Our objective was to demonstrate the utility of scanning force microscopy (SFM) to image cell morphologies and to observe interactions between bacteria and clay minerals. Pseudomonas fluorescens (Migula), strain 2-79 RN(10), was imaged in pure culture and in suspensions of kaolinite and montmorillonite using both SFM and, for comparison purposes, SEM (scanning electron microscopy). Large-scale SFM and SEM images were similar, thus confirming the lack of artifacts introduced by SFM. Images of bacteria in kaolinite suspensions were not obtained with either technique, a result attributed to bacterial isolation in large pores of the kaolinite matrix. Imaging of montmorillonite-bacteria suspensions showed that bacteria were closely associated with the larger mineral particles. In addition, the high resolution of SFM revealed what appeared to be small hexagonal montmorillonite platelets adhering to cell surfaces. The features of SFM allow for this unique ability to image whole cells as well as the microtopography of cell surfaces within hydrated environments. Scanning force microscopy provides valuable information concerning microbial cell morphology and the interaction of bacterial cells with montmorillonite.