Methods

AFM images presented in this study were all obtained with an AFM mounted on an inverted microscope (Bioscope; Digital Instruments) with samples analyzed either in air or in water using a fluid cantilever holder (as indicated). Force curves were conducted in water using DNP-S silicon nitride cantilevers (Digital Instruments) with the force constants measured for individual tips determined using a correlation based on the Cleveland method13 contained in the DI software (v. 4.32). For more details on specific experiments, the reader should consult the indicated references.

Results are presented here for bacteria that include: Pseudomonasputida KT2442, a bacterium that can degrade substituted aromatic compounds; Pseudomonas stutzeri KC, capable of reductively dehalogenating chlorinated aliphatic compounds; and three strains of Escherichia coli that differ in the length of the lipopolysaccharide (LPS) on the cell surface.8,14,15 Two E. coli strains had progressively truncated LPS chain lengths. E. coli strain D21 (wild type) expresses an LPS that extends from the lipid to the outer core region, while mutant strain D21f2 has an LPS chain that is truncated just after the KDO carboxyl group.8 E. coli K-23 strain JM-109 contains these two parts of the LPS plus an O-antigen, and therefore this strain has a full LPS layer.16

Bacteria were anchored to glass slides for AFM imaging using two different techniques. In the first approach, carboxyl groups on the surface of the bacterium were covalently bonded to amino groups of amino-silane compounds on the surface of a glass slide.17 In the second approach, bacteria were placed onto polyethylenimine (PEI; 750,000 Da) coated slides.16,18

16.3 RESULTS AND DISCUSSION 16.3.1 Bacterial Topography

AFM imaging occurs by rastering the tip back and forth across a surface. The AFM software converts the position of the tip to location, and the plane and a height above a plane so that the image can be presented either as a series of lines in three dimensions,

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