Direct characterization of the physicochemical properties of fungal spores using functionalized AFM probes

A new method is described for characterizing the physicochemical properties of native microbial cells by using atomic force microscopy (AFM) with chemically functionalized probes. Adhesion forces were measured, under deionized water, between probes and model substrata functionalized with alkanethiol self-assembled monolayers terminated with OH and CH(3) groups. These were found to be 6 +/- 2 nN (n = 1024), 0.9 +/- 0.4 nN, and approximately 0 nN, for CH(3)/CH(3), CH(3)/OH, and OH/OH surfaces, respectively, and were not significantly influenced by changes of ionic strength (0.1 M NaCl versus deionized water). This shows that functionalized probes are very sensitive to changes of surface hydrophobicity. Using OH- and CH(3)-terminated probes, patterns of rodlets, approximately 10 nm in diameter, were visualized, under physiological conditions, at the surface of spores of Phanerochaete chrysosporium. Multiple (1024) force-distance curves recorded over 500 x 500-nm areas at the spore surface, either in deionized water or in 0.1 M NaCl solutions, always showed no adhesion for both OH- and CH(3)-terminated probes. Control experiments indicated that the lack of adhesion is not due to transfer of cellular material onto the probe, but to the hydrophilic nature of the spore surface.