Humidity-Dependent Bacterial Cells Functional Morphometry Investigations Using Atomic Force Microscope

The effect of a relative humidity (RH) in a range of 93–65% on morphological and elastic properties of Bacillus cereus and Escherichia coli cells was evaluated using atomic force microscopy. It is shown that gradual dehumidification of bacteria environment has no significant effect on cell dimensional features and considerably decreases them only at 65% RH. The increasing of the bacteria cell wall roughness and elasticity occurs at the same time. Observed changes indicate that morphological properties of B. cereus are rather stable in wide range of relative humidity, whereas E. coli are more sensitive to drying, significantly increasing roughness and stiffness parameters at RH 84% RH. It is discussed the dependence of the response features on differences in cell wall structure of gram-positive and gram-negative bacterial cells. 1. Introduction Significant progress in using of atomic force microscope (AFM) as a tool for investigations of eukaryotic and prokaryotic cells has been reached during past decade [1, 2]. In contrast to traditional methods of visualization—scanning electron and optical microscopy—atomic force microscopy offers important benefits: high spatial resolution, real quantitative data acquisition in three dimensions, relatively simple and nondestructive sample preparation procedure, and flexibility in ambient operating conditions (i.e., without the need for a vacuum or gold sputtering) [3]. Besides topographic imaging, AFM makes it possible to probe local surface forces and mechanical properties of biomaterials [4]. In particular, mechanical properties of mammalian [5] and bacterial cells [6] have been measured. Though the method of atomic force microscopy is relatively new, it could become widespread in microbiological studies that use bacteria as sensors, changing their morphological characteristics at various exposures. Thus, AFM has been used to study temperature-dependent morphological alterations of prokaryotic cells [7] and effects of antibiotics on E. coli and S. aureus [8]. It is important to take into consideration that different environmental conditions that often remain unregistered could distort AFM results at investigation of physical and morphological properties of bacterial cells. For example, the humidity of the environment where AFM specimens are left to dry is often ignored [8, 9], though distinct differences in morphology of bacterial cells growing at different relative humidity were observed by De Goffau et al. [10]. Therefore, the development and standardization of AFM methods for preparation and imaging of