Whole Cell Biochemical and Nanomechanical Investigations of Bacillus Using Atomic Force Microscopy

Abstract

Atomic force microscopy (AFM) based adhesion force spectroscopy and elasticity measurements have emerged as powerful tools in the biophysical analysis of cellular systems. Such measurements can now be extended to probe the distribution of specific biomolecules and elasticity at the single cell level. Here, we report on studies using Bacillus cereus, a common food-borne pathogen, as a model system. Using AFM-based adhesion force spectroscopy coupled with lectin probes - wheat germ agglutinin (WGA) and concanavalin A (ConA), we show the spatial mapping of specific cell-surface carbohydrate targets - N-acetylglucosamine (GlcNAc) and mannose/glucose (Glu). We show the compositional change from the vegetative cell to the spore, mapped, and quantified at the nanoscale across single B. cereus cell surfaces. The surface molar ratios of GlcNAc:Glu are ∼4:1 on a vegetative cell surface but display a switch to ∼1:3 on a spore surface. This trend is in excellent agreement with previously reported values using GC-MS and chromatography conducted on bulk samples. Further, we investigated the morphological and nanomechanical transformation of B. cereus in the sporulation process in response to temporal nutrient deprivation conditions. Using AFM imaging and elasticity mapping, we observed the morphogenesis and the progression in elasticity of the endospore and released mature endospore. The elastic modulus increased nearly 300% from the rod-like vegetative cell (1.1± 0.2GPa) to the oval-shape mature spore (5.1±0.3 GPa) due to the formation of spore coat and cortex. Collectively, these investigations demonstrate atomic force microscopy as a versatile single cell technique in microbiology to quantitatively detect and spatially map bacterial surface biomarkers and probe key spatial and temporal changes in surface biochemical and nanomechanical properties during cellular activities.