Comparative Study of NAP-XPS and Cryo-XPS for the Investigation of Surface Chemistry of the Bacterial Cell-Envelope.

Marit Kjærvik,Paul M. Dietrich,Madeleine Ramstedt,Karin Schwibbert,Wolfgang E. S. Unger

doi:10.3389/fchem.2021.666161

Marit Kjærvik, Paul M. Dietrich + Show 3 more

Open Access

https://doi.org/10.3389/fchem.2021.666161

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Abstract

Bacteria generally interact with the environment via processes involving their cell-envelope. Thus, techniques that may shed light on their surface chemistry are attractive tools for providing an understanding of bacterial interactions. One of these tools is Al Kα-excited photoelectron spectroscopy (XPS) with its estimated information depth of <10 nm. XPS-analyses of bacteria have been performed for several decades on freeze-dried specimens in order to be compatible with the vacuum in the analysis chamber of the spectrometer. A limitation of these studies has been that the freeze-drying method may collapse cell structure as well as introduce surface contaminants. However, recent developments in XPS allow for analysis of biological samples at near ambient pressure (NAP-XPS) or as frozen hydrated specimens (cryo-XPS) in vacuum. In this work, we have analyzed bacterial samples from a reference strain of the Gram-negative bacterium Pseudomonas fluorescens using both techniques. We compare the results obtained and, in general, observe good agreement between the two techniques. Furthermore, we discuss advantages and disadvantages with the two analysis approaches and the output data they provide. XPS reference data from the bacterial strain are provided, and we propose that planktonic cells of this strain (DSM 50090) are used as a reference material for surface chemical analysis of bacterial systems.

Highlights

Bacteria are ubiquitous in our environment and contribute to both health and disease depending on circumstances and context
Since PBS, or other buffers, were not used during sample preparation, this signal originated from the bacterial cell envelope
The two methods allow for analysis of the hydrated bacterial cell-envelope of intact bacterial cells

Summary

Introduction

Bacteria are ubiquitous in our environment and contribute to both health and disease depending on circumstances and context. Tools that can provide a description of the surface chemistry of bacterial cells are of great interest for life sciences, especially analysis techniques and methods that can be applied onto intact cells with minimum pre-treatment. In this light, X-ray photoelectron spectroscopy (XPS), in its different applications, is an interesting tool as it provides the chemical composition and binding states of atoms at the near surface of a sample. Rouxhet et al performed detailed work on how to perform this process with a minimum of carbon contamination during sample preparation by freeze drying (Rouxhet et al, 1994)

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