Scratching the Surface: Bacterial Cell Envelopes at the Nanoscale

Abstract

The bacterial cell envelope is essential for viability, the environmental gatekeeper and first line of defense against external stresses. For most bacteria, the envelope biosynthesis is also the site of action of some of the most important groups of antibiotics. It is a complex, often multicomponent structure, able to withstand the internally generated turgor pressure. Thus, elucidating the architecture and dynamics of the cell envelope is important, to unravel not only the complexities of cell morphology and maintenance of integrity but also how interventions such as antibiotics lead to death. To address these questions requires the capacity to visualize the cell envelope in situ via high-spatial resolution approaches. In recent years, atomic force microscopy (AFM) has brought novel molecular insights into the assembly, dynamics, and functions of bacterial cell envelopes. The ultrafine resolution and physical sensitivity of the technique have revealed a wealth of ultrastructural features that are invisible to traditional optical microscopy techniques or imperceptible in their true physiological state by electron microscopy. Here, we discuss recent progress in our use of AFM imaging for understanding the architecture and dynamics of the bacterial envelope. We survey recent studies that demonstrate the power of the technique to observe isolated membranes and live cells at (sub)nanometer resolution and under physiological conditions and to track in vitro structural dynamics in response to growth or to drugs.

AFM Study of Nanoscale Membrane Perturbation Induced by Antimicrobial Lipopeptide C14 KYR

“…Quantitative rupture force measurement with AFM is a promising way to characterize how the lipopeptide affects the mechanical properties of the bacterial surface. The AFM probe as nanoindentation depth plots was used and analyzed according to theoretical models for providing quantitative information on the rupture force of the sample [ 20 ]. These models have been applied toward bacteria, E. coli HB101.…”

“…The organization at the nanoscale of the bacterial cell wall was examined using functionalized AFM tips to address changes in cell morphology, surface roughness, and elasticity after antibiotic treatment. The high resolution and sensitivity of AFM have brought novelties into the real-time effects of antibiotics and external agents in the cell envelope ultrastructure [ 20 ]. Fantner et al developed special AFM cantilevers allowing high-speed tapping-mode AFM, and they observed the kinetics of antimicrobial peptide CM15 degrading individual E. coli cells at high resolution in real time at the nanometer level [ 21 ].…”

Studying the surfaces of bacteria using neutron scattering: finding new openings for antibiotics

“…The advent of atomic force microscopy (AFM) paved the way for the investigation of the properties and behaviour of bacterial surfaces under conditions that are much closer to those relevant to antibiotic action in vivo [36]. AFM has demonstrated the potential to monitor real-time effects of antimicrobials on the surfaces of living bacterial cells under native conditions [37,38] and can achieve sub-second time resolution, enabling single molecule tracking on the bacterial surface [39].…”

AFM in cellular and molecular microbiology

“…High resolution topographic imaging studies of healthy bacteria have shown that the cell surface may vary considerably based on whether a species is Gram-positive or Gram-negative as well as on the presence or absence of surface molecules such as cell wall polysaccharides or S-layers (Viljoen, Foster, Fantner, Hobbs, & Dufrêne, 2020). In Gram-negatives and mycobacteria that have outer membranes masking the peptidoglycan (PG) layer, the cell surfaces are often relatively featureless and smooth (Alsteens et al, 2008;Mathelié-Guinlet, Asmar, Collet, & Dufrêne, 2020;Vassen et al, 2019).…”