Contour and persistence length of Corynebacterium diphtheriae pili by atomic force microscopy

On the strong connection between nanoscale adhesion of Yad fimbriae and macroscale attachment of Yad-decorated bacteria to glycosylated, hydrophobic and hydrophilic surfaces

Yad fimbriae are currently viewed as versatile bacterial adhesins able to significantly mediate host or plant-pathogen recognition and contribute to the persistence of Escherichia coli in both the environment and within hosts. To date, however, the underlying adhesion process of Yad fimbriae on surfaces defined by controlled coating chemistries has not been evaluated on the relevant molecular scale. In this work, the interaction forces operational between Yad fimbriae expressed by genetically modified E. coli and self-assembled monolayers (SAM) differing in terms of charge, hydrophobicity or the nature of decorating sugar units are quantified by Single Molecule Force Spectroscopy (SMFS) on the nanoscale. It is found that the adhesion of Yad fimbriae onto probes functionalized with xylose is as strong as that measured with probes decorated with anti-Yad antibodies (ca. 80 to 300 pN). In contrast, the interactions of Yad with galactose, lactose, mannose, -OH, -NH₂, -COOH and -CH₃ terminated SAMs are clearly non-specific. Interpretation of SMFS measurements on the basis of worm-like-chain modeling for polypeptide nanomechanics further leads to the estimates of the maximal extension of Yad fimbriae upon stretching, of their persistence length and of their polydispersity. Finally, we show for the first time a strong correlation between the adhesion properties of Yad-decorated bacteria determined from conventional macroscopic counting methods and the molecular adhesion capacity of Yad fimbriae. This demonstration advocates the effort that should be made to understand on the nanoscale level the interactions between fimbriae and their cognate ligands. The results could further help the design of potential anti-adhesive molecules or surfaces to better fight against the virulence of bacterial pathogens.

The microbial adhesive arsenal deciphered by atomic force microscopy

Microbes employ a variety of strategies to adhere to abiotic and biotic surfaces, as well as host cells. In addition to their surface physicochemical properties (e.g. charge, hydrophobic balance), microbes produce appendages (e.g. pili, fimbriae, flagella) and express adhesion proteins embedded in the cell wall or cell membrane, with adhesive domains targeting specific ligands or chemical properties. Atomic force microscopy (AFM) is perfectly suited to deciphering the adhesive properties of microbial cells. Notably, AFM imaging has revealed the cell wall topographical organization of live cells at unprecedented resolution, and AFM has a dual capability to probe adhesion at the single-cell and single-molecule levels. AFM is thus a powerful tool for unravelling the molecular mechanisms of microbial adhesion at scales ranging from individual molecular interactions to the behaviours of entire cells. In this review, we cover some of the major breakthroughs facilitated by AFM in deciphering the microbial adhesive arsenal, including the exciting development of anti-adhesive strategies.

Analysis of Homotypic Interactions of Lactococcus lactis Pili Using Single-Cell Force Spectroscopy

Cell surface proteins of Gram-positive bacteria play crucial roles in their adhesion to abiotic and biotic surfaces. Pili are long and flexible proteinaceous filaments known to enhance bacterial initial adhesion. They promote surface colonization and are thus considered as essential factors in biofilm cohesion. Our hypothesis is that pili mediate interactions between cells and may thereby directly affect biofilm formation. In this study, we use single-cell force spectroscopy (SCFS) to quantify the force of the homotypic pili interactions between individual bacterial cells, using different Lactococcus lactis strains producing pili or not as model bacteria. Moreover the force-distance curves were analyzed to determine the physical and nanomechanical properties of L. lactis pili. The results for pili-devoided strains showed a weak adhesion between cells (adhesion forces and work in the range of 100 pN and 7 × 10^-18 J, respectively). On the contrary, the piliated strains showed high adhesion levels with adhesion forces and adhesion work over 200 pN and 50 × 10^-18 J, respectively. The force-extension curves showed multiple adhesion events, typical of the unfolding of macromolecules. These unfolding force peaks were fitted using the physical worm-like chain model to get fundamental knowledge on the pili nanomechanical properties. In addition, SCFS applied to a L. lactis isolate expressing both pili and mucus-binding protein at its surface and two derivative mutants revealed the capacity of pili to interact with other surface proteins including mucus-binding proteins. This study demonstrates that pili are involved in L. lactis homotypic interactions and thus can influence biofilm structuring.

Microbial adhesion and ultrastructure from the single-molecule to the single-cell levels by Atomic Force Microscopy

In the last decades, atomic force microscopy (AFM) has evolved towards an accurate and lasting tool to study the surface of living cells in physiological conditions. Through imaging, single-molecule force spectroscopy and single-cell force spectroscopy modes, AFM allows to decipher at multiple scales the morphology and the molecular interactions taking place at the cell surface. Applied to microbiology, these approaches have been used to elucidate biophysical properties of biomolecules and to directly link the molecular structures to their function. In this review, we describe the main methods developed for AFM-based microbial surface analysis that we illustrate with examples of molecular mechanisms unravelled with unprecedented resolution.

The Role of Glycans in Bacterial Adhesion to Mucosal Surfaces: How Can Single-Molecule Techniques Advance Our Understanding?

Bacterial adhesion is currently the subject of increased interest from the research community, leading to fast progress in our understanding of this complex phenomenon. Resent research within this field has documented the important roles played by glycans for bacterial surface adhesion, either through interaction with lectins or with other glycans. In parallel with this increased interest for and understanding of bacterial adhesion, there has been a growth in the sophistication and use of sensitive force probes for single-molecule and single cell studies. In this review, we highlight how the sensitive force probes atomic force microscopy (AFM) and optical tweezers (OT) have contributed to clarifying the mechanisms underlying bacterial adhesion to glycosylated surfaces in general and mucosal surfaces in particular. We also describe research areas where these techniques have not yet been applied, but where their capabilities appear appropriate to advance our understanding.

Analysis of Corynebacterium diphtheriae macrophage interaction: Dispensability of corynomycolic acids for inhibition of phagolysosome maturation and identification of a new gene involved in synthesis of the corynomycolic acid layer

Corynebacterium diphtheriae is the causative agent of diphtheria, a toxin mediated disease of upper respiratory tract, which can be fatal. As a member of the CMNR group, C. diphtheriae is closely related to members of the genera Mycobacterium, Nocardia and Rhodococcus. Almost all members of these genera comprise an outer membrane layer of mycolic acids, which is assumed to influence host-pathogen interactions. In this study, three different C. diphtheriae strains were investigated in respect to their interaction with phagocytic murine and human cells and the invertebrate infection model Caenorhabditis elegans. Our results indicate that C. diphtheriae is able to delay phagolysosome maturation after internalization in murine and human cell lines. This effect is independent of the presence of mycolic acids, as one of the strains lacked corynomycolates. In addition, analyses of NF-κB induction revealed a mycolate-independent mechanism and hint to detrimental effects of the different strains tested on the phagocytic cells. Bioinformatics analyses carried out to elucidate the reason for the lack of mycolates in one of the strains led to the identification of a new gene involved in mycomembrane formation in C. diphtheriae.

Modeling and Simulating the Dynamics of Type IV Pili Extension of Pseudomonas aeruginosa

Bacteria such as Pseudomonas aeruginosa use type IV pili to move across surfaces. The pili extend, attach to the surface, and then retract to move the bacteria forward. In this article, a coarse-grained model of pilus extension and attachment is developed. Simulations performed at biologically relevant conditions indicate that pilus extension is a quasistatic process such that the pili are able to relax via thermal fluctuations as it is being built and extended. Results are generated for pili with different rigidities ranging from very flexible to very stiff. It is shown that very flexible pili do not extend very far and thus would limit the bacteria to short jumps forward while stiff pili enable much greater displacements. Feasible mechanisms of attachment to the surface are also found to vary greatly between flexible and stiff pili. While it is not always the tip of flexible pili that first makes contact with the substrate, it is likely to be a part of the pili that is close to the tip. Conversely, stiff pili are much more likely to make contact with the substrate via the tip, but if not then the part of the pilus that attaches can be quite far from the tip. These results thus give insight to help resolve current discrepancies in the literature regarding pilus stiffness and the location of adhesins on pili.

Functional characterization of the collagen-binding protein DIP2093 and its influence on host-pathogen interaction and arthritogenic potential of Corynebacterium diphtheriae

Corynebacterium diphtheriae is typically recognized as the a etiological agent of diphtheria, a toxaemic infection of the respiratory tract; however, both non-toxigenic and toxigenic strains are increasingly isolated from cases of invasive infections. The molecular mechanisms responsible for bacterial colonization and dissemination to host tissues remain only partially understood. In this report, we investigated the role of DIP2093, described as a putative adhesin of the serine-aspartate repeat (Sdr) protein family in host-pathogen interactions of C. diphtheriae wild-type strain NCTC13129. Compared to the parental strain, a DIP2093 mutant RN generated in this study was attenuated in its ability to bind to type I collagen, to adhere to and invade epithelial cells, as well as to survive within macrophages. Furthermore, DIP2093 mutant strain RN had a less detrimental impact on the viability of Caenorhabditis elegans as well as in the clinical severity of arthritis in mice. In conclusion, DIP2093 functions as a microbial surface component recognizing adhesive matrix molecules, and may be included among the factors that contribute to the pathogenicity of C. diphtheriae strains, independently of toxin production.

Emergence of complex behavior in pili-based motility in early stages of P. aeruginosa surface adaptation

Pseudomonas aeruginosa move across surfaces by using multiple Type IV Pili (TFP), motorized appendages capable of force generation via linear extension/retraction cycles, to generate surface motions collectively known as twitching motility. Pseudomonas cells arrive at a surface with low levels of piliation and TFP activity, which both progressively increase as the cells sense the presence of a surface. At present, it is not clear how twitching motility emerges from these initial minimal conditions. Here, we build a simple model for TFP-driven surface motility without complications from viscous and solid friction on surfaces. We discover the unanticipated structural requirement that TFP motors need to have a minimal amount of effective angular rigidity in order for cells to perform the various classes of experimentally-observed motions. Moreover, a surprisingly small number of TFP are needed to recapitulate movement signatures associated with twitching: Two TFP can already produce movements reminiscent of recently observed slingshot type motion. Interestingly, jerky slingshot motions characteristic of twitching motility comprise the transition region between different types of observed crawling behavior in the dynamical phase diagram, such as self-trapped localized motion, 2-D diffusive exploration, and super-diffusive persistent motion.

The Nanomechanical Properties of Lactococcus lactis Pili Are Conditioned by the Polymerized Backbone Pilin

Pili produced by Lactococcus lactis subsp. lactis are putative linear structures consisting of repetitive subunits of the major pilin PilB that forms the backbone, pilin PilA situated at the distal end of the pilus, and an anchoring pilin PilC that tethers the pilus to the peptidoglycan. We determined the nanomechanical properties of pili using optical-tweezers force spectroscopy. Single pili were exposed to optical forces that yielded force-versus-extension spectra fitted using the Worm-Like Chain model. Native pili subjected to a force of 0–200 pN exhibit an inextensible, but highly flexible ultrastructure, reflected by their short persistence length. We tested a panel of derived strains to understand the functional role of the different pilins. First, we found that both the major pilin PilB and sortase C organize the backbone into a full-length organelle and dictate the nanomechanical properties of the pili. Second, we found that both PilA tip pilin and PilC anchoring pilin were not essential for the nanomechanical properties of pili. However, PilC maintains the pilus on the bacterial surface and may play a crucial role in the adhesion- and biofilm-forming properties of L. lactis.

Atomic Force Microscopy and pharmacology: from microbiology to cancerology

BACKGROUND

Atomic Force Microscopy (AFM) has been extensively used to study biological samples. Researchers take advantage of its ability to image living samples to increase our fundamental knowledge (biophysical properties/biochemical behavior) on living cell surface properties, at the nano-scale.

SCOPE OF REVIEW

AFM, in the imaging modes, can probe cells morphological modifications induced by drugs. In the force spectroscopy mode, it is possible to follow the nanomechanical properties of a cell and to probe the mechanical modifications induced by drugs. AFM can be used to map single molecule distribution at the cell surface. We will focus on a collection of results aiming at evaluating the nano-scale effects of drugs, by AFM. Studies on yeast, bacteria and mammal cells will illustrate our discussion. Especially, we will show how AFM can help in getting a better understanding of drug mechanism of action.

MAJOR CONCLUSIONS

This review demonstrates that AFM is a versatile tool, useful in pharmacology. In microbiology, it has been used to study the drugs fighting Candida albicans or Pseudomonas aeruginosa. The major conclusions are a better understanding of the microbes' cell wall and of the drugs mechanism of action. In cancerology, AFM has been used to explore the effects of cytotoxic drugs or as an innovative diagnostic technology. AFM has provided original results on cultured cells, cells extracted from patient and directly on patient biopsies.

GENERAL SIGNIFICANCE

This review enhances the interest of AFM technologies for pharmacology. The applications reviewed range from microbiology to cancerology.

Cell envelope of corynebacteria: structure and influence on pathogenicity

To date the genus Corynebacterium comprises 88 species. More than half of these are connected to human and animal infections, with the most prominent member of the pathogenic species being Corynebacterium diphtheriae, which is also the type species of the genus. Corynebacterium species are characterized by a complex cell wall architecture: the plasma membrane of these bacteria is followed by a peptidoglycan layer, which itself is covalently linked to a polymer of arabinogalactan. Bound to this, an outer layer of mycolic acids is found which is functionally equivalent to the outer membrane of Gram-negative bacteria. As final layer, free polysaccharides, glycolipids, and proteins are found. The composition of the different substructures of the corynebacterial cell envelope and their influence on pathogenicity are discussed in this paper.