Shearing and Enrichment of Extracellular Type IV Pili

Pilin antigenic variants impact gonococcal lifestyle and antibiotic tolerance by modulating interbacterial forces

Type 4 pili (T4P) are multifunctional filaments involved in adhesion, surface motility, biofilm formation, and horizontal gene transfer. These extracellular polymers are surface-exposed and, therefore, act as antigens. The human pathogen Neisseria gonorrhoeae uses pilin antigenic variation to escape immune surveillance, yet it is unclear how antigenic variation impacts most other functions of T4P. Here, we addressed this question by replacing the major pilin of a laboratory strain with pilins from clinical isolates. We reveal that the resulting strains vary substantially in their attractive forces. Strongly interacting bacteria form microcolonies while weakly interacting bacteria retain a planktonic lifestyle. In mixed microcolonies, different variant strains segregate in agreement with the differential strength of adhesion hypothesis. By combining structural predictions and laser tweezers experiments, we show that the C-terminal region of the pilin is crucial for attraction. Lifestyle affects growth kinetics and antibiotic tolerance. In the presence of ceftriaxone or ciprofloxacin, the killing kinetics indicate strongly increased tolerance of aggregating strains. We propose that pilin antigenic variation produces a mixed population containing variants optimized for growth, colonization, or survivability under external stress. Different environments select different variants, ensuring the survival and reproduction of the population as a whole.

Structure of the Pseudomonas aeruginosa PAO1 Type IV pilus

Type IV pili (T4Ps) are abundant in many bacterial and archaeal species, where they play important roles in both surface sensing and twitching motility, with implications for adhesion, biofilm formation and pathogenicity. While Type IV pilus (T4P) structures from other organisms have been previously solved, a high-resolution structure of the native, fully assembled T4P of Pseudomonas aeruginosa, a major human pathogen, would be valuable in a drug discovery context. Here, we report a 3.2 Å-resolution structure of the P. aeruginosa PAO1 T4P determined by electron cryomicroscopy (cryo-EM). PilA subunits constituting the T4P exhibit a classical pilin fold featuring an extended N-terminal α-helix linked to a C-terminal globular β-sheet-containing domain, which are packed tightly along the pilus, in line with models derived from previous cryo-EM data of the P. aeruginosa PAK strain. The N-terminal helices constitute the pilus core where they stabilise the tubular assembly via hydrophobic interactions. The α-helical core of the pilus is surrounded by the C-terminal globular domain of PilA that coats the outer surface of the pilus, mediating interactions with the surrounding environment. Comparison of the P. aeruginosa PAO1 T4P with T4P structures from other organisms, both at the level of the pilin subunits and the fully assembled pili, confirms previously described common architectural principles whilst highlighting key differences between members of this abundant class of prokaryotic filaments. This study provides a structural framework for understanding the molecular and cell biology of these important cellular appendages mediating interaction of prokaryotes to surfaces.

Systematic functional analysis of the Com pilus in Streptococcus sanguinis: a minimalistic type 4 filament dedicated to DNA uptake in monoderm bacteria

IMPORTANCE

Type 4 filaments (T4F) are nanomachines ubiquitous in prokaryotes, centered on filamentous polymers of type 4 pilins. T4F are exceptionally versatile and widespread virulence factors in bacterial pathogens. The mechanisms of filament assembly and the many functions they facilitate remain poorly understood because of the complexity of T4F machineries. This hinders the development of anti-T4F drugs. The significance of our research lies in characterizing the simplest known T4F-the Com pilus that mediates DNA uptake in competent monoderm bacteria-and showing that four protein components universally conserved in T4F are sufficient for filament assembly. The Com pilus becomes a model for elucidating the mechanisms of T4F assembly.

Purification of Single-Stranded RNA Bacteriophages and Host Receptors for Structural Determination Using Cryo-Electron Microscopy

Single-stranded RNA bacteriophages (ssRNA phages) are small viruses with a compact genome (~3-4 kb) that infect gram-negative bacteria via retractile pili. These phages have been applied in various fields since their discovery approximately 60 years ago. To understand their biology, it is crucial to analyze the structure of mature virions. Cryo-electron microscopy (cryo-EM) has been employed to determine the structures of two ssRNA phages, MS2 and Qβ. This chapter presents a method for purifying these two phages and their receptor, the F-pilus, to allow examination using cryo-EM.

Bacterial glycocalyx integrity drives multicellular swarm biofilm dynamism

Exopolysaccharide (EPS) layers on the bacterial cell surface are key determinants of biofilm establishment and maintenance, leading to the formation of higher-order 3D structures that confer numerous survival benefits to a cell community. In addition to a specific cell-associated EPS glycocalyx, we recently revealed that the social δ-proteobacterium Myxococcus xanthus secretes a novel biosurfactant polysaccharide (BPS) to the extracellular milieu. Together, secretion of the two polymers (EPS and BPS) is required for type IV pilus (T4P)-dependent swarm expansion via spatio-specific biofilm expression profiles. Thus the synergy between EPS and BPS secretion somehow modulates the multicellular lifecycle of M. xanthus. Herein, we demonstrate that BPS secretion functionally alters the EPS glycocalyx via destabilization of the latter, fundamentally changing the characteristics of the cell surface. This impacts motility behaviors at the single-cell level and the aggregative capacity of cells in groups via cell-surface EPS fibril formation as well as T4P production, stability, and positioning. These changes modulate the structure of swarm biofilms via cell layering, likely contributing to the formation of internal swarm polysaccharide architecture. Together, these data reveal the manner by which the combined secretion of two distinct polymers induces single-cell changes that modulate swarm biofilm communities.

Surface Display of Small Affinity Proteins on Synechocystis sp. Strain PCC 6803 Mediated by Fusion to the Major Type IV Pilin PilA1

Functional surface display of small affinity proteins, namely, affibodies (6.5 kDa), was evaluated for the model cyanobacterium Synechocystis sp. strain PCC 6803 through anchoring to native surface structures. These structures included confirmed or putative subunits of the type IV pili, the S-layer protein, and the heterologous Escherichia coli autotransporter antigen 43 system. The most stable display system was determined to be through C-terminal fusion to PilA1, the major type IV pilus subunit in Synechocystis, in a strain unable to retract these pili (ΔpilT1). Type IV pilus synthesis was upheld, albeit reduced, when fusion proteins were incorporated. However, pilus-mediated functions, such as motility and transformational competency, were negatively affected. Display of affibodies on Synechocystis and the complementary anti-idiotypic affibodies on E. coli or Staphylococcus carnosus was able to mediate interspecies cell-cell binding by affibody complex formation. The same strategy, however, was not able to drive cell-cell binding and aggregation of Synechocystis-only mixtures. Successful affibody tagging of the putative minor pilin PilA4 showed that it locates to the type IV pili in Synechocystis and that its extracellular availability depends on PilA1. In addition, affibody tagging of the S-layer protein indicated that the domains responsible for the anchoring and secretion of this protein are located at the N and C termini, respectively. This study can serve as a basis for future surface display of proteins on Synechocystis for biotechnological applications.IMPORTANCE Cyanobacteria are gaining interest for their potential as autotrophic cell factories. Development of efficient surface display strategies could improve their suitability for large-scale applications by providing options for designed microbial consortia, cell immobilization, and biomass harvesting. Here, surface display of small affinity proteins was realized by fusing them to the major subunit of the native type IV pili in Synechocystis sp. strain PCC 6803. The display of complementary affinity proteins allowed specific cell-cell binding between Synechocystis and Escherichia coli or Staphylococcus carnosus Additionally, successful tagging of the putative pilin PilA4 helped determine its localization to the type IV pili. Analogous tagging of the S-layer protein shed light on the regions involved in its secretion and surface anchoring.