Interplay of a secreted protein with type IVb pilus for efficient enterotoxigenic Escherichia coli colonization

Type IV pili of Enterobacteriaceae species

Type IV pili (T4Ps) are surface filaments widely distributed among bacteria and archaea. T4Ps are involved in many cellular functions and contribute to virulence in some species of bacteria. Due to the diversity of T4Ps, different properties have been observed for homologous proteins that make up T4Ps in various organisms. In this review, we highlight the essential components of T4Ps, their functions, and similarities to related systems. We emphasize the unique T4Ps of enteric pathogens within the Enterobacteriaceae family, which includes pathogenic strains of Escherichia coli and Salmonella. These include the bundle-forming pilus (BFP) of enteropathogenic E. coli (EPEC), longus (Lng) and colonization factor III (CFA/III) of enterotoxigenic E. coli (ETEC), T4P of Salmonella enterica serovar Typhi, Colonization Factor Citrobacter (CFC) of Citrobacter rodentium, T4P of Yersinia pseudotuberculosis, a ubiquitous T4P that was characterized in enterohemorrhagic E. coli (EHEC), and the R64 plasmid thin pilus. Finally, we highlight areas for further study.

Pathogen-specific alterations in intestinal microbiota precede urinary tract infections in preterm infants: a longitudinal case-control study

Urinary tract infections (UTIs) are among the most common late-onset infections in preterm infants, characterized by nonspecific symptoms and a pathogenic spectrum that diverges from that of term infants and older children, which present unique diagnostic and therapeutic challenges. Existing data on the role of gut microbiota in UTI pathogenesis in this demographic are limited. This study aims to investigate alterations in gut microbiota and fecal calprotectin levels and their association with the development of UTIs in hospitalized preterm infants. A longitudinal case-control study was conducted involving preterm infants admitted between January 2018 and October 2020. Fecal samples were collected weekly and analyzed for microbial profiles and calprotectin levels. Propensity score matching, accounting for key perinatal factors including age and antibiotic use, was utilized to match samples from UTI-diagnosed infants to those from non-UTI counterparts. Among the 151 preterm infants studied, 53 were diagnosed with a UTI, predominantly caused by Enterobacteriaceae (79.3%) and Enterococcaceae (19.0%). Infants with UTIs showed a significantly higher abundance of these families compared to non-UTI infants, for both Gram-negative and positive pathogens, respectively. Notably, there was a significant pre-UTI increase in the abundance of pathogen-specific taxa in infants later diagnosed with UTIs, offering high predictive value for early detection. Shotgun metagenomic sequencing further confirmed the dominance of specific pathogenic species pre-UTI and revealed altered virulence factor profiles associated with Klebsiella aerogenes and Escherichia coli infections. Additionally, a decline in fecal calprotectin levels was observed preceding UTI onset, particularly in cases involving Enterobacteriaceae. The observed pathogen-specific alterations in the gut microbiota preceding UTI onset offer novel insight into the UTI pathogenesis and promising early biomarkers for UTIs in preterm infants, potentially enhancing the timely management of this common infection. However, further validation in larger cohorts is essential to confirm these findings.

Mechanism of secretion of TcpF by the Vibrio cholerae toxin-coregulated pilus

Many bacteria possess dynamic filaments called Type IV pili (T4P) that perform diverse functions in colonization and dissemination, including host cell adhesion, DNA uptake, and secretion of protein substrates-exoproteins-from the periplasm to the extracellular space. The Vibrio cholerae toxin-coregulated pilus (TCP) and the enterotoxigenic Escherichia coli CFA/III pilus each mediates export of a single exoprotein, TcpF and CofJ, respectively. Here, we show that the disordered N-terminal segment of mature TcpF is the export signal (ES) recognized by TCP. Deletion of the ES disrupts secretion and causes TcpF to accumulate in the V. cholerae periplasm. The ES alone can mediate export of Neisseria gonorrhoeae FbpA by V. cholerae in a T4P-dependent manner. The ES is specific for its autologous T4P machinery as CofJ bearing the TcpF ES is exported by V. cholerae, whereas TcpF bearing the CofJ ES is not. Specificity is mediated by binding of the ES to TcpB, a minor pilin that primes pilus assembly and forms a trimer at the pilus tip. Finally, the ES is proteolyzed from the mature TcpF protein upon secretion. Together, these results provide a mechanism for delivery of TcpF across the outer membrane and release into the extracellular space.

Mechanism of assembly of type 4 filaments: everything you always wanted to know (but were afraid to ask)

Type 4 filaments (T4F) are a superfamily of filamentous nanomachines - virtually ubiquitous in prokaryotes and functionally versatile - of which type 4 pili (T4P) are the defining member. T4F are polymers of type 4 pilins, assembled by conserved multi-protein machineries. They have long been an important topic for research because they are key virulence factors in numerous bacterial pathogens. Our poor understanding of the molecular mechanisms of T4F assembly is a serious hindrance to the design of anti-T4F therapeutics. This review attempts to shed light on the fundamental mechanistic principles at play in T4F assembly by focusing on similarities rather than differences between several (mostly bacterial) T4F. This holistic approach, complemented by the revolutionary ability of artificial intelligence to predict protein structures, led to an intriguing mechanistic model of T4F assembly.

Structural basis for the toxin-coregulated pilus-dependent secretion of Vibrio cholerae colonization factor

Colonization of the host intestine is the most important step in Vibrio cholerae infection. The toxin-coregulated pilus (TCP), an operon-encoded type IVb pilus (T4bP), plays a crucial role in this process, which requires an additional secreted protein, TcpF, encoded on the same TCP operon; however, its mechanisms of secretion and function remain elusive. Here, we demonstrated that TcpF interacts with the minor pilin, TcpB, of TCP and elucidated the crystal structures of TcpB alone and in complex with TcpF. The structural analyses reveal how TCP recognizes TcpF and its secretory mechanism via TcpB-dependent pilus elongation and retraction. Upon binding to TCP, TcpF forms a flower-shaped homotrimer with its flexible N terminus hooked onto the trimeric interface of TcpB. Thus, the interaction between the minor pilin and the N terminus of the secreted protein, namely, the T4bP secretion signal, is key for V. cholerae colonization and is a new potential therapeutic target.

The role of CFA/I in adherence and toxin delivery by ETEC expressing multiple colonization factors in the human enteroid model

Enterotoxigenic Escherichia coli (ETEC) is a primary causative agent of diarrhea in travelers and young children in low-to-middle-income countries (LMICs). ETEC adhere to intestinal epithelia via colonization factors (CFs) and secrete heat-stable toxin (ST) and/or heat-labile toxin (LT), causing dysregulated cellular ion transport and water secretion. ETEC isolates often harbor genes encoding more than one CF that are targets as vaccine antigens. CFA/I is a major CF that is associated with ETEC that causes moderate-to-severe diarrhea and plays an important role in pathogenesis. The Global Enteric Multicenter Study finding that 78% of CFA/I-expressing ETEC also encode the minor CF CS21 prompted investigation of the combined role of these two CFs. Western blots and electron microscopy demonstrated growth media-dependent and strain-dependent differences in CFA/I and CS21 expression. The critical role of CFA/I in adherence by ETEC strains expressing CFA/I and CS21 was demonstrated using the human enteroid model and a series of CFA/I- and CS21-specific mutants. Furthermore, only anti-CFA/I antibodies inhibited adherence by global ETEC isolates expressing CFA/I and CS21. Delivery of ST and resulting cGMP secretion was measured in supernatants from infected enteroid monolayers, and strain-specific ST delivery and time-dependent cGMP production was observed. Interestingly, cGMP levels were similar across wildtype and CF-deficient strains, reflecting a limitation of this static aerobic infection model. Despite adherence by ETEC and delivery of ST, the enteroid monolayer integrity was not disrupted, as shown by the lack of decrease in transepithelial electrical resistance and the lack of IL-8 cytokines produced during infection. Taken together, these data demonstrate that targeting CFA/I in global clinical CFA/I-CS21 strains is sufficient for adherence inhibition, supporting a vaccine strategy that focuses on blocking major CFs. In addition, the human enteroid model has significant utility for the study of ETEC pathogenesis and evaluation of vaccine-induced functional antibody responses.

Cryo-EM structure of an extracellular Geobacter OmcE cytochrome filament reveals tetrahaem packing

Electrically conductive appendages from the anaerobic bacterium Geobacter sulfurreducens were first observed two decades ago, with genetic and biochemical data suggesting that conductive fibres were type IV pili. Recently, an extracellular conductive filament of G. sulfurreducens was found to contain polymerized c-type cytochrome OmcS subunits, not pilin subunits. Here we report that G. sulfurreducens also produces a second, thinner appendage comprised of cytochrome OmcE subunits and solve its structure using cryo-electron microscopy at ~4.3 Å resolution. Although OmcE and OmcS subunits have no overall sequence or structural similarities, upon polymerization both form filaments that share a conserved haem packing arrangement in which haems are coordinated by histidines in adjacent subunits. Unlike OmcS filaments, OmcE filaments are highly glycosylated. In extracellular fractions from G. sulfurreducens, we detected type IV pili comprising PilA-N and -C chains, along with abundant B-DNA. OmcE is the second cytochrome filament to be characterized using structural and biophysical methods. We propose that there is a broad class of conductive bacterial appendages with conserved haem packing (rather than sequence homology) that enable long-distance electron transport to chemicals or other microbial cells.

Impaired intestinal stem cell activity in ETEC infection: enterotoxins, cyclic nucleotides, and Wnt signaling

Enterotoxigenic Escherichia coli (ETEC) in humans and animals colonizes the intestine and thereafter secrets heat-stable enterotoxin (ST) with or without heat-labile enterotoxin (LT), which triggers massive fluid and electrolyte secretion into the gut lumen. The crosstalk between the cyclic nucleotide-dependent protein kinase/cystic fibrosis transmembrane conductance regulator (cAMP or cGMP/CFTR) pathway involved in ETEC-induced diarrhea channels, and the canonical Wnt/β-catenin signaling pathway leads to changes in intestinal stem cell (ISC) fates, which are strongly associated with developmental disorders caused by diarrhea. We review how alterations in enterotoxin-activated ion channel pathways and the canonical Wnt/β-catenin signaling pathway can explain inhibited intestinal epithelial activity, characterize alterations in the crosstalk of cyclic nucleotides, and predict harmful effects on ISCs in targeted therapy. Besides, we discuss current deficits in the understanding of enterotoxin-intestinal epithelial cell activity relationships that should be considered when interpreting sequelae of diarrhea.

PilB from Streptococcus sanguinis is a bimodular type IV pilin with a direct role in adhesion

Type IV pili (T4P) are functionally versatile filaments widespread in prokaryotes, composed of type IV pilins and assembled by conserved multiprotein machineries. It remains unclear how such rather simple filaments can be so versatile. Our structure/function analysis of PilB, a minor pilin of Streptococcus sanguinis T4P, offers an elegant explanation for this paradox. We show that PilB is a modular pilin with a bulky module “grafted” onto a small pilin module, which directly mediates adhesion of S. sanguinis to host cells/proteins. This evolutionary tinkering strategy appears to be prevalent in bacteria since a global analysis reveals that modular pilins are widespread and exhibit an astonishing variety of architectures.

Type IV pili (T4P) are functionally versatile filamentous nanomachines, nearly ubiquitous in prokaryotes. They are predominantly polymers of one major pilin but also contain minor pilins whose functions are often poorly defined and likely to be diverse. Here, we show that the minor pilin PilB from the T4P of Streptococcus sanguinis displays an unusual bimodular three-dimensional structure with a bulky von Willebrand factor A–like (vWA) module “grafted” onto a small pilin module via a short loop. Structural modeling suggests that PilB is only compatible with a localization at the tip of T4P. By performing a detailed functional analysis, we found that 1) the vWA module contains a canonical metal ion–dependent adhesion site, preferentially binding Mg²⁺ and Mn²⁺, 2) abolishing metal binding has no impact on the structure of PilB or piliation, 3) metal binding is important for S. sanguinis T4P–mediated twitching motility and adhesion to eukaryotic cells, and 4) the vWA module shows an intrinsic binding ability to several host proteins. These findings reveal an elegant yet simple evolutionary tinkering strategy to increase T4P functional versatility by grafting a functional module onto a pilin for presentation by the filaments. This strategy appears to have been extensively used by bacteria, in which modular pilins are widespread and exhibit an astonishing variety of architectures.

Caldicellulosiruptor bescii Adheres to Polysaccharides via a Type IV Pilin-Dependent Mechanism

Biological hydrolysis of cellulose above 70°C involves microorganisms that secrete free enzymes and deploy separate protein systems to adhere to their substrate. Strongly cellulolytic Caldicellulosiruptor bescii is one such extreme thermophile, which deploys modular, multifunctional carbohydrate-acting enzymes to deconstruct plant biomass. Additionally, C. bescii also encodes noncatalytic carbohydrate binding proteins, which likely evolved as a mechanism to compete against other heterotrophs in carbon-limited biotopes that these bacteria inhabit. Analysis of the Caldicellulosiruptor pangenome identified a type IV pilus (T4P) locus encoded upstream of the tāpirins, that is encoded by all Caldicellulosiruptor species. In this study, we sought to determine if the C. bescii T4P plays a role in attachment to plant polysaccharides. The major C. bescii pilin (CbPilA) was identified by the presence of pilin-like protein domains, paired with transcriptomics and proteomics data. Using immuno-dot blots, we determined that the plant polysaccharide xylan induced production of CbPilA 10- to 14-fold higher than glucomannan or xylose. Furthermore, we are able to demonstrate that recombinant CbPilA directly interacts with xylan and cellulose at elevated temperatures. Localization of CbPilA at the cell surface was confirmed by immunofluorescence microscopy. Lastly, a direct role for CbPilA in cell adhesion was demonstrated using recombinant CbPilA or anti-CbPilA antibodies to reduce C. bescii cell adhesion to xylan and crystalline cellulose up to 4.5- and 2-fold, respectively. Based on these observations, we propose that CbPilA and, by extension, the T4P play a role in Caldicellulosiruptor cell attachment to plant biomass.IMPORTANCE Most microorganisms are capable of attaching to surfaces in order to persist in their environment. Type IV (T4) pili produced by certain mesophilic Firmicutes promote adherence; however, a role for T4 pili encoded by thermophilic members of this phylum has yet to be demonstrated. Prior comparative genomics analyses identified a T4 pilus locus possessed by an extremely thermophilic genus within the Firmicutes Here, we demonstrate that attachment to plant biomass-related carbohydrates by strongly cellulolytic Caldicellulosiruptor bescii is mediated by T4 pilins. Surprisingly, xylan but not cellulose induced expression of the major T4 pilin. Regardless, the C. bescii T4 pilin interacts with both polysaccharides at high temperatures and is located to the cell surface, where it is directly involved in C. bescii attachment. Adherence to polysaccharides is likely key to survival in environments where carbon sources are limiting, allowing C. bescii to compete against other plant-degrading microorganisms.

Bridging Bacteria and the Gut: Functional Aspects of Type IV Pili

Cell-surface-located proteinaceous appendages, such as flagella and fimbriae or pili, are ubiquitous in bacterial communities. Here, we focus on conserved type IV pili (T4P) produced by bacteria in the intestinal tract, one of the most densely populated human ecosystems. Computational analysis revealed that approximately 30% of known intestinal bacteria are predicted to produce T4P. To rationalize how T4P allow intestinal bacteria to interact with their environment, other microbiota members, and host cells, we review their established role in gut commensals and pathogens with respect to adherence, motility, and biofilm formation, as well as protein secretion and DNA uptake. This work indicates that T4P are widely spread among the known members of the intestinal microbiota and that their contribution to human health might be underestimated.

The Vibrio cholerae minor pilin TcpB mediates uptake of the cholera toxin phage CTXφ

Virulent strains of the bacterial pathogen Vibrio cholerae cause the diarrheal disease cholera by releasing cholera toxin into the small intestine. V. cholerae acquired its cholera toxin genes by lysogenic infection with the filamentous bacteriophage CTXφ. CTXφ uses its minor coat protein pIII, located in multiple copies at the phage tip, to bind to the V. cholerae toxin-coregulated pilus (TCP). However, the molecular details of this interaction and the mechanism of phage internalization are not well-understood. The TCP filament is a polymer of major pilins, TcpA, and one or more minor pilin, TcpB. TCP are retractile, with both retraction and assembly initiated by TcpB. Consistent with these roles in pilus dynamics, we hypothesized that TcpB controls both binding and internalization of CTXφ. To test this hypothesis, we determined the crystal structure of the C-terminal half of TcpB and characterized its interactions with CTXφ pIII. We show that TcpB is a homotrimer in its crystallographic form as well as in solution and is present in multiple copies at the pilus tip, which likely facilitates polyvalent binding to pIII proteins at the phage tip. We further show that recombinant forms of TcpB and pIII interact in vitro, and both TcpB and anti-TcpB antibodies block CTXφ infection of V. cholerae Finally, we show that CTXφ uptake requires TcpB-mediated retraction. Our data support a model whereby CTXφ and TCP bind in a tip-to-tip orientation, allowing the phage to be drawn into the V. cholerae periplasm as an extension of the pilus filament.

The structure of PilA from Acinetobacter baumannii AB5075 suggests a mechanism for functional specialization in Acinetobacter type IV pili

Type IV pili (T4P) are bacterial appendages composed of protein subunits, called pilins, noncovalently assembled into helical fibers. T4P are essential, in many bacterial species, for processes as diverse as twitching motility, natural competence, biofilm or microcolony formation, and host cell adhesion. The genes encoding type IV pili are found universally in the Gram-negative, aerobic, nonflagellated, and pathogenic coccobacillus Acinetobacter baumannii, but there is considerable variation in PilA, the major protein subunit, both in amino acid sequence and in glycosylation patterns. Here we report the X-ray crystal structure of PilA from AB5075, a recently characterized, highly virulent isolate, at 1.9 Å resolution and compare it to homologues from A. baumannii strains ACICU and BIDMC57, which are C-terminally glycosylated. These structural comparisons revealed that PilA^AB5075 exhibits a distinctly electronegative surface chemistry. To understand the functional consequences of this change in surface electrostatics, we complemented a ΔpilA knockout strain with divergent pilA genes from ACICU, BIDMC57, and AB5075. The resulting transgenic strains showed differential twitching motility and biofilm formation while maintaining the ability to adhere to epithelial cells. PilA^AB5075 and PilA^ACICU, although structurally similar, promote different characteristics, favoring twitching motility and biofilm formation, respectively. These results support a model in which differences in pilus electrostatics affect the equilibrium of microcolony formation, which in turn alters the balance between motility and biofilm formation in Acinetobacter.