Pili of Mycobacterium tuberculosis: current knowledge and future prospects

Glycan-mediated adhesion mechanisms in antibiotic-resistant bacteria

Bacterial adhesins play a central role in host-pathogen interactions, with many specifically targeting glycans to mediate bacterial colonization, influence infection dynamics, and evade host immune responses. In this review, we focus on bacterial pathogens identified by the World Health Organization as critical threats to public health and in urgent need of new treatments. We summarize glycoconjugate targets identified in the literature across 19 bacterial genera and species. This comprehensive review provides a foundation for the development of innovative therapeutic strategies to effectively combat these pathogens.

Tad pili contribute to the virulence and biofilm formation of virulent Aeromonas hydrophila

Type IV pili (T4P) are versatile proteinaceous protrusions that mediate diverse bacterial processes, including adhesion, motility, and biofilm formation. Aeromonas hydrophila, a Gram-negative facultative anaerobe, causes disease in a wide range of hosts. Previously, we reported the presence of a unique Type IV class C pilus, known as tight adherence (Tad), in virulent Aeromonas hydrophila (vAh). In the present study, we sought to functionalize the role of Tad pili in the pathogenicity of A. hydrophila ML09-119. Through a comprehensive comparative genomics analysis of 170 A. hydrophila genomes, the conserved presence of the Tad operon in vAh isolates was confirmed, suggesting its potential contribution to pathogenicity. Herein, the entire Tad operon was knocked out from A. hydrophila ML09-119 to elucidate its specific role in A. hydrophila virulence. The absence of the Tad operon did not affect growth kinetics but significantly reduced virulence in catfish fingerlings, highlighting the essential role of the Tad operon during infection. Biofilm formation of A. hydrophila ML09-119 was significantly decreased in the Tad operon deletant. Absence of the Tad operon had no effect on sensitivity to other environmental stressors, including hydrogen peroxide, osmolarity, alkalinity, and temperature; however, it was more sensitive to low pH conditions. Scanning electron microscopy revealed that the Tad mutant had a rougher surface structure during log phase growth than the wildtype strain, indicating the absence of Tad impacts the outer surface of vAh during cell division, of which the biological consequences are unknown. These findings highlight the role of Tad in vAh pathogenesis and biofilm formation, signifying the importance of T4P in bacterial infections.

ESAT-6 a Major Virulence Factor of Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb), the causative agent of human tuberculosis (TB), is one of the most successfully adapted human pathogens. Human-to-human transmission occurs at high rates through aerosols containing bacteria, but the pathogen evolved prior to the establishment of crowded populations. Mtb has developed a particular strategy to ensure persistence in the host until an opportunity for transmission arises. It has refined its lifestyle to obviate the need for virulence factors such as capsules, flagella, pili, or toxins to circumvent mucosal barriers. Instead, the pathogen uses host macrophages, where it establishes intracellular niches for its migration into the lung parenchyma and other tissues and for the induction of long-lived latency in granulomas. Finally, at the end of the infection cycle, Mtb induces necrotic cell death in macrophages to escape to the extracellular milieu and instructs a strong inflammatory response that is required for the progression from latency to disease and transmission. Common to all these events is ESAT-6, one of the major virulence factors secreted by the pathogen. This narrative review highlights the recent advances in understanding the role of ESAT-6 in hijacking macrophage function to establish successful infection and transmission and its use as a target for the development of diagnostic tools and vaccines.

The Flp type IV pilus operon of Mycobacterium tuberculosis is expressed upon interaction with macrophages and alveolar epithelial cells

The genome of Mycobacterium tuberculosis (Mtb) harbors the genetic machinery for assembly of the Fimbrial low-molecular-weight protein (Flp) type IV pilus. Presumably, the Flp pilus is essential for pathogenesis. However, it remains unclear whether the pili genes are transcribed in culture or during infection of host cells. This study aimed to shed light on the expression of the Flp pili-assembly genes (tadZ, tadA, tadB, tadC, flp, tadE, and tadF) in Mtb growing under different growth conditions (exponential phase, stationary phase, and dormancy NRP1 and NRP2 phases induced by hypoxia), during biofilm formation, and in contact with macrophages and alveolar epithelial cells. We found that expression of tad/flp genes was significantly higher in the stationary phase than in exponential or NRP1 or NRP2 phases suggesting that the bacteria do not require type IV pili during dormancy. Elevated gene expression levels were recorded when the bacilli were in contact for 4 h with macrophages or epithelial cells, compared to mycobacteria propagated alone in the cultured medium. An antibody raised against a 12-mer peptide derived from the Flp pilin subunit detected the presence of Flp pili on intra- and extracellular bacteria infecting eukaryotic cells. Altogether, these are compelling data showing that the Flp pili genes are expressed during the interaction of Mtb with host cells and highlight a role for Flp pili in colonization and invasion of the host, subsequently promoting bacterial survival during dormancy.

Exploiting pilus-mediated bacteria-host interactions for health benefits

Surface pili (or fimbriae) are an important but conspicuous adaptation of several genera and species of Gram-negative and Gram-positive bacteria. These long and non-flagellar multi-subunit adhesins mediate the initial contact that a bacterium has with a host or environment, and thus have come to be regarded as a key colonization factor for virulence activity in pathogens or niche adaptation in commensals. Pili in pathogenic bacteria are well recognized for their roles in the adhesion to host cells, colonization of tissues, and establishment of infection. As an 'anti-adhesive' ploy, targeting pilus-mediated attachment for disruption has become a potentially effective alternative to using antibiotics. In this review, we give a description of the several structurally distinct bacterial pilus types thus far characterized, and as well offer details about the intricacy of their individual structure, assembly, and function. With a molecular understanding of pilus biogenesis and pilus-mediated host interactions also provided, we go on to describe some of the emerging new approaches and compounds that have been recently developed to prevent the adhesion, colonization, and infection of piliated bacterial pathogens.

Lectins of Mycobacterium tuberculosis - rarely studied proteins

The importance of bacterial lectins for adhesion, pathogenicity, and biofilm formation is well established for many Gram-positive and Gram-negative bacteria. However, there is very little information available about lectins of the tuberculosis-causing bacterium, Mycobacterium tuberculosis (Mtb). In this paper we review previous studies on the carbohydrate-binding characteristics of mycobacteria and related Mtb proteins, discussing their potential relevance to Mtb infection and pathogenesis.

The Biosynthesis and Structures of Bacterial Pili

To interact with the external environments, bacteria often display long proteinaceous appendages on their cell surface, called pili or fimbriae. These non-flagellar thread-like structures are polymers composed of covalently or non-covalently interacting repeated pilin subunits. Distinct pilus classes can be identified on basis of their assembly pathways, including chaperone-usher pili, type V pili, type IV pili, curli and fap fibers, conjugative and type IV secretion pili, as well as sortase-mediated pili. Pili play versatile roles in bacterial physiology, and can be involved in adhesion and host cell invasion, DNA and protein secretion and uptake, biofilm formation, cell motility and more. Recent advances in structure determination of components involved in the various pilus systems has enabled a better molecular understanding of their mechanisms of assembly and function. In this chapter we describe the diversity in structure, biogenesis and function of the different pilus systems found in Gram-positive and Gram-negative bacteria, and review their potential as anti-microbial targets.

A structural overview of mycobacterial adhesins: Key biomarkers for diagnostics and therapeutics

Adherence, colonization, and survival of mycobacteria in host cells require surface adhesins, which are attractive pharmacotherapeutic targets. A large arsenal of pilus and non-pilus adhesins have been identified in mycobacteria. These adhesins are capable of interacting with host cells, including macrophages and epithelial cells and are essential to microbial pathogenesis. In the last decade, several structures of mycobacterial adhesins responsible for adhesion to either macrophages or extra cellular matrix proteins have been elucidated. In addition, key structural and functional information have emerged for the process of mycobacterial adhesion to epithelial cells, mediated by the Heparin-binding hemagglutinin (HBHA). In this review, we provide an overview of the structural and functional features of mycobacterial adhesins and discuss their role as important biomarkers for diagnostics and therapeutics. Based on the reported data, it appears clear that adhesins are endowed with a variety of different structures and functions. Most adhesins play important roles in the cell life of mycobacteria and are key virulence factors. However, they have adapted to an extracellular life to exert a role in host-pathogen interaction. The type of interactions they form with the host and the adhesin regions involved in binding is partly known and is described in this review.

Bacterial pili, with emphasis on Mycobacterium tuberculosis curli pili: potential biomarkers for point-of care tests and therapeutics

CONTEXT

Novel biomarkers are essential for developing rapid diagnostics and therapeutic interventions Objective: This review aimed to highlight biomarker characterisation and assessment of unique bacterial pili.

METHODS

A PubMed search for bacterial pili, diagnostics, vaccine and therapeutics was performed, with emphasis on the well characterised pili.

RESULTS

In total, 46 papers were identified and reviewed.

CONCLUSION

Extensive analyses of pili enabled by advanced nanotechnology and whole genome sequencing provide evidence that they are strong biomarker candidates. Mycobacterium tuberculosis curli pili are emphasised as important epitopes for the development of much needed point-of-care diagnostics and therapeutics.

Analysis of the contribution of MTP and the predicted Flp pilus genes to Mycobacterium tuberculosis pathogenesis

Mycobacterium tuberculosis (Mtb) is one of the world's most successful pathogens. Millions of new cases of tuberculosis occur each year, emphasizing the need for better methods of treatment. The design of novel therapeutics is dependent on our understanding of factors that are essential for pathogenesis. Many bacterial pathogens use pili and other adhesins to mediate pathogenesis. The recently identified Mycobacterium tuberculosis pilus (MTP) and the hypothetical, widely conserved Flp pilus have been speculated to be important for Mtb virulence based on in vitro studies and homology to other pili, respectively. However, the roles for these pili during infection have yet to be tested. We addressed this gap in knowledge and found that neither MTP nor the hypothetical Flp pilus is required for Mtb survival in mouse models of infection, although MTP can contribute to biofilm formation and subsequent isoniazid tolerance. However, differences in mtp expression did affect lesion architecture in infected lungs. Deletion of mtp did not correlate with loss of cell-associated extracellular structures as visualized by transmission electron microscopy in Mtb Erdman and HN878 strains, suggesting that the phenotypes of the mtp mutants were not due to defects in production of extracellular structures. These findings highlight the importance of testing the virulence of adhesion mutants in animal models to assess the contribution of the adhesin to infection. This study also underscores the need for further investigation into additional strategies that Mtb may use to adhere to its host so that we may understand how this pathogen invades, colonizes and disseminates.