The Mycobacterium tuberculosis capsule: a cell structure with key implications in pathogenesis

Abstract

Bacterial capsules have evolved to be at the forefront of the cell envelope, making them an essential element of bacterial biology. Efforts to understand the Mycobacterium tuberculosis (Mtb) capsule began more than 60 years ago, but the relatively recent development of mycobacterial genetics combined with improved chemical and immunological tools have revealed a more refined view of capsule molecular composition. A glycogen-like α-glucan is the major constituent of the capsule, with lower amounts of arabinomannan and mannan, proteins and lipids. The major Mtb capsular components mediate interactions with phagocytes that favor bacterial survival. Vaccination approaches targeting the mycobacterial capsule have proven successful in controlling bacterial replication. Although the Mtb capsule is composed of polysaccharides of relatively low complexity, the concept of antigenic variability associated with this structure has been suggested by some studies. Understanding how Mtb shapes its envelope during its life cycle is key to developing anti-infective strategies targeting this structure at the host-pathogen interface.

Keywords: Mycobacterium tuberculosis; arabinomannan; capsule; α-glucan.

Figure 1.. Visualization of Mtb capsule by electron microscopy.

(A) Micrograph of ultra-thin sections of Mtb grown in the absence of detergent for 24 h processed for transmission electron microscopy (TEM) [82]. Scale bar is 100 nm. (B) Model of the Mtb cell envelope as observed from (A). Note the trilaminar appearance of membrane organization. (C) Micrograph of ultra-thin sections of Mtb grown in the absence of detergent for 5 days processed for TEM. Note the capsule enlargement when compared with (A). Scale bar is 500 nm. (D) Electron micrograph of an Mtb cell immunostained with an anti-LAM monoclonal antibody processed for negative staining. Secondary nano-gold antibody size is 10 nm. The capsule appears as fragmented clusters of electron-dense material surrounding the bacterium. Scale bar 500 nm. (E) Scanning electron micrograph of Mtb grown in the absence of detergent. Arrow indicates the capsular material adhered to the bacterial clump. Scale bar 500 nm. (F) Cryo-electron micrograph of Mtb grown in the absence of detergent. Scale bar 100 nm. Reproduced with permission from [38].

Figure 2.. The Mtb cell envelope.

The cell envelope of Mtb comprises four main layers: (i) the plasma membrane or inner membrane, (ii) the peptidoglycan–arabinogalactan complex (AGP), (iii) an assymetrical outer membrane or ‘mycomembrane’, that is covalently linked to AGP through the mycolic acids, and (iv) the external capsule [22]. The inner leaflet of the outer membrane is composed of long chain (C60–C90) fatty acids, with the outer leaflet mostly consisting of a variety of non-covalently attached (glyco)lipids and lipoglycans, including trehalose mono- (TMM) and dimycolates (TDM), sulfoglycolipids (SGL), phosphatidylinositol mannosides (PIMs), lipomannan (LM) and lipoarabinomannan (LAM), and lipoproteins/proteins some of which are glycosylated [63,64,159]. The structural organization of this matrix is responsible for the low permeability of the Mtb cell envelope [23,24]. LAM is presumed to be non-covalently associated with the cell membrane, although it has also been found in the mycomembrane. Different studies indicate that the lipoprotein LprG could be assisting the transfer of membrane-associated LAM to the mycomembrane [66,67]. The capsule is mostly made of neutral polysaccharides including a major glycogen-like α-glucan and lower amounts of arabinomannan (AM) and mannan. Both AM and mannan are structurally and chemically similar to the cell-wall-associated LAM and LM. We hypothesize that the action of a missing enzyme could be responsible for the conversion of LAM and LM into AM and mannan, respectively. This enzyme could function at the level of the mycomembrane or the extracellular vesicles, which were shown to be carriers of LAM [68].

Figure 3.. Chemical structure and biosynthetic pathway of α-glucan from Mtb.

(A) Structure of intracellular (=glycogen-like) and extracellular (=capsular) α-glucan from Mtb. The polymer consists of linear α-1,4-glucan chains comprising ca. seven glucose units, each bearing mostly only one α-1,6-branch. This results in a polymer that exhibits a significantly lower degree of branching, and hence a less arboreal structure, compared with glycogen from other bacteria or mammals. (B) Pathway for biosynthesis of glycogen-like and capsular α-glucan in Mtb (adopted from [54]). The two essential enzymes GlgE and GlgB co-operate in an iterative process to produce a structurally distinct α-glucan as depicted in A using maltose 1-phosphate as substrate. The polymer is produced intracellularly and then partially secreted to build the capsule. The substrate maltose 1-phosphate is synthesized on two alternative routes which are interconnected by the shared use of ADP-glucose and allows rechanneling of ADP-glucose and buffering of total maltose 1-phosphate formation.

Figure 4.. Chemical structure of Mtb mannan (M) and arabinomannan (AM).

Details on the biosynthetic pathway can be found in [21,22].