The Francisella tularensis Polysaccharides: What Is the Real Capsule?

Abstract

SUMMARY Francisella tularensis is a tier 1 select agent responsible for tularemia in humans and a wide variety of animal species. Extensive research into understanding the virulence factors of the bacterium has been ongoing to develop an efficacious vaccine. At least two such virulence factors are described as capsules of F. tularensis: the O-antigen capsule and the capsule-like complex (CLC). These two separate entities aid in avoiding host immune defenses but have not been clearly differentiated. These components are distinct and differ in composition and genetic basis. The O-antigen capsule consists of a polysaccharide nearly identical to the lipopolysaccharide (LPS) O antigen, whereas the CLC is a heterogeneous complex of glycoproteins, proteins, and possibly outer membrane vesicles and tubes (OMV/Ts). In this review, the current understanding of these two capsules is summarized, and the historical references to “capsules” of F. tularensis are clarified. A significant amount of research has been invested into the composition of each capsule and the genes involved in synthesis of the polysaccharide portion of each capsule. Areas of future research include further exploration into the molecular regulation and pathways responsible for expression of each capsule and further elucidating the role that each capsule plays in virulence.

Bacterial capsules: Occurrence, mechanism, and function

“…However, some bacterial capsules are atypical: the capsule of Yersinia pestis is a protein polymer composed of 17-kDa subunits 11 , 12 , and the capsule of Bacillus anthracis consists of D-glutamic acid 13 . Additionally, the O-antigen capsule and the capsule-like complex (CLC) are identified as capsules of Francisella tularensis ( F. tularensis ) 14 . The O-antigen capsule is composed of mannose, rhamnose, and dideoxy sugars, whereas the CLC comprises proteins and carbohydrates 15 .…”

Host-pathogen interaction in arthropod vectors: Lessons from viral infections

“…Surface components of bacteria may serve as PAMPs through recognition by TLRs (Toll-like receptors). Certain pathogens, for example Francisella tularensis mask their surfaces (to evade immune sensing) by synthesizing a carbohydrate-based capsule that inhibits the antibody and complement deposition on the cell wall, and provide protection against microbicidal host responses (for instance, opsonization) ( 111 ). Some bacteria, for example, B. burgdorferi and Anaplasma use host lipids (as a building block for the biogenesis of their membranes) that helps in bypassing host immune responses by avoiding the immune cells ( 108 , 112 ).…”

Lipopolysaccharides and outer membrane proteins as main structures involved in complement evasion strategies of non-typhoidal Salmonella strains

“…In later studies, also Wilson et al (2011) have demonstrated that the expression of Vi antigen reduces C3b opsonization to the bacterial cell surface. In recent years, more and more attention has been paid to group IV polysaccharide capsules, which have a high degree of similarity in structure with the O-antigen of the LPS molecule (Caboni et al, 2015;Freudenberger Catanzaro and Inzana, 2020;Marshall and Gunn, 2015;Patrick et al, 2009). It has been shown that in Salmonella the O-antigen capsules are important in the processes of adhesion and in the persistence of bacteria in the environment (Barak et al, 2007;Crawford et al, 2008).…”