Identification and characterization of a novel capsule-like complex surface antigen of Francisella tularensis

Abstract

Francisella tularensis is a highly virulent zoonotic pathogen that is the causative agent of tularemia in humans. Two subspecies of F. tularensis are the most virulent in humans: tularensis (type A) and holarctica (type B), with less than 10 organisms via aerosol of a type A strain having the ability to cause fatal infection. Over the last decade much research has been done on the pathogenesis of this unique intracellular bacterium and many different virulence factors have been identified. The goal of this dissertation has been to identify and characterize the capsule-like complex (CLC) surface antigen of F. tularensis, and to determine its role in virulence and immunoprotection in a mouse model. In addition, I have investigated the role of CLC in biofilm formation.

The CLC appears as a negatively staining material surrounding F. tularensis cells during transmission electron microscopy (TEM). I found that the CLC in the type B live vaccine strain (LVS) could be significantly diminished by deleting two glycosyl transferase genes (LVSΔ1423-22) in the putative polysaccharide locus, FTL_1432-FTL_1421. In addition, I determined that the CLC was not a typical polysaccharide capsule, but was in fact composed of over 50 proteins and glycoproteins including known virulence determinants, such as GroEL, DnaK, and ClpB. Upon further evaluation of the CLC, I determined that it was composed of an increase in production of outer membrane vesicles and tubules (OMV/T). These OMV/T appeared to be self-aggregating into what I visualized through TEM as the CLC. LVSΔ1423-22 was attenuated in the mouse model, and BALB/c mice immunized with CLC and adjuvant were protected against challenge with LVS. In addition to virulence, the CLC appears to play a role in biofilm formation and development. F. tularensis type B strains lacking the surface antigens CLC or CLC and O-antigen, develop a 2-7-fold more robust biofilm than the parent strains. The biofilm matrix contains a glucan-like EPS, proteins, and extracellular DNA, and further characterization may lead to determining if the biofilm acts as an environmental survival mechanism for F. tularensis.

In summary, the CLC appears to be a novel surface antigen composed of upregulated OMV/T that is present in type A and B F. tularensis. Deficiency in CLC contributes to increased biofilm formation that could contribute to the survival of F. tularensis in a wide range of environmental niches. Furthermore, the CLC contributes to virulence of type B strains and elicits a protective immune response to type B challenge. A CLC-deficient type A strain could be a candidate for a new live vaccine strain, and therefore further investigation of such a mutant is warranted.