The Role of Capsular Polysaccharide in Acinetobacter baumannii Virulence

Author: Jennifer Singh

Singh, Jennifer, 2022 The Role of Capsular Polysaccharide in Acinetobacter baumannii Virulence, Flinders University, College of Science and Engineering

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The Gram-negative opportunistic bacterium Acinetobacter baumannii is a significant cause of hospital-borne infections worldwide. Alarmingly, the rapid development of antimicrobial resistance coupled with remarkable ability of isolates to persist on surfaces for extended periods of time has led to infiltration of A. baumannii into our healthcare environments. Treatment options for A. baumannii infections are becoming increasingly limited, and the urgency to develop effective infection control strategies and therapies to combat infections is apparent.

Capsular polysaccharide is comprised of tightly packed repeating oligosaccharide units and forms a barrier around the bacterial cell providing protection from environmental stressors found in the nosocomial environment as well as host immune responses during infection. Additionally, capsule has been shown to contribute resistance to a range of clinically relevant antimicrobial compounds. An increased understanding of the contribution of capsule to the pathobiology of A. baumannii is required to determine its feasibility as a target for new strategies to combat drug resistant infections.

Phenomenal diversity exists between capsule structures (K types) identified for various A. baumannii strains, with over 150 distinct capsule biosynthesis gene clusters have been identified to date. In this study, capsule related phenotypes of twelve A. baumannii strains, representing nine distinct capsule types were assessed to identify correlations between capsule structure and phenotype. Cell surface hydrophobicity, acid stress tolerance, lysozyme resistance and serum resistance were compared between the twelve strains. A correlation was found between capsule charge and cell surface hydrophobicity, but no relationships were identified between K type and any other phenotype assessed.

Manipulation of the A. baumannii genome is essential for investigating the role capsule plays in persistence, resistance, and host immune evasion. Genetic manipulation of A. baumannii has historically been difficult, though recent advances in cloning methodologies have led to improvements. For some bacterial strains, such as Streptococcus pneumoniae, K type is highly correlated to disease potential. The gold standard for studying the role of K type on disease potential is assessing ‘capsule swapped’ isogenic strains expressing heterologous K types. For most A. baumannii strains, the K loci, which encodes genes specific to capsule production, is located between two highly conserved genes, lldp and fkpA Several attempts were made to make capsule swapped variants of A. baumannii strains ATCC 17978 and 04117201 but ultimately proved unsuccessful.

The contribution of capsule to A. baumannii environmental persistence and host immune evasion was assessed by generating acapsular mutants with one or more gene/s required for capsule production removed. Three acapsular mutants, removing the initial transferase (ΔitrA2), variable region genes (Δcps), and capsule polymerase (Δwzy), were generated in the A. baumannii strain ATCC 17978. The elimination of capsule production altered the response of A. baumannii to various stressors including growth in acidic pH, the osmotic stress response, resistance to human complement and lysozyme killing, macrophage phagocytosis, growth under iron limitation, and resistance to antimicrobial compounds. This study has defined a role for capsule in A. baumannii survival in adverse environments and highlights the importance of capsule in evasion of the host immune system and resistance to antimicrobial compounds. Notably, the response of acapsular mutants to some stressors varied considerably depending on what gene/s were removed, demonstrating that abrogation of capsule production at different stages of synthesis can differentially affect stress responses.

Keywords: Bacteria, Acinetobacter baumannii, capsular polysaccharide

Subject: Microbiology & Infectious Diseases thesis

Thesis type: Doctor of Philosophy
Completed: 2022
School: College of Science and Engineering
Supervisor: Melissa Brown