Diverse Roles for the Extracellular Glycome of Extra-intestinal Pathogenic Escherichia coli in Serum Resistance and more

Abstract

Extra-intestinal pathogenic Escherichia coli (ExPEC) is a major cause of urinary tract infections, bacteraemia, and sepsis. This laboratory, among others, has previously shown that prototypic urosepsis strain CFT073 is serum-resistant, with virulence factors such as the exopolysaccharide capsule and other extracellular polysaccharides imparting resistance to the complement system. The aims of this study were to further characterise the regulation and interplay of lipopolysaccharide (LPS) and capsule in CFT073 serum resistance, as well as identify other virulence factors implicated in serum resistance. This study found that LPS-mediated surface charge is a major determinant of cell integrity, association of capsule with the cell and subsequently, serum resistance. This study also found that a secreted factor provides protection from the bactericidal effects of serum. Haemolysin HlyA, a pore-forming toxin which can be secreted or remain cell-associated, is encoded by CFT073 and many other sepsis isolates, and was investigated as the potential secreted factor. A CFT073 hlyA mutant was shown to be significantly more sensitive to 50% serum than the wild-type, implicating haemolysin in serum resistance. However, further analysis showed that secreted HlyA does not contribute to serum resistance and thus it is the cell-associated LPS-bound HlyA which confers serum resistance to CFT073 through facilitating capsule retention. This study has identified a conserved synergy between LPS and capsules, which exists in ExPEC and perhaps other Gram-negative species. Targeting LPS through polymyxin antibiotics, phage depolymerase or other treatments would thereby reduce capsule association and may be of potential therapeutic benefit in conjunction with antibiotic treatment to prevent the dissemination of UTI into sepsis.