# Role for RTX-family toxin HlyA of extraintestinal pathogenic Escherichia coli in serum resistance

## TL;DR

This study shows that a toxin called HlyA helps a type of E. coli resist destruction by the immune system's serum, possibly by interacting with the bacteria's protective capsule.

## Contribution

The study identifies HlyA as a novel virulence factor promoting serum resistance in CFT073 ExPEC through interactions with surface capsule.

## Key findings

- A CFT073 hlyA mutant is significantly more sensitive to 50% serum than the wild-type.
- HlyA may provide resistance to serum by interacting with the cell-associated capsule and other surface factors.
- Culture supernatants protect CFT073 from serum killing, suggesting a secreted factor is involved.

## Abstract

Extraintestinal pathogenic Escherichia coli (ExPEC) is a major cause of urinary tract infections, bacteraemia, and sepsis. CFT073 is a prototypic, urosepsis isolate of sequence type (ST) 73. ST73 isolates are associated with higher virulence scores than other pandemic clonal groups, such as ST131. This laboratory, among others, has previously shown that strain CFT073 is serum-resistant, with virulence factors such as the exopolysaccharide capsule and other extracellular polysaccharides imparting resistance to the complement system. In this study, it was shown that culture supernatants were protective in standardized serum killing assays, when compared to cultures standardized in fresh medium. Diluting cultures in fresh medium in place of conditioned medium significantly increased sensitivity of CFT073 to serum, indicating that a secreted factor may provide resistance to serum. Haemolysin, a pore-forming toxin, is secreted by CFT073 in a calcium-dependent manner. This study found that a CFT073 hlyA mutant is significantly more sensitive to 50% serum than the wild-type, implicating haemolysin in the response of CFT073 to serum. In addition to acting as a toxin upon secretion, it has been shown previously that HlyA forms a complex with lipopolysaccharide (LPS), which permits modulation of host immune responses by HlyA whilst cell-associated. The effect of HlyA on capsule expression and serum resistance was examined and characterized in this study, with results indicating that perhaps the HlyA–LPS complex interacts with surface capsule. This study is the first to identify haemolysin as a virulence factor promoting resistance to serum in CFT073, acting whilst associated with the cell.

A common exotoxin of extraintestinal pathogenic Escherichia coli (ExPEC) was shown in this study to contribute to serum resistance, potentially through interactions with surface capsule and other surface-associated factors in an ExPEC prototype, CFT073.