# Serum susceptibility of Escherichia coli and its association with patient clinical outcomes

## TL;DR

This study introduces a new flow cytometry method to test how well Escherichia coli survives in human serum, finding high variability among bloodstream infection isolates.

## Contribution

A novel high-throughput flow cytometry-based serum bactericidal assay for E. coli bloodstream isolates is developed and validated.

## Key findings

- The mean proportion of E. coli BSI isolates surviving 25% serum exposure was 0.68 with high variability.
- No significant associations were found between serum resistance and clinical outcomes like mortality or septic shock.
- The new flow cytometry-based assay reduced hands-on work compared to traditional methods.

## Abstract

The innate immune system eliminates bloodstream pathogens such as Escherichia coli in part through complement protein deposition and subsequent bacterial death (i.e., “serum killing”). Some E. coli strains have developed mechanisms to resist serum killing, though the extent of variation in serum killing among bloodstream infection (BSI) isolates and the clinical impact of this variation is not well understood. To address this issue, we developed a novel assay that uses flow cytometry to perform high throughput serum bactericidal assays (SBAs) with E. coli BSI isolates (n = 183) to define the proportion of surviving bacteria after exposure to serum. We further determined whether E. coli resistance to serum killing is associated with clinical outcomes (e.g., in-hospital attributable mortality, in-hospital total mortality, septic shock) and bacterial genotype in the corresponding patients with E. coli BSI. Our novel flow cytometry-based SBA performed similarly to a traditional SBA, though with significantly decreased hands-on bench work. Among E. coli BSI isolates, the mean proportion that survived exposure to 25% serum was 0.68 (Standard deviation 0.02, range 0.57–0.93). We did not identify associations between E. coli resistance to serum killing and clinical outcomes in our adjusted models. Together, this study describes a novel flow cytometry-based approach to the bacterial SBA that allowed for high-throughput testing of E. coli BSI isolates and identified high variability in resistance to serum killing among a large set of BSI isolates.