A micro luminescence-based assay to measure serum susceptibility in Escherichia coli

Abstract

Neonatal meningitis Escherichia coli (NMEC) is the leading cause of gram-negative meningitis in neonates, and it contributes to neonatal morbidity and mortality globally. The prototypic strain of NMEC, E. coli RS218 possesses the K1 capsule and has been widely employed in the study of NMEC pathogenesis. Previously, our laboratory has utilised relatively large volumes of culture to assay serum bactericidal activities, to garner valuable insights into bacterial immune evasion strategies. However, these methods can be labour intensive, time consuming and are not easily adaptable for high throughput applications. To overcome these limitations, a smaller volume real-time assay was developed. Bacteria were cultured in 100 µl volumes and were sub-inoculated for logarithmic growth. A slow kinetic absorbance assay was established on the Optima Fluorostar microplate reader, which enabled the accurate and reliable measurement of bacterial growth. Subsequently, bioluminescence was incorporated into the assay to facilitate the measurement of bacterial viability in real-time. Bacteria were rendered bioluminescent via electroporation of the pIlux plasmid or alternatively by the addition of exogenous beetle luciferin and recombinant firefly luciferase. The utility of these approaches in the determination of bacterial complement evasion is reported below.