Background: Successful establishment of a controlled human infection (‘challenge’) model of GAS pharyngitis holds great promise in accelerating vaccine development and understanding of GAS infection and immunity. A well-defined challenge bacterial inoculum is central to building a model.
Methods: The aim of this work was to explore growth, adhesion, invasion, viability, and delivery characteristics of the primary emm75 GAS challenge strain (311024), and two back-up emm12 strains (611020 and 611025).
Results: All candidate strains grew well in a chemically-defined medium free of animal products and adhered to pharyngeal-derived Detroit-562 cells. Optimal adherence occurred at early-logarithmic phase, and streptococci were harvested at this time point for storage. All three strains maintained their adherence to Detroit cells following immediate thaw from storage at -80°C for 7 days. Viability of the frozen stock cultures was maintained at 61 days post-freezing. The properties of different swabs for pharyngeal delivery of the inoculum were explored, with a Dacron swab found to be most suitable.
Conclusions: This work highlights important parameters relevant for safe and reliable deployment of a GAS inoculum in a human challenge model of GAS pharyngitis.