Fig. 8. Bacterial cell–cell and cell–host interaction models.

a–d S. pneumoniae strain sfCSPr that expresses GFP in response to CSP-1 was mixed with strain ADP112 that produces CSP-1 upon IPTG induction. A 40:1 mixture of ADP112:sfCSPr was encapsulated into agarose droplets, the oil removed, and then fluorescence and brightfield microscopy images were captured after 3 -h culture in the absence (a) and the presence (b) of 1 mM IPTG (white arrows highlight GFP-expressing sfCSPr microcolonies). Subsequent FACS analysis of agarose droplets accurately represents the predicted GFP signal frequency in the absence (c) and the presence (d) of 1 mM IPTG. e, f Gelled agarose droplets can be re-encapsulated into another droplet to form a second layer of agarose. Agarose droplets containing sfCSPr were re-encapsulated into media containing 1% agarose in the absence (e) or presence (f) of 560 ng/ml CSP-1. After 2 h of culture, the oil was removed and brightfield and fluorescence imaging revealed no GFP expression for the untreated sample, but positive GFP expression for the CSP-1 treated sample (black arrow highlights non-induced sfCSPr, white arrow highlights GFP-expressing sfCSPr, white asterisk indicates the inner droplet, black asterisk inidcates the outer droplet layer). g–j Agarose/hydrogel droplets containing Yersinia pseudotuberculosis (Yptb) were exposed to murine bone marrow-derived macrophages (BMDMs). Yptb strain IP2666 GFP + was grown in droplets overnight and visualized by fluorescence (g) and brightfield (h) microscopy. After a 1-h incubation, BMDMs can attach to oil-free empty droplets (i) or droplets containing Yptb cells (j) (white arrows indicate Yptb cells and black arrows indicate BMDMs).