Fluorescent labelling of intracellular bacteria in living host cells

The fluorescent reagent, CellTracker, labels metabolically-active cells and was used here to label Chlamydia in vivo during their exponential phase of growth in infected cells. HeLa cells infected with C. psittaci were labell ed with the CellTracker reagents between 15 and 48 h post-infection. The fl uorescent label accumulated in the host-cell membrane compartment (inclusio n) within which Chlamydia reside and replicate, and was also incorporated b y the bacteria. Labelling with the CellTracker affected neither the growth nor the differentiation of the chlamydiae, and labelled chlamydiae isolated from infected cells were infectious. Our results demonstrate that the Cell Tracker could become a valuable tool for in vivo labelling of obligate intr acellular parasites for which no genetic tools exist. (C) 2000 Elsevier Sci ence BN. All rights reserved.