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.