Morphological features of intracellular Y. pestis in antibiotic-free media.

RAW264.7 and DH82 cells infected with Y. pestis strain KIM6+ for 30 min in RPMI-1640 with 10% FBS media, washed with PBS to remove extracellular bacteria, and then incubated in HBSS with 10% FBS were sampled at 2.5 and 5 h p.i. for light microscopic examination by staining with Wright Giemsa stain. Arrows indicate filamentous Y. pestis. Images are presented at 1,000× magnification.