TABLE 1.

T. whipplei survival requires acidic pH^a

Agent	Vacuolar pH	Cell viability (%)	Cell infection
Control	4.7 ± 0.3	98 ± 2	696 ± 32
NH4Cl (100 nM)	6.45 ± 0.15	85 ± 6	70 ± 12∗
N-Ethylmaleimide (25 mM)	6.35 ± 0.2	87 ± 5	108 ± 15∗
Bafilomycin A1 (0.1 nM)	6.5 ± 0.15	90 ± 4	81 ± 11∗
Chloroquine (2 mM)	6.35 ± 0.2	89 ± 6	37 ± 8∗

^aHeLa cells were incubated with T. whipplei (bacterium-to-cell ratio, 100:1) for 4 h and then with pH-neutralizing drugs for 48 h. The vacuolar pH was measured by ratiometric analysis of DM-NERF fluorescence intensities, and mean pH values were calculated using a pH reference curve. The cell viability was determined by trypan blue exclusion. Intracellular bacteria were revealed by indirect immunofluorescence, and their viability was assessed by using a combination of SYTO 9 and propidium iodide. The infection by viable organisms was expressed as the mean index (absolute value of infection index) ± SD of three experiments. ∗, P < 0.01.