TABLE 1.

Uptake of M. tuberculosis by human monocyte-derived macrophages following exposure to different pHs

Time of exposure to acid (h)a	Uptake (% of initial inoculum) of M. tuberculosis at pH:				Uptake (%) at pH 6 withb:
	5.0	6.0	7.2	8.0	Serum (10%)	No serum
1	20 ± 4	23 ± 6	17 ± 6	15 ± 8	ND	ND
2	47 ± 6c	49 ± 5c	19 ± 4	16 ± 5	ND	ND
4	57 ± 14c	59 ± 7c	18 ± 6	20 ± 4	48 ± 5	49 ± 3
24	51 ± 9c	48 ± 7c	19 ± 5	20 ± 6	ND	ND

^aM. tuberculosis H37Rv in 7H9 broth was exposed to different conditions for 1, 2, 4, and 24 h. After exposure, the bacteria were centrifuged at 4°C and resuspended in RPMI 1640 with 10% heat-inactivated autologous serum, and the concentration was adjusted to 5 × 10⁶ bacteria/ml (multiplicity of infection [MOI] of 10). Phagocytosis was carried out for 1 h. Results using an MOI of 1 were similar and are not shown. The experiment was repeated four times. Results are means ± standard deviations.

^bBacteria (5 × 10⁶) were incubated at pH 6 for 4 h prior to the assay. Then, bacteria were washed at 4°C and incubated with macrophages in the absence or presence of 10% autologous serum or were heat inactivated for 1 h. The uptake of bacteria cultured under laboratory conditions by macrophages increased from 16% ± 3% to 24% ± 4% of the inoculum (P < 0.05). ND, Not done.

^cP < 0.05 compared with uptake of M. tuberculosis incubated at pH 7.2 for the same period of time.