FIGURE 4.

Glucose as well as glutamine alone can supply ample energy for host-cell invasion by T. gondii. A, invasion efficiency of the indicated parasite strains in culture medium supplemented with dialyzed serum (10%) and glutamine (2 mm, if any). Fresh syringe-released extracellular parasites were used to infect human fibroblast cells (37 °C, 5% CO₂, 60 min), followed by staining with α-TgSag1 and α-TgGap45 antibodies. Glutamine-free samples were incubated without the amino acid for 30 min before invasion assay. B, synchronized host-cell invasion assay. Fresh tachyzoites were suspended in high K⁺ buffer and allowed to settle on host-cell monolayers for 30 min before exchanging buffers (high to low K⁺). Parasites were stained, as described in panel A. C, effect of known inhibitors of mitochondrial electron transport (atovaquone, 0.1 μm) and of glutamine catabolism (DON, 2 μm) on invasion rates of different parasite strains. Host cells were washed with ice-cold PBS before infection, and fresh medium supplemented with individual inhibitors was added at the time of invasion assay. D, ATP contents of extracellular parasites after incubation (37 °C, 5% CO₂, 30 min) in culture medium supplemented with dialyzed FCS (10%) and glutamine (2 mm, if any). Relative declines are shown as percentages. The viability of all parasite strains was nearly 100%, which remained unaltered by glutamine depletion (not shown). Statistical significance (Student's t test) was measured separately for each group by comparing to the parental strain (panels A and B) or to the ATP levels of individual strains under control conditions (panel C) (*, p < 0.05; **, p < 0.01; ***, p < 0.001).