RH strain Toxoplasma gondii tachyzoites that had naturally lysed their host cells were allowed to infect new host cells for a limited amount of time; subsequent parasite cell divisions were observed closely. On the basis of 4 independent trials, the estimated time to first cell division was 9.8 hr postinfection (PI) and was quite variable (95% confidence interval [CI]: 3.1–16.5 hr PI). The estimated time to second cell division was 14.9 hr PI and was less variable (95% CI: 12.1–17.7 hr PI). Few parasites divided before 6 hr PI in these 4 trials. When tachyzoites were derived by forced lysis (scraping an infected host cell culture and passing it through 27-gauge needles), the first parasite cell division occurred much more rapidly than had been observed in any of the trials with parasites derived by natural lysis. When parasites derived by forced lysis were held away from host cells for 3 hr PI, the first cell division was delayed in a manner similar to that seen in parasites derived by natural lysis. No differences were observed in the timing of the second cell division of parasites derived by forced lysis whether or not they had been held away from cells. These studies demonstrate that the conditions to which tachyzoites are exposed during transit from one host cell to another can affect the kinetics of parasite cell division in the new host cell.
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