Comparison of intracellular growth and extracellular survival between strains of T. gondii. (A) Comparison of intracellular growth rate between RH-ERP2009 and GT1. Monolayers of HFF cells were infected with parasites, grown for 30 h, fixed, and stained with MAb DG52 to detect surface antigen SAG1. The average numbers of parasites present per vacuole was determined by microscopic examination and counting of 50 vacuoles/coverslip. GT1 contained significantly more vacuoles with 16 parasites/vacuole (*, P ≤ 0.05), while the number of vacuoles with 32 parasites was significantly higher in RH-ERP (**, P ≤ 0.005). Values indicate means ± the SEM (n = three separate experiments with three coverslips each). (B) Comparison of extracellular survival between RH-ERP2009, RH-JSR, and both low- and high-passage GT1. The number of tachyzoites surviving after natural egress was determined by incubation of parasites for at 37°C with CO2 in complete medium and subsequent plaque assay. The viability of RH-ERP2009 was higher at the outset and viability dropped by 50% over the next 24 h (**, P ≤ 0.005, comparison of 2 h versus 24 h). The initial viability of GT1 was significantly lower at 0 h after egress (**, P ≤ 0.005), and this dropped further to almost negligible levels by 12 h after egress (**, P ≤ 0.005, comparison of 2 h versus 12 h). There was also a significant difference in the viability of low passaged GT1-LP and RH-JSR isolates versus GT1 even at early time points (*, P < 0.05). Values are means ± the SEM (n = three experiments).