Growth of and competition between Neospora caninum and Toxoplasma gondii in vitro

Competitive interactions between Neospora caninum and Toxoplasma gondii wer e studied because both species appear to have identical ecological niches i n vitro. Tachyzoites of N. caninum (NC-1 isolate) and T. gondii (RH isolate ) were compared in three in vitro studies: (I) rate of penetration of host cells; (2) generation time; and (3) competition between the two species whe n grown together in the same flask and allowed to compete for space. When t achyzoites of the two species were inoculated onto human foreskin fibroblas ts, 3.24-times more N, caninum tachyzoites penetrated cells by 1 h p.i. At 3 h p.i., there were 2.87-times more N. caninum intracellular tachyzoites t han T. gondii tachyzoites. The generation times for N. caninum (NC-1 isolat e) and T. gondii (RH isolate) were approximately 14-15 h and 8-10 h, respec tively. Before exponential growth occurred, both species displayed a lag pe riod, which was 10-12 h for N. caninum and 8-10 h for T. gondii. To observe competition, equal numbers of tachyzoites of each species were mixed and i noculated into flasks of host cells, and the monolayers were allowed to pro ceed to >90% lysis before the next transfer. Competition was analysed for 3 1 days by labelling samples of each flask with a species-specific monoclona l antibody and determining the ratio of each species. In all trials, T. gon dii outcompeted N. caninum. By 4 days p.i., 70% of the tachyzoites were T. gondii; this percentage increased to 97% by 23 days p.i. When the starting inoculum contained 75% N. caninum and 25% T. gondii tachyzoites, T, gondii was still competitively superior. When infected monolayers that were labell ed with T. gondii-specific antibodies were examined, it was noted that both species can occupy and undergo endodyogeny in the same host simultaneously . (C) 1999 Australian Society for Parasitology Inc. Published by Elsevier S cience Ltd. All rights reserved.