Tetratox: Tetrahymena pyriformis population growth impairment endpoint - A surrogate for fish lethality

A short-term, static protocol using the common freshwater ciliate Tetrahymena pyriformis (strain GL-C) is reported. The 50% impairment growth concentration (IGC(50)) is the endpoint of choice. Cultures are reared in 50 mL of a semi-defined medium in 250-mL Erlenmeyer flasks. Definitive test replicates consist of a minimum of 5 different concentrations of each test material. Duplicate flasks are inoculated to an initial density of approximate to 2500 cells/mL with log-growth-phase ciliates. Following 40 h of incubation at 27 +/- 1 degrees C, population density is measured spectrophotometrically and 50% effect levels are determined Toxicity data for 250 chemicals representing several mechanisms of toxic action are reported and compared to fish toxicity data. As demonstrated by the relationship, Log (IGC(50)(-1)) = 0.77 log (LC50-1)-0.40; r(2) = .750; s = .546; F = 744, there is favorable similarity in, toxic potency between T. pyriformis (log [IGC(50)(-1)] and fish, Pimephales promelas, survivability (log [LC50-1]). However, toxicokinetics and/or toxicodynamic differences between the two systems result in either potency or mechanism of action variances for several chemical classes.