Preliminary findings on the use of protozoa (Tetrahymena thermophila) as models for ocular irritation testing in rabbits.

The ciliated protozoan Tetrahymena thermophila (ATCC 30008) was used as an indicator organism in the development of an in vitro test to replace in vivo testing of chemicals in the eyes of rabbits. Fifty microliters of Tetrahymena suspension were mixed with fifty microliters of varying dilutions of test chemicals. After 2 minutes, a bacteriological loopful of the mixture was examined microscopically to evaluate the motility of the organism. The minimum dilution allowing nearly 100% typical cell motility was recorded as was the maximum dilution causing nearly 100% cell death. The reciprocals of these two dilutions were added together, and the higher this number, the more toxic the compound. The results were compared to published reports of rabbit eye irritancy studies. The preliminary in vitro testing indicated a close correlation with in vivo testing with many, but not all chemicals tested. The incidence of false negatives was minimal.