A Comparison of Different Criteria for Determining the Effects of Antibiotics on Tetrahymena pyriformis E. *

Abstract

A study was made of the effects of Chloromycetin, Aureomycin and Terramycin on growth of Tetrahymena pyriformis E. Growth was measured by turbidimetry, cell count and cell volume. Data were analyzed to compare growth rates, time to attain half-maximal growth and growth after 4 days of incubation. Antibiotic levels which yielded approximately 50% inhibition of optical density were as follows: (1) Aureomycin: 1.9 × 10 −5 M at 4 days and 5.8 × 10 −5 M for the number of days to reach half-maximal growth; 50% depression of growth rate was never attained since levels greater than 5.8 × 10 −5 M were lethal; 4.9 to 5.8 × 10 −5 M Aureomycin resulted in approximately 40% inhibition; (2) Chloromycetin: 12.4 × 10 −5 M at 4 days and 23.2 to 31 × 10 −5 M for growth rate and time to reach half-maximum; and (3) Terramycin: 1 × 10 −4 M at 4 days, 1.5 × 10 −4 M for growth rate, and 1.5 to 2 × 10 −4 M for time required to reach the half-maximal point. The diverse results obtained are discussed. Those concentrations of an antibiotic which showed approximately 50% inhibition of growth rate by turbidimetric measurement were then studied for their effect on number of cells during growth. Aureomycin was found to be 66% inhibitory at 4 days, but only 28 and 26% suppressive for growth rate and days needed to attain a half-maximal population, respectively. Chloromycetin reduced growth 82% at 4 days, inhibited rate of growth by 37% and suppressed time to reach the half-maximal level by 39%. Terramycin was calculated to be 50% inhibitory at 4 days, 16% suppressive for growth rate and inhibited the time to reach half-maximal population by 24%. It is also demonstrated that Chloromycetin reduces the size of individual Tetrahymena cells during growth in the culture medium as well as during starvation in buffer. The divers results reported in the literature on the influence of a given antibiotic on Tetrahymena are shown to be caused not only by strain and medium variability, but also, to a great extent, by differences in the methods of measurement and analysis employed. It is suggested that despite certain inherent errors, turbidimetric growth rate is the method of choice for measuring the influence of inhibitors upon total protoplasmic mass.