Effects of ultrasonic radiation on growth and fermentation in the yeast, Saccharomyces cerevisiae

Abstract

Samples from young and older cultures of S. cerevisiae, Strain 15.56, suspended to standard optical density in buffer solution, were exposed to CW or pulsed ultrasonic radiation for different periods. Frequency was i Mc; a plastic lens focussed the beam into a region in which average intensity, over an area of 0.78 cm 2, was about 10.5 W/cm 2. Pulsed radiation, repetition-rate 600/sec, was applied in 25% and 50% duty cycles. Effects were estimated by determination of fermentation rate and growth behaviour, in special flasks. Ten minutes' irradiation of young cells with CW ultrasound produced greater effects than longer exposure to pulsed radiation. Fermentation was markedly inhibited; the lag phase of cultures from irradiated cells was prolonged, and the relative growth rate depressed. Counts of killed cells (stained with eosin) showed that inhibition of fermentation could be accounted for on the basis of population reduction. This decrease in effective inoculum explains also the prolonged lag phase, but not the depressed growth rates. Growth inhibition may be caused by direct, sublethal damage to surviving cells, or by the production of toxic substances in the medium during irradiation. Cells from older cultures were essentially unaffected by radiation. Heating of the suspension is not the factor causing damage; other physical effects were not evaluated. The ineffectiveness of pulsed radiation is attributed to the provision of recovery periods between pulses.