Escherichia coli viability in an isochoric system at subfreezing temperatures.
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abstract
In comparison with isobaric (constant pressure) freezing, isochoric (constant volume) freezing reduces potential mechanical damage from ice crystals and exposes stored biological matter to a lower extracellular concentration, at the price of increased hydrostatic pressure. This study evaluates the effects of isochoric freezing to low temperatures and high pressures on Escherichia coli (E. coli) survival. The viability of E. coli was examined after freezing to final temperatures between -5C and -20C for periods from 0.5h to 12h, with recovery periods from 0h to 24h. Freezing for up to two hours to -10C and -15C had little effect on the percentage of viable E. coli, relative to the controls. However, after two hours of exposure at -20C, when left to recover for 24h, a 75% reduction in survival is observed. Furthermore, after 12h of isochoric freezing at -15C and -20C, E. coli population is reduced by 2.5 logs while freezing to these temperatures in conventional isobaric atmospheric conditions reduces population by only one log. This suggests that the combination of low temperature and high pressure experienced during isochoric freezing close to the triple point may be more detrimental to biological matter survival than the combination of elevated concentration, low temperature, and ice crystallization experienced during conventional freezing, and that this effect may be related to the time of exposure to these conditions.
