Mutation induction in bacteria by freeze-drying

Abstract

The stability of the deoxyribonucleic acid (DNA) double helix to freezing and thawing both in vitro and in vivo has been well documented. Shikama (13), using thermal denaturation techniques, showed that purified DNA solutions were not affected by freeze-thaw procedures. Ashwood-Smith ( 1) presented evidence of the genetic stability of bacteria frozen and thawed under several different conditions. No mutation induction in Escherichiu coli was evident and no change in a wide spectrum of antibiotic resistance could be demonstrated after a long series of freeze-thaw selection procedures. Swartz (14), however, has published evidence that freezing and thawing ‘of E. coli produced single-strand breaks in DNA in vivo, but this has not been confirmed by other workers ( 1). The best evidence at this time, therefore, is that freezethaw procedures, per se, do not produce genetic damage, and this statement is substantiated by evidence presented in this paper. Genetic changes in bacteria induced by freeze-drying have not been reported, although Webb (16) has published data on the genetic effects of exposing E. co& in an aerosol to different degrees of humidity. The highest number of mutants was observed when the relative humidity of the aerosol was approximately 40%. Bridges (personal communication) has indicated that stocks of freeze-dried bacterial preparations have higher background reversion