Effects of freezing on UV inactivation of waterborne microorganisms

摘要

This work was carried out to investigate the response of Escherichia coli, Enterococcus faecalis, and spores of Bacillus subtilis to ultraviolet (UV) irradiation after freezing. The effect of freezing temperature and freeze thaw cycles on UV inactivation efficiency was investigated using a collimated beam apparatus. Freezing affected the response of all test microbes; overall, a lower UV inactivation was observed in the freezing-treated bacteria. The greatest UV inactivation decrease, approximately 1.0 log unit, occurred in the freezing-treated E. coli. The average inactivation level of E. faecalis after freezing was 0.3-0.5 log unit lower than that of the control, and the difference reached approximately 0.1-0.2 log for B. subtilis spores. Under certain conditions, the significantly lower UV inactivation was observed in E. faecalis after surviving freezing, and the freezing-treated spores became more resistant to UV. Freezing temperature did not have a profound effect on the response of freezing-treated test organisms to UV. Freeze thaw cycles affected the response of E. coli and E. faecalis but not B. subtilis. The results suggested that UV inactivation could be less effective on waterborne microorganisms preexposed to freezing.