Purpose: The objectives of this study were to compare the UV-C sensitivity of various pathogenic and attenuated E. coli O157:H7 strains, and to investigate the mechanism of UV-C disinfection.
Methods: Planktonic cells of six pathogenic E. coli O157:H7 strains associated with recent outbreaks of foodborne illnesses, and four attenuated E. coli O157:H7 strains were exposed to varying doses (0-913 mJ/cm2) of UV-C. The mechanism of UV-C damage on two pathogenic E. coli strains with different UV-C sensitivities was evaluated using real time PCR assays with ethidium monoazide (EMA) pre-treatment.
Results: Results showed that the attenuated E. coli strains had similar overall UV-C sensitivity as the pathogenic counterparts. The UV-C inhibition on the PCR amplification of DNA correlated well with UV-C dose, as indicated by the cycle threshold (Ct), regardless of EMA pre-treatment. The Ct values increased linearly (R2=0.98) with increasing UV-C doses in the 0-203 mJ/cm2 dose range. The difference in UV-C sensitivity between strains RM6535 and EDL933 (i.e., the most UV-C sensitive and resistant pathogenic strains, respectively) was reflected by increases in Ct values, viz., increases in Ct values as a function of UV-C dose were significantly (P<0.05) higher for RM6535 than for EDL 933. EMA-PCR analysis indicated that the cell membrane was affected only at high doses when UV-C inactivated more than 6 log CFU/ml of bacteria.
Significance: Overall, our results suggest that the attenuated E. coli strains, on average, had similar sensitivities to UV-C as the pathogenic strains. Furthermore, real-time PCR assays can be used to assess DNA damage caused by UV-C.