Effects of UVC Treatment on Biofilms of Escherichia coli Strains Formed at Different Temperatures and Maturation Periods

In the present study, the biofilm formation and ultraviolet-C (UVC) resistance characteristics of Escherichia coli isolated from an occluded biliary stent were compared with those of four E. coli O157:H7 strains (ATCC 35150, 43889, 43890, and 43895). To evaluate biofilm formation, the E. coli isolated from a stent and four E. coli O157:H7 strains were incubated at 37, 25, and 15 °C for 7 days, revealing that peak biofilm formation occurred at 37 °C (day 1), 25 °C (day 3), and 15 °C (day 5), with the stent-isolated strain consistently exhibiting significantly higher biofilm cell counts than the others (p < 0.05). The UVC treatment was less effective at reducing viable biofilm cells as the formation temperature decreased, with the stent-isolated E. coli biofilm formed at 15 °C showing the lowest reduction levels. Exopolysaccharide quantification revealed that all E. coli strains produced more extracellular polymeric substances (EPSs) at lower temperatures, with the stent-isolated E. coli biofilm formed at 15 °C showing significantly higher EPS levels than the other strains (p < 0.05), potentially explaining its greater UVC resistance. Based on these results, it was confirmed that the biofilm formed by the E. coli isolated from the stent at 15 °C exhibited the highest resistance to UVC, which can be attributed to its elevated exopolysaccharide production. This study demonstrates that both temperature and maturation period significantly influence E. coli biofilm characteristics and provides valuable insights into E. coli isolated from the stent, which may pose a risk of cross-contamination in food-related environments.