Resistance of various STEC strains and serogroups to UV radiation and effect of nalidixic acid adaptation

Shigatoxin-producing Escherichia coli (STEC) are one of the important pathogens of concern in various foods and beef in particular. Ultra violet radiation (UV) is an important non thermal treatment that can be very effective in controlling STEC. Current research was aimed at studying the resistance of various STEC to UV and determining suitability of nalidixic acid adapted (Nal) STEC strains for UV inactivation studies. A total of 20 parent strains and 20 Nal strains of E. coli O157:H7, O26, O45, O103, O104, O111, O121 and O145 along with nonpathogenic surrogates (ATCC 1427 and 1428) were used in this study (individual strain and cocktail). Pure cultures of the respective pathogens were treated with UV at three different intensities (1.04, 1.92 and 3.02 mW/cm2). The D values (time needed for one log reduction at a particular intensity) were determined from the linear portion of the inactivation curves. The results show that among the five E. coli O157:H7 strains tested, strain 5 was the most resistant strain with a D value of 86.8 s at 1.04 mW/cm2 UV intensity and E. coli O104:H4 was the most resistant non-O157 serogroup with D value of 65.8 s. An overall comparison of the entire parent STEC strains shows that the serogroup E. coli O157:H7 was the most resistant one to UV. It was observed that the D values of parent strains except for E. coli O157:H7 strain E932 were significantly higher (p < 0.05) than the Nal strains indicating that Nal strains were significantly less resistant to UV than their respective parent strains. Surrogates tested in this study were also significantly less resistant than the parent strains. When comparing inactivation of STEC with the same amount of total energy but at different UV intensities, data shows that high intensity short time UV treatments were more effective than low intensity long time treatments for the STEC inactivation.