Abstract
Escherichia coli, the most important foodborne pathogen worldwide, can enter a dormant state called viable but nonculturable (VBNC) state under various stress conditions. Hence, it escapes detection by traditional culturing methods. The present study used Propidium monoazide (PMA)-Polymerase chain reaction (PCR) assay in order to verify whether E. coli, especially O157 strains, converted to the VBNC state under the influence of cool-warm and freeze-thaw stresses. 120 beef liver samples were collected from local markets. The samples that were positive to pathogenic E. coli were treated with cooling (4 °C)-warm and freezing (-18 °C)-thaw cycles to induce the VBNC state. Samples that demonstrated the inability to form colonies on EMB agar were tested by PMA-PCR assays to confirm the existence of VBNC E. coli. 9 samples (7.5 %) of 120 beef liver samples were positive to pathogenic E. coli by microbiological tests. Cool-warm and freeze-thaw stresses on the 9 samples led to the inability of 3 of them to grow on EMB agar plates. By using PMA-PCR assay, we found that 4 °C cool-warm cycle induced E. coli into VBNC state at a higher percentage ratio than -18 °C freeze-thaw cycle. Even though further experiments are still required, in rare cases, at -18 °C, E. coli VBNC may lose expression of genes responsible for viability and metabolic activity. Additionally, our study evaluated the PMA-PCR assay as a rapid and accurate method to verify the existence of VBNC-associated foodborne E. coli O157.

Key words: E. coli O157; Food-borne diseases; freeze-thaw cycle; Polymerase chain reaction; Propidium Monoazide; VBNC state