Evidence of lethal and sublethal injury in food‐borne bacterial pathogens exposed to high‐intensity pulsed‐plasma gas discharges.

Abstract

Aims: To apply scanning electron microscopy, image analysis and a fluorescent viability stain to assess lethal and sublethal injury in food‐borne bacteria exposed to pulsed‐plasma gas discharges (PPGD).

Methods and Results: The fluorescent redox probe 5‐cyano‐2,3‐ditolyl tetrazolium chloride (CTC) was used for enumerating actively respiring cells of Campylobacter jejuni, Escherichia coli, Listeria monocytogenes, Staphylococcus aureus and Salmonella enterica serovar Typhimurium that were suspended in sterile water at 4°C and exposed to separate PPGD and heat treatments. While there was good agreement between use of respiratory staining (RS) and direct‐selective agar plate counting (PC) for enumerating untreated bacteria, there were c. 1 and 3 log‐unit differences in surviving cell numbers per millilitre for test organisms subjected to PPGD and heat treatments respectively, when enumerated by these different viability indicators. PPGD‐treated bacteria were markedly altered at the cellular level when examined by scanning electron microscopy. Conclusions: Use of this RS method revealed that substantial subpopulations of test bacteria rendered incapable of forming colonies by separate PPGD and heat treatments may remain metabolically active.