Combination methods for microbial decontamination

Novel methods to improve the microbiological quality and to extend the shelf life of foods would be advantageous. In this study, the killing effect of UV, laser, microwave radiation, conventional heating and ozone was investigated, alone and in various combinations, on saline suspensions and agar plate cultures of Listeria monocytogenes, Salmonella typhimurium, Campylobacter jejuni, Shewanella putrefaciens, Pseudomonas fragi, Micrococcus luteus and on E. coli (lux) as an indicator organism. E. coli (lux) was the most sensitive to the effect of UV, whereas M. luteus was the most resistant to UV and Nd:YAG laser radiation. S. putrefaciens was the most sensitive bacterium to Nd:YAG laser radiation. With microwave treatment, a temperature between 70-71°C was the critical point for killing bacteria by microwave energy, although there was evidence of an athermal effect of microwave on bacteria. Ozone was effective against the bacteria used, although the killing of bacteria on the foodstuffs was less significant than killing on plates. The killing effect of Nd:YAG laser and CO2 laser was also investigated on different bacteria on agar plates. Higher frequencies of the Nd:YAG laser resulted in improved clearing effects and, with the CO2 laser, continuous wave always showed better clearing compared to pulsed wave. In comparison of the two laser types, the energy density needed for the Nd:YAG laser was approximately 300 times more than that needed by the CO2 laser to produce the same clear area on the agar plates. Sequential treatment of bacterial suspensions by UV, microwave/conventional heating and Nd:YAG laser gave much greater killing than the sum of the effect of the three treatments alone. Also, greater killing was apparent with the order: laser, microwave/conventional heating and UV compared to the order: microwave/conventional heating, UV and laser. Under standard conditions, the priority of the order L+H+UV over the order H+UV+L was seen consistently through the different experiments and the difference was statistically significant. Differences between the best and worst orders of treatment were increased when more severe treatment conditions were used. Results showed that killing by the sequential treatments on bacteria applied to smoked salmon was almost the same as that for the sum of the three treatments alone. The bioluminescent bacterial strain, E. coli (lux), was investigated as an indicator organism as part of a possible real-time method of measuring the efficiency of the different treatments and combinations. UV reduced the viability of the bacterium by about 8 logs, but the light output immediately after treatment was not significantly affected by UV treatment. In contrast, laser treatment and conventional heating reduced the light output dramatically without greatly reducing the subsequent viable count. These observations highlight limitations of the use of the bioluminescence technique as a realtime monitor of bacterial viable cell numbers. However, under standard conditions, perhaps with a more highly bioluminescent organism, it is possible that the method could be useful in the study of particular decontamination processes. In this study, the killing mechanisms by different treatments were investigated. It was shown that cell constituents released by one method of treatment could protect bacteria against subsequent treatments. In another investigation, release of nucleic acid and protein by different treatments varied and in general, the greater the killing effect produced, the greater the release of material. Only killing by UV did not release a significant amount of nucleic acid and protein. These results suggest that each treatment caused different types of damage and has a different killing mechanism. A quick freezing of the bacterial suspensions after microwave treatment increased their susceptibility to the killing effect of subsequent treatment(s). This method could be suggested as a part of a decontamination procedure in the food processing industry but needs more investigation. Laser, microwave or conventional heating sensitise the bacteria to lysis by SDS, but these effects were lower for ozone and minimal for UV treatment. L. monocytogenes was highly sensitive to SDS and also there was a synergistic effect between SDS and other treatments on killing of the bacterium. So, SDS or similar detergents could be used in decontamination of seafood factories or other materials and surfaces. By scanning and transmission electron microscopy, no gross ultrastructural changes to the internal structures of the cell or rupture of the cell-envelope of E. coli were observed with the different treatments.