Listeria monocytogenes - inactivation by essential oils and high pressure, and contribution of genetic variation to stress resistance

A variety of non-thermal mild preservation techniques have lately been introduced in food production, or are currently being investigated thoroughly for potential use in food processing. Two of these novel preservation techniques are the use of essential oil compounds isolated from plants and of High Hydrostatic Pressure (HHP) treatment. Applications of these preservation methods, alone or combined, have shown to reduce the viable numbers of microorganisms occurring in foods, and in addition, are able to preserve the nutritional and sensory characteristics of the product. The objectives of this thesis were to asses the antimicrobial potential of these combined processing techniques against the foodborne bacterium Listeria monocytogenes , which can cause serious disease with a relatively high mortality rate. In addition, to get more insight in the primary cellular targets that are affected upon HHP treatment of L. monocytogenes , we examined the mechanism underlying increased HHP tolerance of an isolate of this organism.Chapter 1 gives a short introduction on L. monocytogenes. Furthermore, mild and novel preservation techniques in food processing are addressed, with special emphasis on the use of essential oil compounds from plants, and the use of High Hydrostatic Pressure (HHP). General characteristics and the mode of action of these preservation techniques are described, followed by resistance mechanisms that bacteria use to counteract their potentially lethal action. This chapter is concluded with the combined use of these preservation methods, and an outline of this thesis.In Chapter 2 , it is demonstrated that various plant-derived volatiles, namely, S-carvone, carvacrol, cinnamaldehyde, thymol, and decanal, have the ability to reduce the viable numbers of L. monocytogenes if used in combination with mild heat treatment, while these treatments alone were not affecting the viability of this pathogen. These findings show that essential oil compounds can play