**3. Dynamics of the** *Salmonella* **population: Ecological factors**

Foods are generally considered ecosystems made up of a habitat and a community of living organisms (biocenosis) that can influence each other and, in turn, be influenced by the habitat itself. Given the minute size of the microorganisms, the environment that affects them, at least in a solid matrix, is very small – of the order of a few millimeters or centimeters – so very heterogeneous physical and chemical conditions can exist in food (different conditions between the surface and the inside). In addition, a succession of microbial communities can be observed in food. The original microbial load, largely depending on the initial sources of contamination, is then replaced by new microbial communities that depend on the set of factors that appear during production and conservation processes. The factors that influence the development and survival of contaminating microorganisms are: (i) intrinsic factors, i.e. the characteristics of the food, arising from its composition or structure, pH levels, water activity (Aw) and redox potential (OR-value); (ii) environmental, extrinsic factors which come into action during the processing or storage of the food (storage or treatment temperature; relative humidity, light, storage environment); they may affect the intrinsic factors; (iii) implicit factors derive from the interaction between populations during manufacturing or storage. They can either be positive, such as mutualism and commensalism, or negative, such as competition, antagonism and parasitism.

Although it is possible to control the growth or survival of an individual or a group of microorganisms acting on only one of these factors, this is not always desirable, because it could have excessive consequences on the sensorial and nutritional qualities of food. As modern consumers are increasingly in demand of foods that look "natural" or "fresh", that are safe and have a relatively long shelf life, it is often necessary to act using a combination of factors, each of which is present at sublethal levels, but that, together, ensure the desired level of control. Therefore, instead of using a single barrier, combinations of barriers are used (the so called "hurdle effect") which cause, if the exposure to these conditions is prolonged, such damage that the microorganisms irreparably lose the ability to multiply, reaching their inactivation. *Vice versa*, if the microorganisms are subjected to lower levels of stress (sublethal conditions), they can adapt by activating a number of protection mechanisms, synthesizing proteins and other substances that improve their resistance to the stress in question or different stress. In recent decades, specific mathematical models have been implemented to describe the phenomena such as the growth, the production of metabolites and the death of the microorganisms found in various conditions, useful both for the conservation and for the hygienic safety of food. Predictive models, in particular, see to formulate mathematical models using adequate experimental designs which should provide information about the danger or conservation of food and about the possibility of growth, death or production of a toxin from pathogenic microorganisms. So, in view of the previous observations, we can predict microbial behaviour in similar environmental conditions (Ross & McMeekin, 1994). These approaches, however, are not disadvantage free, such as the variation of strains and the biomolecular knowledge for understanding which factors are responsible for pathogenicity. As regards *Salmonella* the ranges of the factors that favour their growth, death or survival are shown in **Table 2**.


Table 2. Limits and optimum growth in relation to intrinsic and extrinsic factors for Salmonella spp. Notes. \*: Some serotypes. (Source: ICMSF, 1996; amended).
