**7. Economic importance and societal**

The economic importance of these diseases is considerable. In the U.S., economists have estimated the annual costs of salmonellosis between 400 million and U.S. \$ 3.5 billion for the entire U.S. economy. They took into account the medical costs and lost productivity (Frenzen et al. 1999; Sarwari et al., 2001). Europe, whereas 95% of salmonellosis is food borne, annual costs range between 560 million and 2.8 billion euros. A single case of salmonellosis is estimated, in turn, to a value between EUR 24 and EUR 3.8 million. This estimate refers to cases where the patient dies of infection (European Parliament and Council of the European Union, 2001). In Africa, there are no data on the annual cost of salmonellosis.

Use Thyme Essential Oils for the Prevention of Salmonellosis 313

eighteenth century, more than 10 essential oils were used. In modern history, the therapeutic properties of essential oils have an increasing importance. Aromatherapy has been used to describe the healing properties of essential oils. Actually, we recognize that essential oils have

Among the plant species estimated by botanists (800 000 to 1 500 00), only 10% are classified as "aromatic". Aromatic plants synthesize and secrete trace amounts of aromatic essence through hair, secretory pockets or channels. Types capable of developing the components of essential oils are distributed in a limited number of families, Myrtaceae, Lauraceae, Rutaceae, Lamiaceae, Asteraceae, Cupressaceae, Poaceae, Zingiberaceae, such as Piperaceae [Bruneton, 1999]. About 20,000 of plant species in the world are used for food, cosmetics, chemical, pharmaceutical and therapeutic food. Among the 4,000 plant species existing in

The AFNOR NF T 75-006 (AFNOR, 1986) defines the essential oil as "a product made from a vegetable raw material, either by steam or by mechanical means from the exocarp, Citrus, or by dry distillation. Essential oils (EO), also called "essences" are aromatic substances, volatile and oily consistency, contained in plants [Balz, 1986 - Lardry and Haberkorn, 2003]. Most plants contain (Eo), but usually in lower quantities. Only plants known as "aromatic" produce essential oils in sufficient quantity. They are usually concentrated in a particular area of the plant such as leaves, bark or fruit, and generally when they occur in various organs from the same plant, they have different compositions (Conner, 1993). The synthesis and accumulation of essential oils, classified as secondary metabolites, are generally in the specialized histological structures, often located on or near the surface of the plant (Brunechon, 1987): pockets (citrus exocarp ) of a storage (eucalyptus), secretory canals or blisters containing resin (conifers), or glands in cuticular (conical epidermal cells on the flowers of Rosaceae), trichomes or secretory glandular trichomes on the leaves of solanaceous or Lamiaceae (Gershenzon, 2000) (Figure 8). It is important to note that several categories of these secretory tissues can coexist simultaneously in the same species, even within the same organ (Fahn, 1979 - Fahn, 1988). For example, to the Lamiaceae family, it is within the secretory hairs, in Myrtaceae in pockets secretory or secretory channels in Asteraceae. Essential oils can be stored in various organs of the plant: flowers (oregano), leaves (lemon grass, eucalyptus), bark (cinnamon), wood (rosewood, sandalwood), roots (vetiver), rhizomes (sweet flag), fruits (star anise) and seeds (caraway). Essential oils are complex mixtures consisting of several compounds, mainly terpenes. Terpenes are formed by one or more isoprene units (Tedder, 1970; Brunechon, 1987), constituting a diverse family both structurally and functionally. Mainly mono-and sesquiterpenes (with 10 and 15 carbon atoms) are the most encountered the diterpenes (20 carbon atoms). Essential oils can also contain aliphatic or aromatic compounds. These terpenoids have an ecological role in plant interactions, such as allelopathic agents. They can be inhibitor of germination, but also during

plant-animal interactions, as a protective agent against predators such as insects.

molecular compound, and therefore generally better (Tedder, 1970).

distillation or expression of the peel fruit (Tedder, 1970;Brunechon, 1987).

They are also involved, through their characteristic odors in the attraction of pollinators (Langenheim, 1969). Some plants may have an odor similar due to a common molecule present in significant amounts in the essential oil. According to the economic environment, it would be profitable to produce plant species may provide an essential oil with high

Herbs that produce essential oils have been the subject of various researches particularly in the field of perfumery. A range of products to smell more or less pronounced depending on the concentration of volatile compounds collected from the essential oils produced by steam

pharmacological, psychological and physiological effects in humans.

Morocco, more than 280 plants are currently operating.
