**2. General properties of the genus** *Salmonella*

*Salmonella* are Gram negative, short plump shaped rods, nonsporeforming, noncapsulated, aerobic and facultatively anaerobic organisms and classified under the family Enterobacteriaceae (Freeman, 1985).

*Salmonella* nomenclature has changed many times and still is not stable. The genus *Salmonella* was previously differentiated into two species: *Salmonella enterica* and *Salmonella bongori*. However, a new species, *Salmonella subterranea* was identified and validated (Shelobolina et al., 2004; Validation List No: 102, 2005). Among them, the species *Salmonella enterica (S. enterica)* is further divided into the six subspecies *S. enterica* subsp. *enterica* (I), *S. enterica* subsp. *salamae* (II), *S. enterica* subsp. *arizonae* (IIIa), *S. enterica* subsp. *diarizonae* (IIIb), *S. enterica* subsp. *houtenae* (IV), and *S. enterica* subsp. *indica* (VI). Formerly, *S. bongori* was the subspecies V, but later considered as a separate species (Fluit, 2005).

Fermentation of selected substances, such as dulcitol, malonate, sorbitol, d-tartrate, galacturonate, mucate, salicine, ONPG, and lactose, as well as production of enzymes such as gelatinase, -glutamyl-transferase or -glucuronidase, but also lysis by phage O1 allow a differentiation between the different species and subspecies (Le Minor 1984).

Furthermore, the genus composed of over 2500 serotypes differentiated according to three different types of surface antigens discussed bellow in more detail. 99% of these serotypes belong to *S. enterica* and nearly 60% of them are in *S. enterica* subsp. *enterica*. The average DNA sequence similarity between *Salmonella* serotypes is 96-99% (Edwards et al., 2002).
