**2. Brief history, morphology, physical and biochemical characteristics**

In mid of nineteenth century, *Salmonella* was first reported by Eberth, which was followed by Gaffky who isolated and demonstrated that Bacillus causes human typhoid fever [15]. In 1885, Theobald Smith and Daniel Elmer Salmon from the gut of pigs isolated Bacillus infected with swine fever (hog cholera) [15, 16]. An American pathologist, Dr. Daniel Elmeri Salmon, in collaboration with Smith gave the name *Salmonella* [17]. Most reference centres of *Salmonella* all over the world, including Centers for Disease Control (CDC), use *Salmonella* nomenclature system of World Health Organization (WHO) [18].

#### Salmonella *Infection and Pathogenesis DOI: http://dx.doi.org/10.5772/intechopen.102061*

*Salmonella* are anaerobic, chemo-organotrophic, rod-shaped with size 0.2–1.5 × 2–5 μm and are Gram negative in nature [19]. Except a few serovars *viz S. choleraesuis*, all other members of this genus produce hydrogen sulphide and majority of them do not perform lactose fermentation [20]. This crucial trait has been used to produce a number of selective and differential media for *Salmonella* culture, isolation, and presumptive identification. *Salmonella*-Shigella agar (SS), brilliant green agar (BGA), xylose lysine deoxycholate (XLD) agar, Hektoen enteric (HE) agar, MacConkey agar, lysine iron agar (LIA), and triple sugar iron (TSI) agar are among the media frequently used [21, 22].

*Salmonella* is non-fastidious, as outside the living hosts it can grow and multiply in a variety of environments. *Salmonella* is heat-sensitive, and is frequently killed at temperatures of 70°C or above. The majority of serotypes thrive and grow in temperatures ranging from 5 to 47°C with an optimum of 32 to 35°C. Few serotypes, however, may thrive at temperatures as low as 2–4°C and as high as 54°C [23]. *Salmonella* grow at pH ranging from 4 to 9, with optimum range of 6.5–7.5. *Salmonella* require high water activity of about 0.99–0.94 for survival. At pH greater than 3.8, water activity greater than 0.94 and temperature higher than 70°C, it shows no growth [23]. While almost all serotypes do not make indole, hydrolyze urea, or deaminate phenylalanine or tryptophan, the majority of serotypes rapidly convert nitrate to nitrite, ferment a range of carbohydrates with acid production [20].
