**4. Characteristic**

*Salmonella* belongs to the family *Enterobacteriaceae,* a gram-negative, facultative anaerobic and rod-shaped bacterium. The bacteria are 3–5 μm long and 0.7–1.5 μm wide. They are commonly motile with peritrichous flagella that help the bacteria to move, aerogenic, grow on nutrient agar, glucose-fermenting, non-lactose fermenting, often gas producer, urease-negative, citrate-utilising, oxidase-negative, potassium cyanide-negative and acetylmethyl carbinol-negative [26, 27].

Some serovars have peculiarities that are a mutant of normal motile serovars and can change to non-motile. The majority of isolates expressing H antigen exist in two phases: a motile phase I and a non-motile phase II. A Cragie tube can be used to switch non-motile cultures into the motile phase after they have been established in the primary culture [28]. Most *Salmonella* strains are prototrophic and can grow in a minimal medium utilising glucose as the sole carbon energy source and ammonium ion as a nitrogen source. Some host-adapted serovars (e.g., Typhi, Paratyphi A, Gallinarum, Sendai and Abortusovis) are auxotrophic and require one or more growth factors. The biochemical characteristic of *Salmonella* is shown in **Table 2**. Most species produce hydrogen sulphide, which can be detected by growing them on media containing ferrous sulphate, such as triple sugar iron (TSI). However, certain serovars, such as *S.* Typhi, never produce gas from glucose.

*Salmonella* lives predominantly in the intestines of animals and have adapted to live with their hosts [29]. *Salmonella enterica* subsp. *enterica* inhabits warm-blooded animals, whereas all other *S. enterica* subspecies and *S. bongori* live in cold-blooded animals and rarely infect humans. In terms of the types of hosts infected, *S. enterica*  subsp. *enterica* serovars can be clustered to host-adapted, host-restricted and generalhost [30]. **Table 3** lists out the host range of *Salmonella*. The host-adapted *Salmonella* infects habitually a single host but is capable of causing disease in another animal. Host-restricted *Salmonella* infects only a single host, while general-host *Salmonella*


*Note: +: more than 90% positive reactions; −: less than 10% positive reactions; d: 10–90% strains positive; ONPG: ortho-nitrophenyl-β-D-galactopyranoside.*

#### **Table 2.**

*Biochemical characteristics of* Salmonella *species and subspecies.*

has the capability of infecting a variety of animals. However, the disease's progression may vary depending on the host [31].

Most *Salmonella* grows at a temperature of 7–48°C with the optimal growth at 37°C. However, some strains are capable of withstanding extremely low temperature, 2°C, or high temperature, 54°C [32]. Unfortunately, they are not commonly heat resistant and usually die within 1–10 min at 60°C and less than 1 min at 70°C.

The water activity (aw) of foods influences the time and temperature needed to kill *Salmonella* and reduces the effectiveness of the heat treatment. *Salmonella* needs high water activity (aw) between 0.94 and 0.99, optimally at 0.995 but can survive in foods with low aw [33]. Low-aw foods, such as nuts, flour, butter and chocolate, can extend the time and temperature required to kill the bacteria [34]. Some rare serotypes, such as *S*. Senftenberg strain 775 W, has 10–30 times more heat resistant than *S*. Typhimurium in low-aw food products with high carbohydrate or high fat [35]. *Salmonella* grows at a pH value of 4–9 with the optimum growth at a pH value of 6.5–7.5 [36].

Salmonella*: The Critical Enteric Foodborne Pathogen DOI: http://dx.doi.org/10.5772/intechopen.103900*


#### **Table 3.**

*Host range of* Salmonella *enterica subsp. enterica serovars.*
