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

*Salmonella* is a genus of pathogenic bacteria named by Lignières in 1900, after the veterinarian pathologist and microbiologist Daniel Elmer Salmon, who isolated the agent and associated it with a disease for the first time [6]. These bacteria are part of the Enterobacteriaceae family and are morphologically composed of non-sporeforming, Gram-negative, facultatively anaerobic, rod-shaped bacteria, with optimum growth temperature between 35°C and 37°C [7].

Currently, *Salmonella* spp. is divided into two species: *Salmonella enterica* and *Salmonella bongori*. The first species is divided into six subspecies with a Roman numeral, as follows: *enterica* (serogroup I), *salamae* (serogroup II), *arizonae* (serogroup IIIa), *diarizonae* (serogroup IIIb), *houtenae* (serogroup IV), and *indica* (serogroup VI) [8, 9].

*Salmonella bongori* (serogroup V) has 23 serotypes and *S. enterica* has more than 2500 serotypes (*S. enterica* subsp. *enterica* serogroup I = 1547, *S. enterica* subsp. *salamae* serogroup II = 513, *S. enterica* subsp. *arizonae* serogroup IIIA = 100, *S. enterica* subsp. *diarizonae* serogroup IIIb = 341, *S. enterica* subsp. *houtenae* serogroup IV = 73, and *S. enterica* subsp. *indica* serogroup VI = 13 [10]. This characterization of species and subspecies into serotypes is based on the model proposed by Kauffman-White from differences observed in flagellar (H), capsular (K), and somatic (O) antigens [11].


#### **Table 1.**

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

The species and subspecies of *Salmonella* also have distinguishing biochemical characteristics (**Table 1**). These bacteria are catalase-positive and oxidase-negative and can form hydrogen sulfide through the enzyme cysteine desulfhydrase, which promotes sulfur reduction. Moreover, they can reduce nitrite to nitrate and use citrate as an energy source. In contrast, they do not produce indole or hydrolyze urea [6].

*Salmonella* is a bacterium of worldwide geographical distribution and, therefore, many animal species, including wild animals, can act as a reservoir of its various serovars [12]. Wild and domestic animals and humans can be affected by any of the more than 2500 different serovars [13]. *S. enterica* subsp. *enterica* determines infections mainly in warm-blooded animals [11], chiefly mammals [14], and is associated with most of the world's foodborne diseases [11]. Nevertheless, different serovars of this subspecies have been isolated from exotic reptile kept as pets, as we will report throughout this chapter. The other subspecies of *S. enterica* are uncommon for humans and are usually found in cold-blooded animals and environmental samples [14]. Similarly, *S. bongori* is more common in cold-blooded animals, especially reptiles, and in the environment [6], but can also infect humans [15].

*Salmonella* habitat, based on the host's specificity and clinical manifestations, can be characterized as follows: a. *highly adapted to humans*, corresponding to serotypes *S*. Typhi, and *S*. Paratyphi A, B, and C; b. *highly adapted to animals*, responsible for paratyphoid fever in animals, consisting of *S*. Dublin (cattle), *S*. Choleraesuis and *S*. Typhisuis (pigs), *S*. Abortusequi (equines), and *S*. Pullorum and *S*. Gallinarum (birds); and c. *zoonotic Salmonella*, which affect humans and domestic and wild animals indistinctly and are involved in food poisoning and gastroenteritis. This third group is more representative of public health due to its high morbidity and mortality rates [6, 9].

Notably, *Salmonella* spp. can survive in the environment, mainly in organic matter, and can continue infecting for 280 days in soils used for cultivation, 120 days in pastures, 30 days in bovine feces, and 28 days in bird fecal matter [6, 16]. Moreover, it adheres to the surface of plant roots and survives for long period underground [17]. This occurs because these bacteria, which inhabit the intestinal tract of humans and animals, are eliminated in the feces and can then contaminate both water and soil. Furthermore, in aquatic ecosystems, *Salmonella* can adhere to sediments [18] and survive in high densities in these systems and water after 56 days [19]. In this regard, sediments provide a protective layer for enteric bacteria from a nutrient reserve and prevent stress from the aquatic environment [20].
