**2. History background**

*Salmonella* was a prevalent pathogen that infected the digestive tracts of both human and animals. *Salmonella* contamination in food, water and the natural environment is mainly caused by faecal contamination in the environment (**Figure 1**). Some *Salmonella* serovars are host-specific; for instance, serovars Typhi and Paratyphi A can only be colonised in humans, serovar Abortusovis in sheep and serovar Gallinarum in fowl. *Salmonella* serovars also induce infectious syndromes distinct to their type; for example, the highly adapted serovar Typhi causes systemic infection called typhoid exclusively in humans. Serovar Typhimurium causes non-typhoidal salmonellosis (NTS) in human, which is one of the most prevalent serotypes responsible for infections, including acute gastroenteritis in humans [1] and animal species like hens [4], pig [5] and mice [6]. Serovar Abortusovis causes high rates of abortion in flocks, ewes, sheep and goat [7, 8], and serovar Dublin originally discovered in cattle, which adapted to infect other animals such as bovines and fox [9–11]. Infection by serovar Dublin in human is rare but causes rather severe invasive bloodstream infection [10].

In 1880, Karl Eberth discovered a bacillus-like pathogen in the spleen and Peyer's patches of typhoid patients. He was a student of the famous Rudolf Virchow [12]. Four years later (in 1884), Georg Gaffky, a German microbiologist, successfully grew the pure culture of the bacterium [13]. Theobald Smith was the first to discover what would be later known as *Salmonella enterica* (var. Choleraesuis) while working in the Veterinary Division of the United States Department of Agriculture (USDA) as a research laboratory assistant in a department headed by Daniel Elmer Salmon, the veterinary pathologist. At first, the agent responsible for swine fever or hog cholera was thought to be caused by *Salmonella* Choleraesuis, prompting both Salmon and Smith to name the bacterium "Hog-cholerabacillus" [14]. In fact, Salmon and Smith were first to discover and isolate *S.* Choleraesuis from pigs in 1886. Incidentally, it was not until 1900 that the name and genus *Salmonella* was used when it was proposed by Joseph Leon Lignières and named after Daniel Elmer Salmon as an honorific attribute to the discovery made by his group [15].

#### **Figure 1.**

*Schematic diagram showing important structural components of* Salmonella *Typhi (Source: Hu* & *Kopecko, 2003 [3]).*
