Salmonella is a gram-negative bacillus with a variety of morphologies, usually short, rod-shaped, nonspore-free, and surrounded flagella[1]. The bacterium is widely distributed in nature and is often found in animal feces, soil, food, and water sources. They are capable of infecting a variety of hosts, including humans, poultry, livestock, and wildlife[2, 3]. Salmonella is a genus comprising two species: S. enterica and S. bongori. S. enterica is further categorized into seven subspecies: I (S. enterica subsp. enterica), II (S. enterica subsp. salamae), IIIa (S. enterica subsp. arizonae), IIIb (S. enterica subsp. diarizonae), IV (S. enterica subsp. indica), VI (S. enterica subsp. houtenae), and VII (S. enterica subsp. VII (Newport))[4].
Salmonella possesses a complex antigenic structure, generally classified into three types: bacterial (O) antigen, flagellar (H) antigen, and surface (Vi) antigen. To date, more than 40 O antigens and 114 H antigens have been identified among the known salmonellae, leading to the identification of more than 2,600 serotypes of Salmonella. At least 50% of these serotypes belong to S. enterica subsp. enterica[4, 5]. Salmonella serotypes are generally divided into typhoid and nontyphoidal serotypes, based on whether they cause systemic disease or localized gastroenteritis[1, 6]. Most Salmonella serotypes colonize multiple hosts (e.g., S. enterica serotypes, S. Typhimurium, and S. Enteritidis); however, a relatively small number have adapted to unique hosts (e.g., S. enteritis serovar Typhi, which infects only humans)[7, 8]. Causative serotypes vary from continent to continent, with typhoidal Salmonellae (serotypes Typhi and Paratyphi A) common in Southeast Asia and non-typhoidal Salmonella serovars prevalent in Africa[9].
As a precondition for colonization to the host cell, Salmonella are equipped with a series of virulence factors for binding to host surfaces, such as fimbrial adhesins like type 1 fimbriae (Fim), plasmid-encoded fimbriae (Pef), long polar fimbriae (Lpf), thin aggregative fimbriae (Agf), non-fimbrial autotransported adhesins MisL, and Type 1 Secretion System -secreted adhesins[10]. Salmonella pathogenicity islands 1 (SPI-1) enables invasion of host cells, by the most recent common ancestor of all Salmonella subspecies, and SPI-2, for replication in macrophages, during species divergence of S. enterica from S. bongori[11]. In addition, Salmonella utilize the Type 3 Secretion System dependent effector protein secretion into the host-cell cytosol to facilitate their invasion, intracellular survival and replication, and also manipulate host-cell signaling cascades[12].
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