Salmonella enterica subsp. enterica

Salmonella enterica subsp. enterica serovars are defined based on their somatic (O) and flagellar (H) antigens, with over 2,600 serovars in total; only about 50 of these serovars are common causes of infections in humans [2]. Most of these serovars are found in the environment and survive in plants, water, and soil; many serovars have broad host ranges that allow them to colonize different species in mammals, birds, reptiles, amphibians, and insects. Zoonotic diseases, like Salmonella, spread between the environment and people[3].

A number of techniques are currently used to differentiate between serotypes. These include looking for the presence or absence of antigens, phage typing, molecular fingerprinting and biotyping, where serovars are differentiated by which nutrients they are able to ferment. A possible factor in determining the host range of particular serovars is phage-mediated acquisition of a small number of genetic elements that enable infection of a particular host.[4] It is further postulated that serovars which infect a narrow range of species have diverged from ancestors with a broad host range, and have since specialised and lost the ability to infect some hosts.[5]

The CDC publishes a Salmonella Annual Report with a list of serovars most commonly associated with human illness, the top 10 serovars are listed below[6]:

Studies have concluded most strains of Salmonella enterica subsp. enterica serovars possess serotype-specific virulence plasmids. These are plasmid-associated virulence characterized by low-copy-number plasmids and depending on the serovar, its size ranges from 50 to 100 kb[7]. Serovar Enteritidis, which is the most common serovar isolated in human clinical cases, has also been found to produce endotoxins, coded by the stn and slyA genes, that attribute to the pathogenicity of Enteritidis[8].

In November 2016, a new strain of extensively drug resistant (XDR) Salmonella enterica serovar Typhi emerged in Pakistan, primarily from the cities of Hyderabad and Karachi.[9] Multidrug resistant strains have been present since the late 1970s in Africa and Asia.[10] These XDR strains are resistant to all antibiotic treatment options: chloramphenicol, ampicillin, trimethoprim-sulfamethoxazole, fluoroquinolones, and third-generation cephalosporins. The outbreak has been ongoing since 2016.[11]

The serovars can be designated fully or in a shortened form.[13] The short form lists the genus, Salmonella, which is followed by the capitalized not italicized serovar, e.g. Salmonella Typhi[14] whereas, full designation for Salmonella Typhi is Salmonella enterica subsp. enterica serovar Typhi.[15] Each serovar can have many strains, as well, which allows for a rapid increase in the total number of antigenically variable bacteria.[16]

Invasive strains of non-typhoidal Salmonella, such as Salmonella Typhimurium ST313 have recently been labelled as causing emerging diseases in Africa. Key host immune deficiencies associated with HIV, malaria and malnutrition have contributed to a wide spread of this disease and the need to use expensive antimicrobial drugs in the poorest health services in the world.[17] But also bacterial factors, such as upregulated activity of the virulence gene pgtE, due to a single nucleotide polymorphism (SNP) in its promoter region, have been shown to have a great impact upon the pathogenesis of this particular Salmonella sequence type.[18]