Salmonella enterica subsp. enterica is a subspecies of Salmonella enterica, the rod-shaped, flagellated, aerobic, Gram-negative bacterium. Many of the pathogenic serovars of the S. enterica species are in this subspecies, including that responsible for typhoid.
Serovars
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. 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.
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. 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.
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:
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. 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.
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. Multidrug resistant strains have been present since the late 1970s in Africa and Asia. 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.
Metabolism
Genetic evidence suggests that the serovars can be divided into two groups – one which causes enteric infection and has a broad repertoire of metabolic capabilities, and one which usually causes invasive infection, often in a narrow range of hosts, and shows degradation of anaerobic metabolic pathways. It is thought that these metabolic capabilities are important for obtaining nutrients in the challenging and nutrient-limited inflamed gut environment.
Survival in the stomach
Under normal circumstances, Salmonella enterica infections range from 10^6 and 10^7 bacilli There are certain scenarios that can occur to decrease the infection risk. These include, a higher pH in the stomach, gastric resection, and treatment with anti acid buffering. It is important to note that if the stomach has a lower pH, then this helps as a defensive technique to potentially fight off this infection.
Salmonella Enterica subsp. enterica virulence potential can be linked to higher survival within work that is performed outside a living organism from a human gastrointestinal model. "Food Microbiology" evaluated whether Salmonella virulence could potentially be linked to a greater ability to survive consecutive digestive environments. 13 different strains of S.enterica were selected based upon their high and low virulence phenotypes that were obtained from Dr. Daigle's Labratory. Testing included using lettuce as a food source for the strains to examine the passage through the stomach. After passage, both virulence phenotype strains survived but only the high virulence strain had a greater survival rate. These survival rates could be linked to acid and bile resistant genes that are present. Certain capacities within S.enterica that survive in the human digestion tract have a low pH of the stomach and enzymatic secretions that overall affect the virulence.