Salmonella enterica

Lore & Background

Salmonella enterica primarily resides in the intestinal tract of animals and humans and can be found in feedstuff, soil, bedding, litter, and fecal matter. Most cases of salmonellosis are caused by food infected with S. enterica, which often infects cattle and poultry, though other animals such as domestic cats and hamsters have also been shown to be sources of infection in humans. The primary reservoir for the pathogen is poultry, and the proportion of human cases attributed to the consumption of contaminated eggs, chicken, or turkey varies by region and study. Raw chicken eggs and goose eggs can harbor S. enterica, initially in the yolk or the egg's interior via transovarian infection, although most eggs are not infected. As the egg ages at room temperature, the yolk membrane begins to break down and S. enterica can spread further into the egg. Refrigeration and freezing do not kill all the bacteria, but substantially slow or halt their growth. Pasteurizing and food irradiation are used to kill Salmonella for commercially produced foodstuffs containing raw eggs such as ice cream.

Reader's Guide

Salmonella enterica is a leading foodborne pathogen in the United States, causing the most deaths and having the highest cost burden. It is a resilient microorganism capable of surviving long periods of time in hot and dry environments, increasing its effectiveness as a pathogen and making it able to survive the harsh environments of the gastrointestinal tract and farms. S. enterica during prehistory, and a possible role of the Neolithization process in the evolution of host adaptation. Additional reconstructed genomes from colonial Mexico suggest S. enterica as the cause of cocoliztli, an epidemic in 16th-century New Spain. enterica spread explosively across what is now Mexico, killing up to 90% of the Indigenous population over the next century. Children under five, the elderly, and immunosuppressed adults are at increased risk of systemic dissemination. The serogroup S. Typhi is the cause of typhoid fever. Secreted proteins are of major importance for pathogenesis, and the bacterium uses base excision repair and recombinational repair to resist bile salts.

Did You Know?

• The primary reservoir for S. enterica is poultry, and the proportion of human cases attributed to contaminated eggs, chicken, or turkey varies by region and study.
• The serogroup S. Typhi is the cause of typhoid fever.

Taxonomic Architecture and Serovar Diversity

The genus Salmonella sits within the Enterobacteriaceae family and comprises exactly two recognized species: Salmonella bongori and Salmonella enterica. These serovars are defined through the Kauffman–White classification system, which relies on two antigenic markers: the somatic O antigen (a lipopolysaccharide component) and the flagellar H antigen. A full taxonomic designation might read Salmonella enterica subsp. enterica serotype Typhimurium, though clinicians often shorten it to Salmonella Typhimurium. Beyond serotyping, laboratories employ antibiotic sensitivity profiles, pulsed-field gel electrophoresis, multilocus sequence typing, and increasingly whole-genome sequencing to differentiate strains for epidemiological tracking. Historically, the clinical community has split salmonellae into two broad camps: invasive typhoidal strains and non-invasive nontyphoidal strains, a distinction rooted in host preference and the pattern of human disease they produce.

A Century of Discovery and Naming

The story of Salmonella's discovery spans several decades and multiple researchers. Four years later, Georg Theodor Gaffky succeeded in cultivating the pathogen in pure culture. Choleraesuis. Smith worked under Daniel Elmer Salmon, a veterinary pathologist who administered the division. Decades later, in the late 1930s, Australian bacteriologist Nancy Atkinson founded a salmonella typing laboratory in Adelaide, South Australia—one of only three such facilities worldwide.

Clinical Spectrum: From Self-Limiting Illness to Septic Shock

Salmonella species function as intracellular pathogens, and their clinical impact varies dramatically depending on the serotype involved. Typhoidal strains are restricted to human-to-human transmission and can trigger typhoid or paratyphoid fever. In the septic form, the organism breaches into the bloodstream, disseminates to internal organs, and releases endotoxins—a progression that can culminate in life-threatening hypovolemic or septic shock demanding intensive-care management and aggressive antibiotic therapy. Nontyphoidal serotypes, by contrast, are zoonotic: they move between animals and humans and typically confine their damage to the gastrointestinal tract, producing salmonellosis that often resolves without antimicrobial intervention. Most human infections stem from ingesting food contaminated with fecal matter. However, a critical geographic exception exists in sub-Saharan Africa, where nontyphoidal Salmonella can behave invasively, triggering paratyphoid fever that necessitates immediate antibiotic treatment. This regional variation underscores that the same bacterial genus can produce radically different clinical outcomes depending on the host population and ecological context.

Laboratory Detection, Growth Kinetics, and Emerging Methods

In the laboratory, Salmonella presents both practical challenges and useful diagnostic signatures. The organisms are rod-shaped, Gram-negative rods measuring roughly 0.7 to 1.5 micrometers in diameter and 2 to 5 micrometers in length, equipped with peritrichous flagella that confer motility. Most subspecies generate hydrogen sulfide, a trait exploited in the triple sugar iron test using ferrous sulfate-containing media. Cultures often exist in two phenotypic phases—motile and non-motile—and nonmotile isolates can be coaxed into the motile state using a Craigie tube or ditch plate. RVS broth serves as an enrichment medium for clinical samples, while multiplex and real-time PCR assays offer DNA-based detection and subtyping. Newer molecular serotyping platforms such as xMAP and real-time PCR genotype the genes encoding surface antigens, potentially delivering faster results than traditional antibody-based methods. Mathematical growth-kinetic models have been constructed for chicken, pork, tomatoes, and melons, and the bacteria reproduce asexually with a division interval of approximately 40 minutes. Notably, Salmonella can persist in bathroom environments for weeks and is frequently recovered from water sources.