Salmonella

Salmonella is a genus of rod-shaped, (bacillus) Gram-negative bacteria that belongs to the family Enterobacteriaceae. There are two known species: Salmonella enterica and Salmonella bongori. S. enterica is divided into six subspecies that include over 2,650 different serotypes. The bacteria were named after Daniel Elmer Salmon, an American veterinary surgeon.

These tiny organisms are non-spore-forming and mostly motile with tiny hair-like structures called flagella that help them move. They get their energy from oxidation and reduction reactions using organic sources and can survive with or without oxygen.

Certain types of Salmonella can cause illness such as salmonellosis, usually when people eat food contaminated by animal feces. Some kinds, called typhoidal Salmonella, can spread through the body and cause serious conditions like typhoid fever, which needs intensive care and antibiotics. Other types, called nontyphoidal Salmonella, usually affect only the gastrointestinal tract, but in some regions like sub-Saharan Africa, they can cause infections that also need treatment.

Taxonomy

The genus Salmonella belongs to the family of Enterobacteriaceae. It includes two species: S. bongori and S. enterica. The species S. enterica is divided into six subspecies, such as S. e. enterica. These bacteria are identified into many types, called serotypes, based on certain features. This helps scientists study and treat infections caused by Salmonella.

History

Salmonella was first seen in 1880 by Karl Eberth in patients with a serious illness. Later, other scientists learned how to grow the bacteria in a lab. In honor of a scientist named Daniel Elmer Salmon, the bacteria was named Salmonella in 1900.

In the 1930s, a scientist in Australia named Nancy Atkinson helped identify new types of Salmonella and shared her discoveries with the world.

Serotyping

Serotyping is a way to identify types of Salmonella bacteria by using special proteins called antibodies. This helps scientists understand the risk and find where the bacteria might have come from. For example, a study in 2014 found that a type called S. Reading is common in young turkeys but does not often make people sick.

There are newer ways to do serotyping using DNA, like xMAP and real-time PCR. These methods look at the genes instead of using antibodies and can be faster because of improvements in technology.

Detection, culture, and growth conditions

Most types of Salmonella can be found in the digestive systems of humans, animals, and even reptiles. They can spread through food, water, or by handling animals that carry the bacteria. These bacteria can survive in many places, such as dry foods or bathroom surfaces, for long periods.

Salmonella can be detected in a laboratory using special tests and tools. Scientists can also study how quickly the bacteria grow under different conditions. While freezing doesn’t kill Salmonella, heat does. Cooking food to an internal temperature of 75 °C (167 °F) helps prevent Salmonella infections.

Nomenclature

See also: Salmonella enterica and Salmonella enterica subsp. enterica

Salmonella bacteria used to be named based on the diseases they caused or the animals they were found in. Later, scientists realized that these bacteria could not be limited to one host and began naming them after the place they were discovered.

In 1987, scientists decided that all these different names actually belonged to just one species, called Salmonella enterica. They kept the old names as "serotypes" to tell them apart. Today, Salmonella enterica is known to have six subspecies. There are also many different serotypes, like Salmonella Typhimurium, which help scientists identify and study these bacteria. Modern tools, such as DNA testing, are now used to learn even more about these tiny organisms.

Evolution

Analysis of Salmonella enterica genomes shows that ancient mixing of genetic material has occurred, making it hard to trace their full evolutionary history with standard family trees. Gene transfers have been detected between different groups.

GTDB RS202 reports that S. arizonae, S. diarizonae, and S. houtenae should be classified as separate species based on genetic similarity measurements.

Non-Salmonella

In 2005, a third species named Salmonella subterranea was proposed, but it does not actually belong to the Salmonella group. In 2016, it was suggested to be renamed Atlantibacter subterranea, but this change was not officially accepted. Both GTDB and NCBI support this 2016 reassignment.

Pathogenicity

Salmonella species can invade different types of cells in the body, such as epithelial cells, M cells, macrophages, and dendritic cells. They can live inside these cells and cause infection. Most Salmonella infections happen when people eat food contaminated by animal or human feces. There are two main groups of Salmonella: typhoidal and nontyphoidal. Typhoidal types, like Salmonella Typhi, mainly affect humans, while nontyphoidal types can infect many animals and often cause temporary stomach illnesses.

Researchers have studied Salmonella a lot since the 2010s. They found that important infection genes are located in special areas of the bacteria’s DNA called Salmonella pathogenicity islands (SPIs). These help the bacteria interact with the host’s body. Other features, such as tiny structures called plasmids or flagella, and proteins that help form protective layers called biofilms, also play roles in infection.

Typhoidal Salmonella

See also: Typhoid fever and Paratyphoid fever

Typhoid fever is caused by certain types of Salmonella bacteria, such as Salmonella Typhi, that mainly affect humans and some primates. These bacteria can spread from the intestines into the bloodstream and then to other parts of the body like the liver and kidneys. This can lead to serious health problems, including fever, vomiting, and diarrhea, as the body tries to fight the infection. In severe cases, the illness can cause significant fluid loss and affect blood circulation, leading to serious health complications.

Nontyphoidal Salmonella

See also: Salmonellosis

Infection with nontyphoidal types of Salmonella usually causes gastroenteritis, often called food poisoning. People get it by eating food contaminated with the bacteria. Infants, young children, and older adults are more likely to get sick from smaller amounts of the bacteria. The bacteria enter the body through the digestive system and cause inflammation in the intestines, leading to diarrhea.

About 2,000 types of nontyphoidal Salmonella exist and they cause many illnesses each year. Certain groups, like very young children, pregnant women, and those with weaker immune systems, are at higher risk. Foods like chicken, pork, vegetables, sprouts, and processed foods can sometimes carry the bacteria. Organizations such as the FDA, United States Department of Agriculture, and the Food Safety and Inspection Service work to prevent infections through inspections and safety guidelines.

Molecular mechanisms of infection

Salmonella bacteria have different ways of causing infection depending on the type. To start an infection, they need to get past the wall of the intestine. Once through, they use various strategies to make you sick.

When Salmonella travels through the digestive system, it faces many challenges like stomach acid and immune cells. It responds by producing special proteins that help it survive and cause infection. These proteins help the bacteria attach to and enter intestinal cells, where they can multiply. Some types of Salmonella can also spread to other parts of the body through the blood. Two important systems help Salmonella inject harmful proteins into your cells, allowing it to avoid your immune system and cause illness. These systems also help the bacteria cause inflammation and diarrhea.

Resistance to oxidative burst

Salmonella bacteria have a special ability to survive inside certain cells of the immune system called phagocytes. These cells try to fight off infections by producing harmful substances like nitric oxide and oxygen radicals. To survive, Salmonella must resist damage to its DNA. Studies have shown that certain parts of Salmonella’s DNA repair system are very important for the bacteria to handle this damage and cause infections.

Host adaptation

Salmonella enterica has different types, called serotypes, that can infect various animals. Some serotypes, like Typhimurium, can infect many kinds of mammals, while others, like Typhi, can only infect a few hosts. Salmonella adapts to its hosts in two main ways: by losing certain genetic material and by changing its genes (mutation).

In more complex animals, the immune system tries to fight off Salmonella by targeting structures on the bacteria. Some Salmonella strains have lost the genes needed to make these structures, helping them avoid the immune system. Studies show that more harmful serotypes of Salmonella share certain features because they evolved in similar ways to fight host defenses. Scientists think Salmonella may have evolved by gaining genes from other bacteria and by forming new types through special genetic regions called pathogenicity islands.

One serotype, Salmonella sv. Newport, seems to prefer living on plants and is often linked to foodborne illnesses from produce like tomatoes. This strain has a special gene, papA, that helps it survive on tomatoes and is found in other bacteria that live on both plants and animals.

Research

Salmonella bacteria, especially a type called S. enterica serovar Typhimurium, are important for scientific studies. They help researchers learn about how bacteria cause diseases without the risks of studying more dangerous strains. These bacteria have been used in mice to understand how infections affect the intestines.

Scientists have also used Salmonella Typhimurium to develop tools for studying genes. This includes discovering a virus that helps change bacterial genes easily. These tools have helped us understand how bacteria live and grow, and they even led to a simple test to check if substances can cause cancer. Today, scientists are exploring using viruses called phages to control Salmonella and other harmful bacteria in food.

Ancient DNA

Scientists have studied very old DNA and found evidence of Salmonella enterica infections in people who lived up to 6,500 years ago across Western Eurasia. This shows that these bacteria were widespread long ago. They also believe Salmonella enterica may have caused an epidemic called cocoliztli in 16th-century New Spain.