Salmonella

What is 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. The bacteria were named after Daniel Elmer Salmon, an American veterinary surgeon.

These tiny organisms can move with tiny hair-like structures called flagella. They get their energy from oxidation and reduction reactions using organic sources.

Certain types of Salmonella can make people sick, usually when they eat food contaminated by animal feces. Some kinds can cause serious illness that needs special care and antibiotics. Other types usually only affect the gastrointestinal tract.

Taxonomy

The genus Salmonella is part of the Enterobacteriaceae family. It has two species: S. bongori and S. enterica. The species S. enterica has six subspecies, like S. e. enterica. These bacteria are grouped into many types, called serotypes, based on certain features. This helps scientists study and treat infections caused by Salmonella.

History

Salmonella was first found in 1880 by Karl Eberth in people who were very sick. Later, scientists learned how to grow the bacteria in a lab. In 1900, the bacteria was named Salmonella after a scientist named Daniel Elmer Salmon.

In the 1930s, a scientist in Australia named Nancy Atkinson helped find new kinds of Salmonella and told others about them.

Serotyping

Serotyping is a way to identify types of Salmonella bacteria. It uses special proteins called antibodies. This helps scientists learn about the risk and trace where the bacteria came from.

For example, a study in 2014 showed that a type called S. Reading is common in young turkeys but usually does not make people sick.

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

Detection, culture, and growth conditions

Most types of Salmonella live 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 stay in many places, such as dry foods or bathroom surfaces, for a long time.

Salmonella can be found in a laboratory using special tests and tools. Scientists can study how fast the bacteria grow under different conditions. Freezing does not kill Salmonella, but heat does. Cooking food to an internal temperature of 75 °C (167 °F) helps stop Salmonella infections.

Nomenclature

See also: Salmonella enterica and Salmonella enterica subsp. enterica

Salmonella bacteria used to have different names based on the diseases they caused or the animals they were found in. Later, scientists named them after the place they were discovered.

In 1987, scientists found that all these names were actually for one type of bacteria, called Salmonella enterica. They kept the old names as "serotypes" to tell them apart. Today, Salmonella enterica has six subspecies. There are many serotypes, like Salmonella Typhimurium, which help scientists study these bacteria. Modern tools, such as DNA testing, are now used to learn more about these tiny organisms.

Evolution

Analysis of Salmonella enterica shows that ancient mixing of genetic material has occurred, making it hard to trace their full history. 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 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 waste. 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. These bacteria can make people very sick. They can spread from the intestines into the blood and then to other parts of the body. This can cause fever, vomiting, and diarrhea. In very bad cases, it can make a person very weak and cause other health problems.

Nontyphoidal Salmonella

See also: Salmonellosis

Infection with nontyphoidal types of Salmonella can cause gastroenteritis, often called food poisoning. People get it by eating food that has the bacteria in it. Infants, young children, and older adults can get sick even from small amounts of the bacteria. The bacteria enter the body through the digestive system and can make the intestines inflamed, leading to diarrhea.

Many types of nontyphoidal Salmonella exist, and they can cause illnesses. Some groups, like very young children, pregnant women, and those with weaker immune systems, are more likely to get sick. Foods such as chicken, pork, vegetables, sprouts, and processed foods can sometimes have the bacteria. Groups like the FDA, United States Department of Agriculture, and the Food Safety and Inspection Service help keep food safe by checking it and giving safety rules.

Molecular mechanisms of infection

Salmonella bacteria have different ways of making people sick. To start an infection, they need to get past the wall of the intestine. Once through, they use special tricks to make you feel unwell.

When Salmonella moves through the digestive system, it faces challenges like stomach acid and immune cells. It makes special proteins that help it survive and cause infection. These proteins help the bacteria stick to and enter intestinal cells, where they can grow. Some types of Salmonella can also move to other parts of the body through the blood. Two important systems help Salmonella put harmful proteins into your cells, letting it avoid your immune system and cause illness. These systems also help the bacteria create swelling and diarrhea.

Resistance to oxidative burst

Salmonella bacteria can live inside special cells of the immune system called phagocytes. These cells try to stop infections by making harmful substances like nitric oxide and oxygen radicals. To stay alive, Salmonella must protect its DNA from damage. Research shows that parts of Salmonella’s DNA repair system help it survive and cause infections.

Host adaptation

Salmonella enterica has different types, called serotypes, that can infect many 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 some genetic material and by changing its genes (mutation).

In more complex animals, the immune system tries to fight off Salmonella. Some Salmonella strains have lost the genes needed to make certain structures, helping them avoid the immune system. 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, often causes foodborne illnesses from produce like tomatoes. This strain has a special gene, papA, that helps it survive on tomatoes.

Research

Salmonella bacteria, especially a type called S. enterica serovar Typhimurium, are important for scientific studies. They help scientists learn about how bacteria can make people sick, but they are safer to study than more dangerous kinds. These bacteria have been used in mice to understand how infections affect the intestines.

Scientists have used Salmonella Typhimurium to create tools for studying genes. This helped them discover a virus that can change bacterial genes easily. These tools have helped us understand how bacteria live and grow. They also led to a simple test to see if substances can cause cancer. Today, scientists are studying ways to use 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 around a long time ago. They also believe Salmonella enterica may have caused an epidemic called cocoliztli in the 16th-century New Spain.