For most people, a Salmonella infection causes gastroenteritis, an inflammation of the stomach and intestines that can mean several miserable days of diarrhoea, cramps, fever or vomiting. Most people recover within a week. But severe diarrhoea and dehydration can still be dangerous, even while the bacteria remain confined to the gut
In a small minority of patients, however, Salmonella crosses the gut lining and enters the blood. In the current UK outbreak, seemingly caused by eggs imported from abroad,
two people have developed bloodstream infections and one death has been associated with the outbreak. How does a familiar cause of food poisoning escape the gut, and why does this only happen to some people?
Egg consumption is investigators’ strongest lead, although theat food businesses supplied by severalle opportunities for cross-contamination, in which bacteria are transferred between foods or surfaces, at two premises. They have found no link to UK-produced eggs or poultry
Salmonella Enteritidis belongs to a broad group called non-typhoidal Salmonella, which is distinct from the bacteria that cause typhoid and paratyphoid fever. Doctors describe an infection as invasive when Salmonella reaches the blood or another part of the body that is normally free of bacteria. A bloodstream infection can lead to sepsis, a life-threatening reaction in which the immune system begins damaging the body’s organs and tissues
How Salmonella escapes the gut
Genes, which are sections of DNA containing biological instructions, allow Salmonella to build tiny injection systems that work like molecular syringes. One injects proteins into cells lining the gut, causing them to engulf the bacterium. These injection systems are found in many Salmonella bacteria, so their presence cannot explain why one patient develops a bloodstream infection while another has relatively mild diarrhoea. In most people, several layers of immune defence contain the infection in the gut.
A second helps Salmonella alter the membrane-wrapped compartment that forms around it after an immune cell swallows it, allowing some bacteria to survive and multiply. Macrophages are immune cells that engulf and usually destroy microbes. Some Salmonella can survive inside them. This sheltered position is one way the bacteria may move into deeper tissues and the bloodstream, although other routes may contribute
Why some people become more seriously ill
Scientists have identified more than 2,600 types of Salmonella, known as serovars, although fewer than 100 cause most human illness. A strain is a more closely related genetic branch within one of these types. Both types and strains vary in their tendency to spread
Nontyphoidal Salmonella that can invade blood are known as invasive serovars and the most predominant serovars associated with blood infections are Typhimurium,Enteritidis, Choleraesuis and Dublin
Records from routine testing in England illustrate the difference. Among more than 116,000 bacterial samples collected between 2004 and 2015, about 2% of Enteritidis samples came from blood, compared with 35% of Salmonella Dublin samples. Testing patterns affect the figures, but the difference indicates that some types spread beyond the gut much more readily than others
Strains within the same type can also behave differently. Laboratory experiments on two Enteritidis strains, conducted using mouse immune cells, identified genes that help the bacteria survive and multiply inside macrophages. Such experiments identify mechanisms for further study but cannot predict how ill a patient will become
Age and health also shape the outcome. In the English study, Salmonella was found in blood in 6.5% of samples from adults over 65, compared with 1.9% from adults aged 18 to 64. Newborn babies also had an elevated risk. Weakened immune systems and certain long-term health conditions can make an infection harder to contain
The number of bacteria swallowed affects the likelihood of becoming ill. Outbreak-based models suggest that heavier contamination increases the risk, although researchers do not know how strongly the amount swallowed affects the likelihood of an infection reaching the blood
What the current outbreak can tell us
The hospital admission figure deserves attention, but it does not establish that this strain has an unusual ability to cause severe disease. The 38% applies only to patients for whom information was available. Official counts also miss many people whose illness clears before testing, so those with more serious symptoms can be overrepresented. Scientists commenting on the outbreak have urged caution when interpreting the figure
The UK Health Security Agency used whole-genome sequencing, which involves reading and comparing all or nearly all of the bacteria’s genetic code. Bacteria recovered from patients were very closely related and genetically linked to a 2025 outbreak associated with imported eggs. Sequencing can connect cases and help investigators trace their is to cause severe illness
The Food Standards Agency says pregnant women, babies, young children and older people should eat raw or runny hen eggs only if they carry the British Lion mark or come from the Laid in Britain scheme. These schemes require safety measures including vaccinating hens and additional testing
Anyone who keeps being sick and cannot retain fluids, has bloody diarrhoea or continues to show signs of dehydration after using rehydration sachets should call NHS 111. The NHS provides further advice on when to seek urgent help
Investigators will need to combine the bacteria’s genetic code with information about the patients’ health, what and where they ate, and laboratory evidence. Only then can they begin to explain why the infection spread beyond the gut in two patients while most cases caused gastrointestinal illness
- Food poisoning
- Salmonella
- Salmonella outbreak
