Scientists describe how salmonella bacteria spread in humans (w/ Video)

Sep 30, 2010

New findings by National Institutes of Health scientists could explain how Salmonella bacteria, a common cause of food poisoning, efficiently spread in people. In a study published this week in the Proceedings of the National Academy of Sciences, researchers describe finding a reservoir of rapidly replicating Salmonella inside epithelial cells. These bacteria are primed to infect other cells and are pushed from the epithelial layer by a new mechanism that frees the Salmonella to infect other cells or be shed into the intestine.

The Centers for Disease Control and Prevention estimate that infections sicken 40,000 people each year in the United States, though the actual number of infections is likely much higher because many cases are mild and not diagnosed or reported. Currently, Salmonella is the focus of an ongoing U.S. public health investigation into contaminated .

"Unfortunately, far too many people have experienced the debilitating effects of Salmonella, which cause disease via largely unexplained processes, including overactive inflammatory responses," says Anthony S. Fauci, M.D., director of NIH's National Institute of Allergy and Infectious Diseases (NIAID). "This elegant study provides new insight into the origins of that inflammatory disease process."

While much is known about the human infectious cycle of Salmonella, scientists have yet to understand how the bacteria escape the gut to spread infection. line the outer and inner surfaces of the body, such as the skin and gut, and form a continuous protective tissue against infection. But Salmonella have learned how to live inside epithelial cells and use them for their benefit. Salmonella protect themselves within special membrane-bound compartments, called vacuoles, inside gut epithelial cells.

This video is not supported by your browser at this time.
See a time-lapse series showing hyper-replication of Salmonella bacteria (red) in epithelial cells from two to seven hours after infection. Credit: NIAID

Using special high-resolution microscopes to view laboratory-grown human intestinal epithelial cells and laboratory mice infected with Salmonella, an NIAID research group led by Olivia Steele-Mortimer, Ph.D., in collaboration with Bruce Vallance, Ph.D., of the University of British Columbia in Vancouver, discovered a secondary population of Salmonella not confined within a vacuole, but instead moving freely inside the epithelial cells. This reservoir of Salmonella is distinct from vacuolar Salmonella. The bacteria multiply much faster; they have long tail-like projections, called flagella, used to move; and they exhibit a needle complex they use to pierce cells and inject their proteins. With these attributes, this population of Salmonella is genetically programmed to invade new cells.

The scientists observed that epithelial cells containing the hyper-replicating, invasive Salmonella are eventually pushed out of the intestinal tissue into the gut cavity, setting the Salmonella free. The mechanism used to push these Salmonella-infected cells into the body cavity resembles the natural mechanism humans use to shed dying or dead epithelial cells from their gut. The scientists believe that Salmonella have hijacked this mechanism to facilitate their own escape.

The human immune system, however, also senses that these are not normal, dying cells in the gut and triggers a response that includes release of interleukin-18, a small protein that sets off an inflammation cascade. Interleukin-18 also is prominent in chronic intestinal inflammation associated with autoimmune disorders, such as inflammatory bowel disease. The effects of interleukin-18 release provide an explanation for the acute intestinal inflammation associated with Salmonella infections.

The scientists hope their research leads to a treatment that prevents the spread of infection. They are focusing on how this specialized population of Salmonella escapes from its membrane-bound compartment to multiply and swim freely in the cell.

Explore further: Technology translation engine launches 'Organs-on-Chips' company

More information: L Knodler et al. Dissemination of invasive Salmonella via bacterial-induced extrusion of mucosal epithelia. Proceedings of the National Academy of Sciences DOI:10.1073/pnas.1006098107 (2010).

Provided by National Institutes of Health

5 /5 (3 votes)

Related Stories

Salmonella in garden birds responsive to antibiotics

Jun 02, 2008

Scientists at the University of Liverpool have found that Salmonella bacteria found in garden birds are sensitive to antibiotics, suggesting that the infection is unlike the bacteria found in livestock and humans.

Invasion without a stir

Dec 17, 2009

Bacteria of the genus Salmonella cause most food-borne illnesses. The bacteria attach to cells of the intestinal wall and induce their own ingestion by cells of the intestinal epithelium. Up till now, researchers assumed ...

Safe clearance of salmonella

Sep 14, 2010

Individuals with an intact complex gut flora are more likely to clear Salmonella after an infection than individuals with an altered, less complex gut flora. This is suggested by results from a mouse model ...

Snakes poisoned at birth

Feb 23, 2006

Scientists in Germany have found that a significant route of transmission of Salmonella in non egg-laying snakes is from the mother to the offspring during pregnancy and birth.

Iron regulates the TLR4 inflammatory signaling pathway

Oct 05, 2009

Iron is a micronutrient essential to the survival of both humans and disease-causing microbes. Changes in iron levels therefore affect the severity of infectious diseases. For example, individuals with mutations in their ...

Recommended for you

Stem cells from nerves form teeth

1 hour ago

Researchers at Karolinska Institutet in Sweden have discovered that stem cells inside the soft tissues of the tooth come from an unexpected source, namely nerves. These findings are now being published in the journal Nature and co ...

Human brain has coping mechanism for dehydration

15 hours ago

(HealthDay)—Although dehydration significantly reduces blood flow to the brain, researchers in England have found that the brain compensates by increasing the amount of oxygen it extracts from the blood. ...

User comments : 0