Know Your Enemy: Salmonella

Salmonella is one of the most common causes of foodborne illness.

It is estimated that 1.4 million cases of Salmonella infection occur each year in the United States alone with a cost of $3 billion (US dollars)(1).

Although Salmonella can survive on virtually any food or water source, humans typically contract infection from contaminated food of animal origin (meat, eggs and dairy).

There are over 2500 different Salmonella types with Typhimurium and Enteriditis representing the most common culprits of food poisoning. Salmonella Enteriditis contributes to a large percent of human cases of infection due to its ability to infect hen ovarian tissue. As a result, chicken eggs become contaminated with the pathogen. Grade A eggs are associated with 77 to 82% of human cases of Salmonella (2).
As such, reducing the occurrence of Salmonella in poultry flocks is directly linked to a decreased risk for human illness. Interestingly, studies have shown that the risk for human illness is strongly influenced by animal product storage time and temperature rather than the number of bacteria initially found in the product (2).
Unlike its cousin bacteria E. coli that lives on the periphery of host cells, Salmonella is able to invade and survive inside these cells. The walls of our intestinal tract are lined with a specialized type of cell called an epithelial cell. Salmonella assemble large needle-like machines that they use to inject their own proteins directly into epithelial cells. These proteins ultimately function to induce the epithelial cell to engulf the bacteria. Once inside the cell, survival becomes a balancing act, weighing the strength of the host immune defences against bacterial counteracting strategies (3).
Symptoms of disease including fever, vomiting, diarrhoea, nausea and abdominal pain usually begin 12 to 72 hours after infection. The infectious dose (number of bacteria required to cause disease) for Salmonella is between 1,000 and 100,000. Symptoms typically last only 4 days (1).
The symptoms we associate with food poisoning are often caused by our own immune system. Vomiting and diarrhoea are our body’s way of trying to literally flush out the bacteria. This strategy is a double-edged sword since release of the bacteria back into the environment can be advantageous as it allows them to infect new hosts. Salmonella can induce diarrhoea by disrupting the junctions that hold epithelial cells together. This disrupts the epithelial cell layer leading to uncontrolled passage of water into the intestinal track (3). Our body also induces inflammation to help reduce the spread of infection and recruit cells of our immune system to kill the pathogen. Although less common in developed nations, some Salmonella types can even survive and grow inside these recruited immune cells, allowing for their passage through the blood stream and growth in organs such as the spleen and liver. This systemic infection, more commonly known as Typhoid fever, can be fatal if left untreated.
Healthy adults are typically capable of overcoming a Salmonella infection without the need for medical treatment. Children, elderly and immuno-compromised patients are more likely to become seriously ill. Adults requiring drug treatment typically receive a type of antibiotic called fluoroquinolones. These drugs are not generally recommended for children who instead receive cephalosporin antibiotics. The ability to treat Salmonella infections has become increasingly difficult owing to the emergence of resistance to these commonly used antibiotics.
References
1)  World Health Organization. 2011. http://www.who.int/topics/salmonella/en/
2) World Health Organization. Food and Agriculture Organization of the United Nations. Risk Assessments of Salmonella in eggs and broiler chickens. 2002. Microbiological Risk Assessment Series 1.
3) Haraga, A., and Miller, S.. Salmonellae interplay with host cells. Nature Reviews Microbiology. 2008. 6:53.
Suzanne E. Osborne is funded by the Canadian Institute for Health Research (CIHR) and is a recipient of the prestigious Canadian Vanier Scholarship working at the Institute for Infectious Disease Research, Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Canada.
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