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What are Salmonella?
Salmonella are bacteria. The Salmonella consist of a range of very closely related bacteria, many of which cause disease in humans and animals.
What does their name mean?
There is a great deal of confusion over the naming of Salmonella strains (even the people who work on Salmonella are confused!) but in essence, the strains which we will deal with here are generally different serovars of Salmonella enterica.
This means that they all belong to the genus Salmonella, a division that groups similar, though not identical bacteria together. These bacteria are named after the scientist who discovered them, Dr. Daniel Salmon. The majority of the components of these bacteria are identical, and at the DNA level, they are between 95% and 99% identical. (As a comparison E. coli and Salmonella, which are closely related to each other, are about 60-70% identical at the DNA level).
As their name suggests Salmonella enterica are involved in causing diseases of the intestines (enteric means pertaining to the intestine). The three main serovars of Salmonella enterica are Typhimurium, Enteritidis, and Typhi. Each of these is discussed further below. These distinctions are are designed to help scientists distinguish similar bacteria from each other in papers and when discussing the genetics.
To complicate matters, serovars of Salmonella enterica can be subgrouped even further by "phage type". This technique uses the specificity of phage to differentiate between extremely closely related bacteria. Often these bacteria are indistinguishable by other means, and indeed, the reasons for the differences in phage specificity are often not known.
This bacterium is the causative agent of typhoid fever. Although typhoid fever is not widespread in the United States, it is very common in under-developed countries, and causes a serious, often fatal disease. The symptoms of typhoid fever include nausea, vomiting, fever and death. Unlike the other Salmonella discussed below, S. Typhi can only infect humans, and no other host has been identified. The main source of S. Typhi infection is from swallowing infected water. Food may also be contaminated with S. Typhi, if it is washed or irrigated with contaminated water.
Until recently the most common cause of food poisoning by Salmonella species was due to S. Typhimurium. As its name suggests, it causes a typhoid-like disease in mice. In humans S. Typhimurium does not cause as severe disease as S. Typhi, and is not normally fatal. The disease is characterized by diarrhea, abdominal cramps, vomiting and nausea, and generally lasts up to 7 days. Unfortunately, in immunocompromized people, that is the elderly, young, or people with depressed immune systems, Salmonella infections are often fatal if they are not treated with antibiotics.
In the last 20 years or so, S. Enteritidis has become the single most common cause of food poisoning in the United States. S. Enteritidis causes a disease almost identical to the very closely related S. Typhimurium. S. Enteritidis is particularly adept at infecting chicken flocks without causing visible disease, and spreading from hen to hen rapidly. Many people have blamed the recent increase in the rise of S. Enteritidis infections on the use of mass production chicken farms. When tens or hundreds of thousands of chickens live together, die together, and are processed together a Salmonella infection can rapidly spread throughout the whole food chain. A compounding factor is that chickens from a single farm may be distributed over many cities, and even states, and hence Salmonella infections can be rapidly dispersed through millions of people.
How does Salmonella cause disease?
After Salmonella is eaten it passes through the stomach to the intestine. Here, it binds to the wall of the intestine, and through some special proteins that it makes in response to the particular conditions in the intestine it actually penetrates the barrier between us and the outside. Once it has gained access to our insides, it is taken to the liver or spleen. For most other bacteria, this journey would kill them, however Salmonella has evolved mechanisms to prevent our immune system from doing its job efficiently. In the liver, the Salmonella can grow again, and be released back into the intestine.
Of course, not all of the Salmonella pass through the intestinal wall, and many of them are expelled from the intestine in the diarrhea. In regions with poor sanitation, these bacteria can than survive in the soil or in rivers and infect the next person, cow, chicken or mouse that comes along.
Well, you can try the Salmonella Genetic Stock Center.
Most infections with Salmonella are traced back to dairy, poultry and meat products, but Salmonella can grow on just about any food. Chickens and eggs are particular high risk foods.
Other precautions as suggested by the USDA include:
These sites have more information: