Salmonella is part of a large family of bacteria (Enterobacteriaceae) which live inside the human gut, as well as the guts of various other species. More than 2,500 types of salmonella have been identified. All could cause disease in humans, although in practice only a small proportion are responsible for most outbreaks of the disease.
Salmonella is generally divided into two groups- those which specifically target humans, and tend to have the most severe symptoms, and those which target species less specifically, are associated with food poisoning and tend to be milder.
Salmonella typhi alone is responsible for 27 million cases of typhoid a year, and 217,000 annual deaths.
Typhoidal Salmonella will enter the blood stream and into circulation, where it can reach the lymph nodes, gall bladder, liver, spleen and many other parts of the body, where it can enter into human cells. Symptoms start within a few weeks of ingestion, and get progressively worse throughout infection as the greater numbers of bacteria over stimulate the victim’s immune system. The actions of the immune system can result in tissue damage and even death.
Non typhoidal Salmonella will enter into the gut’s epithelial cells (The gut lining) and the immune system is quickly alerted, leading to gut inflammation. This typically results in:
- Shedding of gut cells
- Substantial fluid loss
- Abdominal pains/ cramps
Symptoms of non-typhoidal varients typically start 6-24 hours after infection and last for 4-7 days, before quickly clearing.
Salmonella is typically contracted by ingesting something contaminated by faeces from another infected individual, or carrier. The most common source is polluted water, so Salmonella is most common in areas with poor, or no, water sanitation.
Contact with cattle and chickens are also typical methods of contraction. Bacteria often can be found in raw meat or other products from these animals, as well as their faeces. Pets can also pass the infection, especially amphibians and reptiles.
Some people remain carriers long after their own infection is over.
Occasionally, non typhoidal Salmonella will enter the victim’s blood stream and can cause bacteraemia (when bacteria multiply in the bloodstream), which can be fatal, and in those with weaker immune systems form typhoid-like symptoms. It is estimated that there are 1.3 x10^9 cases and 3 million deaths of non-typhoidal Salmonella each year.
Salmonella move to the gut shortly after being ingested; they are then able to out-compete the bacteria which naturally occur in the gut for nutrients and will start to attack the gut cells.
Salmonella produces tiny syringe-like structures called Type 3 Secretion Systems (T3SSs), which inject proteins from the bacteria into victim’s cells. In the intestines, these T3SS1 proteins will cause the cell membrane to form lumps and eventually absorb the bacterium responsible, and it becomes trapped in a membrane within the cell. T3SS2 is used by bacteria already inside human cells, and produces virulence factors- a type of chemical which allows infection to be ignored- to avoid being killed by the human immune system. Essentially, our cells absorb the bacteria in the hopes of being able to kill them before infection occurs, but the bacteria stop this from occuring once inside; instead of killing them, the pathogens enter into a protected environment.
Proper cooking of any meats and eggs should kill off bacteria housed within our food. Similarly, boiling water before drinking it should prevent any bacteria surviving to be ingested.
The best method to avoid Salmonella food poisoning is just to regularly wach your hands, especially when handling food.
If a mild form of the disease is encountered, generally not much interference is needed. The human immune system is generally pretty effective, and naturally will come up with methods to fight it over time.
In more severe forms, antibiotics are typically used to halt the increase in bacterial presence, and reduce the severity of symptoms. Patients may need rehydration therapy and antibiotics if there are signs that the disease has entered the blood.
Salmonella typhi supposedly has a sugar coating around itself, which stops it being recognised by the immune system, leading to food poisoning. By not getting an immune response, S. typhi can spread through our bodies fairly freely.
There are no vaccines to protect against non-typhoidal Salmonella. Two vaccines are available in the UK against typhoid, and use of them is rrecommended before travelling to anywhere with high typhoid levels. These vaccines only offer limited protection, however. Improved vaccines will be needed in the future.
It may be possible to prevent infection by Salmonella by developing drugs which target the virulence factors that stop the immune system detecting them.