What is Salmonella?

• There are two generally accepted species of the bacterium Salmonella: Salmonella enterica and Salmonella bongori.
• The species we hear most about is Salmonella enterica, which is estimated by the World Health Organization (WHO) to affect over 17 million people worldwide each year.
• Salmonella Typhimurium and Salmonella Typhi are two of over 2,500 types of Salmonella enterica.
• Salmonella Typhimurium and Salmonella Typhi are closely related. However, there are distinct differences in the type and severity of infectious disease they both cause.
• This difference in infections is the result of differences in their genetic makeup.

Salmonella growing on XLD agar.

Image credit: Nathan Reading on Flickr [CC-BY-2.0], via Wikimedia Commons.

Salmonella Typhimurium

• Salmonella Typhimurium can cause infections in humans and animals.
• It is often associated with animals and animal products that are eaten.
• Salmonella Typhimurium can be transferred to humans through raw or undercooked infected food including meat and eggs.
• In poultry, Salmonella Typhimurium is passed from bird to bird, most commonly through their droppings.
• When the poultry are slaughtered for their meat, the Salmonella bacteria are transferred from their gut to the meat, and can also be passed from the bird’s ovaries to their eggs.
• Salmonella Typhimurium causes gastroenteritis (inflammation of the gut) leading to diarrhoea, vomiting, fever, and abdominal cramps. These symptoms may last up to seven days.
• People infected by Salmonella Typhimurium rarely require antibiotics or hospitalisation. However, antibiotics or other treatments may be needed if an individual is unable to fight off the infection themselves due to an underlying condition, such as a compromised immune system.

Salmonella Typhi

• Salmonella Typhi can only infect humans.
• It causes typhoid fever.
• It is common in developing countries where hygiene is poor and the water can be contaminated with sewage.
• The symptoms of typhoid fever include fever, weakness, stomach pains, headache or loss of appetite.
• Without prompt treatment, Salmonella Typhi infection can lead to liver damage, inflammation of the heart, holes in the gut, and internal bleeding.
• If appropriate treatment is not given, typhoid fever is fatal in up to 20 per cent of affected individuals.
• Typhoid is diagnosed by way of a blood or stool sample.
• People with typhoid fever carry the Salmonella Typhi bacteria in their blood and intestines and shed the bacteria in their faeces.
• For individuals travelling to a country where typhoid is common, a vaccination is recommended.

Studying the genetics of Salmonella

• By looking at the DNA of Salmonella bacteria scientists can see what genes are present in different strains to understand how they cause disease and how they could be treated.
• It can also show us how the bacterium’s genome has changed over time to enable it to be resistant to common antibiotics.
• Scientists here at the Sanger Institute have sequenced the Salmonella genome to examine the genes found in different strains of Salmonella and work out how they cause disease.
• Doing this has allowed scientists to identify unique genes found in the individual types.
• For example, through DNA sequencing we know that Salmonella Typhimurium has 479 unique genes and Salmonella Typhi has 609 unique genes.
• Using next-generation sequencing, scientists are able to identify single base (A, C, G and T) changes in the DNA of the bacteria and explore more closely their ability to adapt and exploit their host.

Antiobiotic resistance in Salmonella

• The common use of a limited range of antibiotics has led to the emergence of strains of Salmonella that are resistant to antibiotics.
• The use of antibiotics, starting with penicillin, created a selective pressure on bacteria.
• Researchers believe that as a result bacteria have evolved increasing resistance to the various types of antibiotic.
• The first antibiotic-resistant bacteria were isolated in 1947, just four years after penicillin went into mass-production.
• Genome sequencing at the Sanger Institute has revealed that several species of Salmonella enterica have acquired antibiotic resistance genes over time but have also lost some gene function.
• This suggests that they are adapting specifically to survive within human hosts.