Salmonella and Shigella Differences, Causes, Disease Symptoms

What is Salmonella?

Salmonella are a group of bacteria that can be divided into typhoid Salmonella (Salmonella typhi) which causes typhoid fever and non-typhoid Salmonella. The latter is commonly known for causing salmenollosis which is a type of foodborne intestinal infection contracted after eating food contaminated with the Salmonella bacteria. These types of intestinal infections are more likely in children or the elderly.

What is Shigella?

Shigella are a family of bacteria that cause an infectious intestinal disease known as shigellosis. It is mainly transmitted through contact with an infected person and contaminated food and water. Shigellosis can occur in any age group but is more commonly seen in children. It is one of the common causes of outbreaks of bacillary dysentry.

Differences and Similarities

  • Salmonella and Shigella are different groups of Gram-negative bacteria.
  • There are about 2,200 serotypes of Salmonella bacteria compared to about 40 serotypes of Shigella bacteria.
  • Both Salmonella and Shigella can be transmitted through contaminated food and water, personal contact or even from animals and objects (fomites).
  • The most common way Shigella is transmitted is through direct person-to-person contact whereas ingesting contaminated raw food is the most common route of transmission of Salmonella.
  • Salmonella infection requires a larger infective dose than for Shigella infection. This means that more bacterial cells need to be ingested for salmonellosis than for shigellosis.
  • Salmonellosis and shigellosis are more likely to occur in children under 5 years of age and the elderly.
  • Salmonella infection affects the small and large intestine (enterocolitis) where as Shigella infection affects the colon (colitis).
  • Diarrhea, abdominal pain and fever are the main symptoms in both diseases.
  • Bloody and mucoid diarrhea is more likely in shigellosis than salmonellosis.
  • The symptoms of both diseases resolve within 7 days or less in most people.
  • Death in both diseases are uncommon but is more likely to occur in children with shigellosis.



The most common type of Salmonella bacteria responsible for salmonellosis are Salmonella enteritidis and Salmonella typhimurium. These bacteria tend to live and thrive in the intestines of mammals. Common ways in which fecal matter containing the bacteria is ingested includes :

  • Raw meat and poultry that is contaminated during the slaughtering process.
  • Raw seafood that are harvested from contaminated water.
  • Raw eggs that are infected before the shell forms.
  • Contaminated fruit, vegetables and milk.
  • Contact with a person who is infected or a carrier.
  • Contact with domestic animals and reptiles.

Only about 1,000,000 (one million) bacterial cells are needed to cause an infection. This is known as the infective dose and most of these bacterial cells are destroyed by the stomach acid. Only a few cells survive to pass into the small and large intestine where it causes enterocolitis. Infection can occur with as little as 1,000 (one thousand) bacterial cells in a person with one or more of the following risk factors :

  • Low stomach acid seen in the elderly, people with conditions like atrophic gastritis or using antacids or acid suppressing medication.
  • Weakened immune system in conditions such as HIV/AIDS or with the use of drugs like corticosteroids.

A person is also more likely to be exposed to an infective dose in one or more of the following situations :

  • Regular exposure to pets – mammals and reptiles.
  • International travel particularly to countries with poor sanitation and hygiene procedures.
  • Close contact with many people as in medical institutions, day care centers and crowded residential buildings.
  • Frequent use of antibiotics.
  • Inflammatory bowel disease (IBD).


Most cases of shigellosis are caused by Shigella sonnei and Shigella flexneri species which are also known as group D and group B Shigella. These bacteria are mainly found in the feces of an infected person and can be spread through the following routes :

  • Touching the hands of an infected person who does not practice good hygiene. Contact with the mouth then transfers fecal particles into the gut.
  • Mothers and nannies who touch soiled diapers of infants with shigellosis or shortly after having the infection.
  • Sexual contact with an infected person particularly with certain practices that may allow fecal particles to reach the mouth.
  • Food contaminated with fecal particles transmitted by infected food workers who do not practice proper hygiene.
  • Contaminated water which may be ingested or used to irrigate and wash fruit and vegetables.
  • Swimming or playing in contaminated water.

The infective dose of shigellosis is as low as 200 bacterial cells – the number of cells that can cause shigellosis when ingested. During the acute disease a person passes out 100,000,000 (100 million) cells per gram of stool. Therefore even a minute amount of fecal matter from an infected person is sufficient to cause shigellosis. The chances of developing shigellosis is higher in the following people and situations :

  • Children under 5 years of age, particularly within the 2 to 4 years group.
  • Facilities such as nursing homes and day care centers.
  • Poor sanitary facilities particularly in developing countries.
  • Endemic areas particularly where there is an infestation of flies and in rural communities where contaminated water may be used for irrigation.
  • Homosexual men are at greater risk due certain sexual practices.


The incubation period is typically between 1 to 3 days in salmonellosis and 2 to 4 days in shigellosis. This is the time from when the bacterial cells enter the gut to cause symptoms. The incubation can be as short as 12 hours in both diseases. Salmonella causes inflammation of the small and large intestine – enterocolitis. Shigella causes inflammation of the colon – colitis. The signs and symptoms of salmonellosis and shigellosis are largely the same and usually indistinguishable.

  • Watery diarrhea which may be bloody with mucus in shigellosis but is typically non-bloody in salmonellosis.
  • Abdominal pain is seen in both diseases and is more cramping-like in shigellosis.
  • Fever
  • Malaise

Other symptoms that may be seen particularly in salmonellosis includes :

  • Headache
  • Muscle aches
  • Nausea
  • Vomiting

Bacteremia is a serious complication where the bacteria, particularly with Salmonella infection,  enters the bloodstream and travels to different sites. Here it causes infections of other organs like the lining around the brain and spinal cord (meningitis), bones (osteomyelitis) and inner heart lining (endocarditis).


The treatment for both salmonellosis and shigellosis is largely supportive. This includes :

  • Plenty of fluidsoral rehydration solution.
  • Bed rest while signs and symptoms like fever and abdominal pain are intense.
  • Practice good hygiene so as not to infect others.
  • Eat bland but balanced meals if there is no vomiting despite the presence of diarrhea.

Salmonellosis resolves within 4 to 7 days and shigellosis within 5 to 7 days even without medication provided that a person does not have underlying diseases or weakened immune systems and are properly nourished. The following medication can be used :

  • Antibiotics to destroy the bacteria in the gut.
  • Antidiarrheal drugs can be used for salmonellosis but should be avoided in shigellosis. Use of this medication can prolong the disease.


1. Shigellosis. CDC

2. Salmonella infection. Mayo Clinic

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