by Nicole Evans M.D.

Created on: June 04, 2010   Last Updated: July 31, 2010

Bloody diarrhea can be caused by parasites, chemical ingestion, viruses, autoimmune diseases and bacteria.  Salmonella and Shigella are two types of bacteria that commonly cause bloody diarrhea in humans. 

Though both Shigella and Salmonella cause infectious bloody diarrhea, only a few similarities exist between these two bacteria.  For example, both Salmonella and Shigella cause diarrhea by invading the mucosal lining of the intestines.  Both bacteria are also considered to be non-lactose fermenters and neither Salmonella nor Shigella has the oxidase enzyme, facts that are important in identifying these bacteria on laboratory tests.

Infection with Salmonell or Shigella causes symptoms of cramping, abdominal pain and diarrhea, which is often bloody and contains mucous.  The symptoms of Salmonella infection are unfortunately only prolonged by treatment with antibiotics.  Shigella infection is also usually not treated with antibiotics unless the symptoms are severe or the patient is very young or old.

There are many differences between Salmonella and Shigella.  Salmonella is propelled in one’s body by tail-like flagella, while Shigella bacteria have no flagella but instead use actin polymerization to move.  Despite the lack of the flagella, Shigella is considerably more virulent than Salmonella.  Exposure to at least 100,000 Salmonella bacteria is usually required before a person becomes infected, while an infection with Shigella occurs after exposure to only 10 of these bacteria. 

While Salmonella tends to induce a monocytic immune response in infected hosts, Shigella actually induces most of it’s cellular response by producing a toxin called the Shiga toxin.  This toxin causes cellular destruction by cleaving the ribosomal RNA of the host cells.  The Shiga toxin can cause hemolytic uremic syndrome, or HUS, particularly in children.

Salmonella often infects humans through meat consumption because animals are a reservoir for Salmonella outside of the human environment.  Unlike Salmonella, Shigella does not have an animal reservoir.  Shigella is transmitted to people via the fecal-oral route, in other words people become infected with Shigella when they unknowingly consume small amounts of contaminated feces.

Unlike the majority of gastroenteritis caused by Shigella and Salmonella infection which are left untreated, one specific species of Salmonella causes an illness that is treated with antibiotics.  This illness is Typhoid Fever and is caused by an infection with Salmonella typhi.  The bacteria invades the intestinal mucosa and then makes it’s way in to the regional lymph nodes.

People with Typhoid Fever develop rose spots on their abdomen, fever, diarrhea and headache.  Infection with Salmonella typhi is treated with antibiotics because if left untreated this bacteria can live in the gallbladder chronically.  Common antibiotics used for Typhoid Fever include ciprofloxacin (Cipro) or ceftriaxone.

Salmonella and Shigella are two common bacterial causes of infectious bloody diarrhea.  Individuals with symptoms of Salmonella infection or Shigella infection (often called Shigellosis) should consult their health care provider for further guidance.

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