The Pathobyte Series- Plesiomonas shigelloides: An Emerging Pathogen

Jul 15, 2026

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6 min read

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What is Plesiomonas shigelloides?

Plesiomonas shigelloides is a naturally occurring bacterium found in freshwater environments and aquatic animals. It belongs to the family Enterobacteriaceae and is unique because it is the only species currently recognized within the genus Plesiomonas. Although it is not among the most common causes of foodborne illness, it has been increasingly recognized as an important cause of diarrheal disease worldwide, particularly in tropical and subtropical regions.

Human infections are usually associated with exposure to contaminated freshwater, untreated drinking water, raw or undercooked seafood, or foods that come into contact with contaminated water. Most illnesses affect the digestive tract, but the bacterium can occasionally invade other parts of the body in people with weakened immune systems.

**What is the history behind the name Plesiomonas shigelloides?**

Plesiomonas shigelloides was first identified in the early twentieth century, but its classification has changed significantly as microbiology has advanced.

The organism was initially mistaken for members of the genus Shigella because of similarities in certain laboratory reactions. This resemblance led to the species name shigelloides, meaning "similar to Shigella." Later studies demonstrated that the bacterium is genetically and biologically distinct, resulting in its placement within the separate genus Plesiomonas.

The genus name comes from Greek words meaning "near" or "close to" and "unit," reflecting its early perceived relationship with other Gram-negative bacteria. Modern molecular techniques, including DNA sequencing, have clarified its evolutionary position within the order Enterobacterales.

Advances in molecular diagnostics have also improved recognition of P. shigelloides as an emerging waterborne and foodborne pathogen. In recent decades, the increased use of multiplex PCR panels has revealed that infections may be more common than previously recognized because traditional stool cultures often failed to detect the organism.

**What disease does Plesiomonas shigelloides cause?**

Plesiomonas shigelloides primarily causes acute gastroenteritis, although it can occasionally produce infections outside the gastrointestinal tract.

After entering the digestive system, the bacterium attaches to intestinal cells and produces toxins and other virulence factors that disrupt normal intestinal function. This leads to inflammation and increased fluid secretion into the intestine, resulting in diarrhea.

Most infections resolve within several days without complications. However, symptoms can occasionally persist for weeks, particularly if medical care is delayed or if the individual has underlying health conditions.

Less commonly, the bacterium can cause bloodstream infections, meningitis, cellulitis, septic arthritis, biliary tract infections, and other extraintestinal diseases, especially in individuals with weakened immunity, liver disease, cancer, or blood disorders.

Common symptoms

Symptoms generally appear within one to several days after exposure.

They may include:

Watery diarrhea
Bloody diarrhea in some patients
Abdominal cramps
Nausea
Vomiting
Fever
Headache
Chills
General fatigue
Loss of appetite

Although most cases are mild, severe dehydration can develop in infants, young children, older adults, and people with chronic illnesses.

Who is at higher risk?

Certain groups are more likely to develop severe disease:

Young children
Older adults
People with weakened immune systems
Individuals with liver disease
People with blood disorders
Cancer patients
Organ transplant recipients
Travelers visiting regions with inadequate water sanitation

**How does Plesiomonas shigelloides spread?**

The bacterium spreads primarily through the fecal–oral route, meaning people become infected after ingesting food or water contaminated with fecal material containing the organism.

Unlike respiratory pathogens, P. shigelloides is not spread through coughing or casual contact.

Common sources of infection include:

Source	Examples
Contaminated drinking water	Untreated wells, rivers, lakes
Freshwater recreation	Swimming or accidental ingestion of contaminated water
Seafood	Raw oysters, shellfish, freshwater fish
Fresh produce	Vegetables washed with contaminated water
International travel	Regions with inadequate sanitation
Cross-contamination	Food prepared with contaminated utensils or water

The bacterium naturally inhabits freshwater ecosystems and has been isolated from numerous aquatic animals, including fish, amphibians, reptiles, and shellfish. Because it survives well in aquatic environments, contamination of food during harvesting, transportation, or preparation can introduce the organism into the human food chain.

Human-to-human transmission appears to occur much less frequently than environmental transmission and has not been considered the primary route of spread.

**How is Plesiomonas shigelloides diagnosed?**

Diagnosis is confirmed through laboratory testing because symptoms are similar to those caused by many other gastrointestinal pathogens.

Healthcare providers usually begin by evaluating a person's symptoms, recent travel history, food consumption, and exposure to untreated water.

Laboratory tests may include:

Diagnostic test	Purpose
Stool culture	Identifies the bacterium from stool samples
Multiplex PCR panels	Detect bacterial genetic material rapidly
Blood cultures	Used when bloodstream infection is suspected
Biochemical identification	Confirms the bacterial species after culture
Antimicrobial susceptibility testing	Determines which antibiotics are likely to be effective if treatment is needed

Modern molecular methods have substantially improved detection rates because the bacterium can sometimes be overlooked using conventional stool culture techniques alone.

Additional blood tests may be recommended in severe infections to evaluate dehydration, electrolyte imbalance, or systemic infection.

Key facts at a glance

Feature	Summary
Main disease	Acute gastroenteritis
Primary habitat	Freshwater environments
Common transmission	Contaminated water and seafood
Incubation period	Usually 1–3 days
Most common symptom	Diarrhea
High-risk groups	Young children, older adults, and immunocompromised individuals
Diagnosis	Stool culture and PCR testing

**How is Plesiomonas shigelloides treated?**

Most Plesiomonas shigelloides infections are treated with supportive care because the illness is typically mild and resolves on its own within a few days. The primary focus of treatment is preventing dehydration by replacing fluids and electrolytes lost through diarrhea and vomiting.

For people with mild symptoms, drinking plenty of fluids and using oral rehydration solutions are usually sufficient. Individuals with severe dehydration, persistent vomiting, or signs of invasive infection may require hospitalization for intravenous (IV) fluids and closer monitoring.

Antibiotics are not routinely needed for uncomplicated gastroenteritis. However, healthcare providers may prescribe them for severe, prolonged, or extraintestinal infections, or for patients at high risk of complications, such as those with weakened immune systems. Because antibiotic susceptibility can vary, treatment is ideally guided by laboratory susceptibility testing.

Treatment at a Glance

Clinical Condition	Typical Management
Mild gastroenteritis	Oral fluids, rest, oral rehydration solution
Moderate dehydration	Oral rehydration therapy and medical evaluation
Severe dehydration	Intravenous fluids and electrolyte replacement
Persistent or severe diarrhea	Consider antibiotics based on clinical assessment
Bloodstream or other invasive infection	Targeted antibiotic therapy guided by susceptibility testing

Key Takeaway: Never take antibiotics without medical advice. Unnecessary antibiotic use can contribute to antimicrobial resistance and may not improve recovery in uncomplicated infections.

**How can you prevent Plesiomonas shigelloides infection?**

Preventing Plesiomonas shigelloides infection primarily involves consuming safe food and water and maintaining good hygiene practices. Since the bacterium naturally inhabits freshwater environments and aquatic animals, reducing exposure to contaminated water and properly handling seafood are the most effective preventive measures.

Practical Prevention Tips

Drink treated, filtered, or boiled water, especially when traveling to areas with uncertain water quality.
Avoid swallowing water while swimming in lakes, rivers, or other freshwater bodies.
Cook seafood, shellfish, and freshwater fish thoroughly before eating.
Wash fruits and vegetables with safe, clean water.
Prevent cross-contamination by using separate cutting boards and utensils for raw seafood.
Wash hands thoroughly with soap and water before preparing food and before eating.
Refrigerate seafood promptly to reduce bacterial growth.

Who Should Be Extra Careful?

People who are more vulnerable to severe infection should take additional precautions, including avoiding raw or undercooked seafood and untreated water.

These groups include:

Infants and young children
Older adults
Pregnant individuals
People with chronic liver disease
Individuals receiving chemotherapy
Organ transplant recipients
People with weakened immune systems

Prevention Checklist

Recommended Practice	Benefit
Drink safe water	Reduces exposure to contaminated water
Cook seafood thoroughly	Eliminates disease-causing bacteria
Practice hand hygiene	Prevents transfer of bacteria to food
Wash produce with clean water	Removes potential contaminants
Prevent cross-contamination	Reduces foodborne transmission

Microbe Profile

Characteristic	Description
Gram status	Gram-negative
Shape	Rod-shaped (bacillus)
Spore formation	Non-spore-forming
Motile	Yes, motile by polar flagella
Oxygen requirements	Facultative anaerobe
Optimum temperature	Approximately 35–37°C
Optimum pH	Approximately 6.5–7.5

Taxonomic Classification

Domain: Bacteria

Kingdom: Pseudomonadati

Phylum: Pseudomonadota

Class: Gammaproteobacteria

Order: Enterobacterales

Family: Enterobacteriaceae

Genus: Plesiomonas

Species: Plesiomonas shigelloides

Reference

Janda JM, Abbott SL, McIver CJ. Plesiomonas shigelloides revisited. Clinical Microbiology Reviews. 2016;29(2):349-374. doi:10.1128/CMR.00103-15

Janda JM, Abbott SL. The genus Plesiomonas. In: Dworkin M, Falkow S, Rosenberg E, Schleifer KH, Stackebrandt E, eds. The Prokaryotes. 3rd ed. Springer; 2006.

Janda JM, Abbott SL. Evolving concepts regarding the genus Plesiomonas: an expanding perspective on a human pathogen. Clinical Infectious Diseases. 1998;26(2):266-272.

World Health Organization (WHO). Diarrhoeal Disease Fact Sheet. Updated 2024.https://www.who.int/news-room/fact-sheets/detail/diarrhoeal-disease

Centers for Disease Control and Prevention (CDC). Food Safety.https://www.cdc.gov/food-safety/

Centers for Disease Control and Prevention (CDC). Travelers' Health.https://wwwnc.cdc.gov/travel

Merck Manual Professional Edition. Plesiomonas and Other Gram-Negative Infections.https://www.merckmanuals.com/professional

Ryan KJ, Ray CG, eds. Sherris Medical Microbiology. 8th ed. McGraw-Hill Education; 2022.

Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 10th ed. Elsevier; 2023.

PATHOGEN

PLESIOMONAS SHIGELLOIDES

Frequently Asked Questions

Can Plesiomonas shigelloides cause serious illness?

Yes, although most infections are mild and self-limiting, severe disease can occur in people with weakened immune systems, chronic liver disease, blood disorders, or other underlying health conditions. In rare cases, the bacterium can spread beyond the intestines and cause bloodstream or other invasive infections.

How long does a Plesiomonas shigelloides infection last?

Most people recover within a few days to a week. However, symptoms may persist longer in some individuals, particularly if dehydration develops or medical attention is delayed.

Is Plesiomonas shigelloides contagious?

Person-to-person transmission appears to be uncommon. Most infections occur after consuming contaminated water, seafood, or food prepared with contaminated water.

Can healthy people become infected?

Yes. Healthy individuals can develop gastroenteritis after exposure to contaminated food or water. However, they usually experience a mild illness and recover without complications.

Can Plesiomonas shigelloides infection be prevented?

Yes. Drinking safe water, cooking seafood thoroughly, practicing good hand hygiene, and avoiding cross-contamination during food preparation are the most effective ways to reduce the risk of infection.

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