Infectious Disease and Microbiology – Plesiomonas shigelloides

Overview

Plesiomonas shigelloides is a Gram-negative bacillus found primarily in freshwater environments and soil. Human infection is most commonly associated with gastroenteritis, particularly after exposure to contaminated water or food, and it is recognized as a potential cause of traveler’s diarrhea.

Most intestinal infections are mild and self-limited, but some patients develop an inflammatory or dysentery-like illness with bloody diarrhea, fever, malaise, and fecal leukocytes. Rarely, P. shigelloides causes invasive extraintestinal disease, including bacteremia, soft tissue infection, bone and joint infection, and meningitis.

Classification

Genus: Plesiomonas

Species: Plesiomonas shigelloides

Organism type: Gram-negative bacillus

Microbiologic Characteristics

P. shigelloides is a:

• Gram-negative bacillus

• Facultatively anaerobic organism

• Oxidase-positive bacterium

• Motile organism

• Freshwater-associated enteric pathogen

It is commonly grouped clinically with other water-associated Gram-negative organisms that can produce gastrointestinal disease.

High-Yield Microbiology Pattern

Gram-negative bacillus

• ●

Freshwater exposure

• ●

Acute diarrhea

→ Consider Plesiomonas shigelloides

Incubation Period

The precise incubation period is:

Not clearly established

Evidence from cases of traveler’s diarrhea suggests that symptoms generally develop after a:

Short incubation period of several days

following exposure.

Epidemiology

P. shigelloides is widely distributed in:

Freshwater

and

Soil

It can also be associated with aquatic environments and animals.

Human infection is generally acquired through environmental or food-related exposure rather than sustained person-to-person transmission.

Transmission

Gastrointestinal infection is associated with ingestion of:

• Contaminated water

• Contaminated food

• Raw or inadequately cooked aquatic foods in some settings

Exposure during international travel may lead to:

Traveler’s diarrhea

High-Yield Exposure Pattern

Recent travel

• ●

Freshwater or contaminated food/water exposure

• ●

Acute diarrhea

→ Consider Plesiomonas shigelloides

Gastroenteritis

The most common clinical manifestation is:

Acute gastroenteritis

Disease severity varies considerably.

Some patients develop:

Mild, watery, self-limited diarrhea

while others develop a more inflammatory illness.

Mild Diarrheal Disease

Uncomplicated infection may present with:

• Watery diarrhea

• Abdominal discomfort or cramping

• Nausea

• Malaise

Most mild infections resolve without specific antimicrobial therapy.

Severe or Dysentery-Like Disease

More severe P. shigelloides gastroenteritis may produce:

Bloody diarrhea

associated with:

• Fever

• Malaise

• Abdominal symptoms

• White blood cells in the stool

The presence of blood and fecal leukocytes suggests an:

Inflammatory diarrheal syndrome

High-Yield Clinical Pattern

Traveler

• ●

Acute diarrhea

• ●

Blood and fecal leukocytes

• ●

Freshwater/environmental exposure

→ Consider Plesiomonas shigelloides

Traveler’s Diarrhea

P. shigelloides is one recognized cause of:

Traveler’s diarrhea

Although other organisms are considerably more common, Plesiomonas should be considered when the epidemiologic history includes:

Travel + contaminated food/water + acute gastroenteritis

Extraintestinal Infection

Although gastrointestinal disease is the major manifestation, P. shigelloides can occasionally produce:

Invasive extraintestinal infections

These are much less common than diarrhea.

Bacteremia

Rarely, the organism may enter the bloodstream and cause:

Bacteremia

Systemic infection is more concerning in vulnerable patients and may require antimicrobial treatment.

Skin and Subcutaneous Tissue Infection

The source also reports:

Skin and subcutaneous tissue infections

These may be particularly relevant when damaged tissue is exposed to contaminated environmental water.

Septic Arthritis and Osteomyelitis

Rare musculoskeletal manifestations include:

Septic arthritis

and

Osteomyelitis

These invasive infections require:

Systemic antimicrobial therapy

and may also require drainage or other source-control procedures.

Other Rare Infections

Additional reported manifestations include:

• Cholecystitis

• Endophthalmitis

• Meningitis

These infections are uncommon but demonstrate that P. shigelloides can occasionally behave as an invasive pathogen outside the gastrointestinal tract.

Neonatal Meningitis

One of the most important severe manifestations is:

Meningitis in neonates

Although rare, neonatal P. shigelloides meningitis can be a serious invasive infection.

High-Yield Severe Disease Pattern

Neonate

• ●

Gram-negative bacillary meningitis

• ●

Possible environmental/water-associated organism

→ Consider Plesiomonas shigelloides among rare causes

Diagnosis

The principal diagnostic method is:

Culture

For gastrointestinal disease, the organism can be recovered from:

Stool culture

For invasive disease, specimens depend on the affected site and may include:

• Blood

• Cerebrospinal fluid

• Joint fluid

• Wound or tissue specimens

• Other normally sterile fluids

Laboratory Identification

Laboratory identification is important because P. shigelloides may resemble other enteric Gram-negative organisms.

A useful characteristic is:

Oxidase positivity

This helps distinguish it from many members of the:

Enterobacterales

which are typically oxidase negative.

High-Yield Laboratory Pattern

Enteric-appearing Gram-negative bacillus

• ●

Oxidase positive

• ●

Freshwater association

• ●

Diarrheal illness

→ Think Plesiomonas shigelloides

Treatment

For uncomplicated gastroenteritis:

Antibiotics are usually unnecessary

because many infections are:

Mild and self-limited

The cornerstone of treatment is:

Fluid and electrolyte replacement

Rehydration

Management should focus on correcting losses caused by diarrhea.

This includes:

Oral rehydration

when tolerated.

More severe dehydration may require:

Intravenous fluid and electrolyte replacement

Antimicrobial Therapy

The source lists a:

Fluoroquinolone

as an antimicrobial treatment option.

Antibiotic therapy may be considered particularly for:

• Severe disease

• Persistent diarrhea

• Dysentery-like illness

• Significant systemic symptoms

• Invasive extraintestinal infection

Selection should be guided by the clinical syndrome and susceptibility information when available.

Additional Treatment Options

The source lists:

• Trimethoprim–sulfamethoxazole

• Chloramphenicol

• Carbapenems

• Aminoglycosides

For serious invasive infection, therapy should be selected according to:

Culture and antimicrobial susceptibility results

whenever possible.

Source Control

For focal invasive infections such as:

Septic arthritis

or

Deep soft tissue infection

antimicrobial therapy may need to be combined with:

Drainage or surgical source control

when clinically indicated.

Plesiomonas vs. Shigella

Despite the species name:

shigelloides

Plesiomonas shigelloides is not a Shigella species.

Plesiomonas shigelloides

→ Freshwater-associated

→ Oxidase positive

→ Motile

→ Can cause watery or bloody diarrhea

Shigella

→ Primarily human intestinal pathogen

→ Oxidase negative

→ Nonmotile

→ Characteristically causes inflammatory dysentery

Plesiomonas vs. Aeromonas

Both organisms may be associated with:

Freshwater

and can cause:

Gastrointestinal disease

Plesiomonas shigelloides

→ Diarrhea, including traveler’s diarrhea

→ Rare invasive infections

→ Neonatal meningitis is a notable severe manifestation

Aeromonas species

→ Freshwater exposure

→ Gastroenteritis

→ Particularly important in wound and soft tissue infections after water exposure

Plesiomonas vs. Vibrio

Plesiomonas

→ Classically associated with freshwater

→ Traveler’s diarrhea possible

Vibrio

→ Many clinically important species are strongly associated with marine or brackish water

→ Raw seafood exposure is an important epidemiologic clue for several species

Thus, the type of water exposure can help narrow the differential diagnosis.

Prevention

Prevention centers on reducing exposure to contaminated food and water.

Important measures include:

• Drinking safe water

• Appropriate food hygiene

• Properly cooking aquatic foods

• Careful food and water practices during travel

• Hand hygiene

High-Yield Clinical Pattern

Freshwater exposure

• ●

Short incubation of a few days

• ●

Traveler’s diarrhea

• ●

Watery or bloody diarrhea

• ●

Oxidase-positive Gram-negative bacillus

→ Think Plesiomonas shigelloides

Exam Essentials

Genus: Plesiomonas

Species: P. shigelloides

Type: Gram-negative bacillus

Metabolism: Facultatively anaerobic

Oxidase: Positive

Motility: Motile

Reservoir: Freshwater and soil

Incubation: Uncertain; probably a few days in many cases

Major disease: Gastroenteritis

Diarrhea: May range from mild watery disease to bloody inflammatory diarrhea

Stool finding in severe disease: Fecal leukocytes may occur

Travel association: Cause of traveler’s diarrhea

Rare infections: Bacteremia, skin/soft tissue infection, septic arthritis, osteomyelitis, cholecystitis, endophthalmitis, and meningitis

Important severe association: Neonatal meningitis

Diagnosis: Culture

Mild infection: Usually supportive treatment only

Essential therapy: Fluids and electrolytes

Source antimicrobial: Fluoroquinolone

Additional source agents: TMP-SMX, chloramphenicol, carbapenems, and aminoglycosides

Important laboratory clue: Oxidase positive, unlike typical Enterobacterales

Major environmental clue: Freshwater exposure

Key clinical pearl: Plesiomonas shigelloides is a freshwater-associated, oxidase-positive Gram-negative bacillus that primarily causes gastroenteritis, including traveler’s diarrhea. Most cases require only fluid and electrolyte replacement, but bloody inflammatory diarrhea can occur, and rare invasive infections—including particularly serious neonatal meningitis—should be recognized.