Vibriões e curvos

Plesiomonas shigelloides

Straight Gram-negative bacilli

Identification & Growth

Identification

Oxidase-positive with glucose fermentation and a positive inositol test
Lysine, ornithine and arginine decarboxylases all positive — the typical triad

Growth Conditions

MacConkey (lactose-negative) and blood agar, 35 °C, 24 h

Clinical Significance

Gastroenteritis after water or raw fish; bacteremia in immunosuppressed patients

Intrinsic Resistance

Intrinsic resistance to ampicillin and, in some isolates, to aminopenicillin-inhibitor combinations

Bench Alerts

Can be confused with Shigella owing to cross-reacting serology — the oxidase test resolves it

Clinical Notes

Clinical presentations

Acute gastroenteritis with watery or, less commonly, dysenteric-type diarrhea, generally self-limited.
Rare extraintestinal infections, including bacteremia, neonatal meningitis and soft-tissue infection, mainly in immunocompromised hosts.
Frequent association with international travel and ingestion of raw seafood or untreated water.

Specimens and collection

Stool culture on enteropathogen-selective media; the laboratory should be alerted when there is specific clinical suspicion, as it is not routinely screened for.
Blood culture and CSF in invasive presentations, especially in neonates and immunocompromised patients.
Record travel history and raw seafood consumption on the request form to guide diagnostic suspicion.

Epidemiology and at-risk populations

Environmental organism of fresh and estuarine water in tropical and subtropical regions; associated with contaminated fish and seafood.
Higher incidence in travelers returning from endemic areas and in young children.
Invasive disease is more frequent in neonates, older adults and patients with liver disease or immunosuppression.

Resistance and therapeutic implications

Gastroenteritis is generally self-limited and does not require antimicrobials, except in severe, prolonged cases or in at-risk populations.
Intrinsic resistance to penicillins via beta-lactamase production; fluoroquinolones, trimethoprim-sulfamethoxazole and third-generation cephalosporins are usually active.
No species-specific breakpoints in EUCAST/BrCAST; guide therapy by MIC and clinical correlation, documenting the limitation in the report.

Bench and reporting notes

Oxidase-positive, motile Gram-negative rod that can be confused with Shigella on agglutination testing due to somatic antigen cross-reaction.
Grows on MacConkey agar as a lactose non-fermenter; biochemical confirmation or MALDI-TOF prevents misidentification.
Fecal isolation should be correlated with the clinical picture, as it can occur in asymptomatic carriers in endemic areas.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Plesiomonas shigelloides in the lab?

Plesiomonas shigelloides is identified through oxidase-positive with glucose fermentation and a positive inositol test, lysine, ornithine and arginine decarboxylases all positive — the typical triad.

What are the intrinsic resistances of Plesiomonas shigelloides?

This organism is naturally resistant to intrinsic resistance to ampicillin and, in some isolates, to aminopenicillin-inhibitor combinations. These drugs should not be reported as susceptible.

Where is Plesiomonas shigelloides commonly found?

It is typically associated with gastroenteritis after water or raw fish; bacteremia in immunosuppressed patients.

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