Enterobacterales

Shigella spp.

Gram-negative bacilli

Identification & Growth

Identification

Oxidase-negative, non-motile, lactose-negative, H₂S-negative, lysine-negative, no gas production
Serological confirmation by group (A–D)

Growth Conditions

MacConkey/SS agar: colourless colonies; 35–37 °C, 24 h

Clinical Significance

Dysentery with mucus and blood, outpatient and daycare settings

Intrinsic Resistance

Penicillin G, macrolides, clindamycin, glycopeptides, daptomycin

Bench Alerts

Aminoglycosides and first-/second-generation cephalosporins should not be reported (clinical failure despite in vitro activity)
Report outbreaks; SXT and ampicillin resistance rates are high in Brazil

Clinical Notes

Clinical presentations

Causes bacillary dysentery with bloody diarrhea, tenesmus, fever, and crampy abdominal pain.
Shigella dysenteriae type 1 produces Shiga toxin, which can cause hemolytic-uremic syndrome.
Complications include toxic megacolon, seizures in children, and postinfectious reactive arthritis.
Bacteremia is rare but may occur in malnourished or immunocompromised patients.

Specimens and collection

Fresh stool culture or rectal swab is the specimen of choice, requiring prompt processing due to organism lability.
Transport in Cary-Blair medium is recommended when processing delay exceeds two hours.
Multiplex molecular enteric pathogen panels increase detection sensitivity compared to culture alone.

Epidemiology and at-risk populations

Person-to-person fecal-oral transmission with a low infectious dose favors outbreaks in daycare centers and closed institutions.
Young children in low-income countries bear the greatest morbidity and mortality from shigellosis.
Outbreaks among men who have sex with men have been associated with multidrug-resistant strains.

Resistance and therapeutic implications

Resistance to ampicillin, trimethoprim-sulfamethoxazole, and fluoroquinolones is widely disseminated globally.
Strains resistant to azithromycin and third-generation cephalosporins have been reported, limiting oral therapeutic options.
Empiric treatment should consider local resistance patterns before selecting an antimicrobial.

Bench and reporting notes

Non-lactose-fermenting colonies on MacConkey or XLD agar suggest Shigella, requiring biochemical and serologic confirmation.
Differentiation from enteroinvasive Escherichia coli can be challenging, requiring additional biochemical or molecular testing.
Confirmed cases are notifiable in many jurisdictions due to outbreak potential.

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 Shigella spp. in the lab?

Shigella spp. is identified through oxidase-negative, non-motile, lactose-negative, h₂s-negative, lysine-negative, no gas production, serological confirmation by group (a–d).

What are the intrinsic resistances of Shigella spp.?

This organism is naturally resistant to penicillin g, macrolides, clindamycin, glycopeptides, daptomycin. These drugs should not be reported as susceptible.

Where is Shigella spp. commonly found?

It is typically associated with dysentery with mucus and blood, outpatient and daycare settings.

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