Lab diagnosis of shigella

Specimen Collection

Ideal specimen: Fresh stool.

• Unsatisfactory: Rectal swabs.

• Transport media: Sach’s buffered glycerol saline.

Wet Mount

Findings: Large number of pus cells, erythrocytes, and macrophages.

Culture

Inhibiting commensals: Use enrichment broth and selective media.

• Enrichment broths:

• Selenite F broth

• Tetrathionate broth

• Gram-negative broth

• Selective media:

• Mildly selective: MacConkey agar → Pale, NLF colonies.

• Highly selective:

• DCA (Deoxycholate citrate agar) → Colorless NLF colonies.

• XLD agar (Xylose lysine deoxycholate) → Red colonies without black center.

Culture Smear and Motility

Smear findings: Gram-negative, short bacilli.

• Nonmotile, noncapsulated, nonsporing.

Identification Methods

Catalase: Positive (except S. dysenteriae serotype-1).

• Oxidase: Negative.

• ICUT tests:

• Indole: Negative.

• Citrate: Negative.

• Urease: Negative.

• TSI: Alkaline/acid, gas absent, H₂S absent.

• Mannitol fermentation: All species positive except S. dysenteriae.

• Automated methods:

• VITEK: Identifies species.

• MALDI-TOF: Poor differentiation from E. coli.

Serotyping

Confirm genus: Slide agglutination with polyvalent antisera.

• Identify species: Group-specific antisera.

• Identify serotypes: Type-specific antiser

Additional Typing and Testing

Colicin typing: For S. sonnei.

• Antimicrobial susceptibility: Mueller-Hinton agar, disk diffusion test.