GPR: 2020 - Week 45

From MGH Learn Pathology

Clinical history

A 72 year-old woman with mixed connective tissue disease (SLE and Sjogren's, on mycophenolate and prednisone) presents with acute abdominal pain and vomiting. Laboratory testing reveals a slight leukocytosis (WBC 14) and eosinophilia. A routine stool culture is performed.


Registered users must be logged in to access the interactive quiz.


  • Intestinal nematode, 180-380 um long, infects ~30-100 million people worldwide.
  • Autoinfective life cycle: skin - lungs (via the blood) - intestine - penetrates the intestinal mucosa to re-enter bloodstream.
  • More severe presentations, higher organism burden in immunosuppressed patients
  • Tracking phenomenon
    • Motile Strongyloides larvae crawl along the agar, dragging normal gut bacteria behind with them.
    • Same bacterial hitchhiking occurs in patients, who can present with recurrent gram-negative bacteremia when larvae penetrate gut mucosa.




  • Salmonella is a gram-negative rod that looks like typical gut flora on a blood agar plate.
  • Most common food-borne pathogen in the U.S.
  • Presents most commonly with abdominal pain and fever in adults.
  • Can be preliminarily identified in stool using the selective and differential Hektoen-Enteric agar, where Salmonella colonies grow as blue-green colonies with black center due to H2S production.



Clostridioides difficile

  • Obligate anaerobe, would not grow on a routine stool culture.
  • Important cause of antibiotic-associated pseudomembranous colitis and hospital-acquired diarrhea.
  • Usually diagnosed with a rapid antigen test, which detects the C. difficile antigen and toxin.
  • In rare cases when it’s cultured, it can be identified grossly by its very strong horsebarn-like smell and unique greenish/chartreuse fluorescence under UV light.

  • Denise Bernier and the Microbiology Technologists
  • George Eng, MD, PhD (C. difficile image)
  • Will Rothwell, MD, PhD (Salmonella image)
  • Panosian KJ, Marone P, and Edberg SC. “Elucidation of Strongyloides stercoralis by Bacterial-Colony Displacement.” Journal of Clinical Microbiology, 1986.

This week's Gross Pathology Roundup was presented by Melis Anahtar MD, PhD on 2020-11-02.