An 80 year old male with a PMH of myelodysplasia with progression to AML and neutropenic fever presented with severe leg pain and violaceous leg lesion spanning a large portion of the left medial thigh and knee. There was no reported trauma. A radiograph demonstrated significant gas/air tracking in the tissues of the left medial thigh. He was given vanc/zosyn/clinda and subsequently transferred to UVMMC for further evaluation. He underwent debridement in the OR and fluid was sent for culture. Primary Gram stain showed no neutrophils and few gram positive bacilli. The blood agar plate and leg lesion are shown below.
This organism was identified on MALDI-MS as Clostridium septicum, consistent with the spreading colonies with irregular margins on blood agar. Box-car GPB morphology was present on gram stain, and although subterminal spores can be an additional clue to the identify C. septicum, they were not seen in this particular case.
Important diagnostic features of Clostridium septicum in the microbiology lab include:
- GPB with rare subterminal spores (Image 3)
- Gray/white colonies with irregular margins, translucent, β-hemolytic
- Can show swarming growth pattern in < 24 hours on Schaedler plate (Image 4)
- This swarming phenomenon is due to the ability to differentiate into giant hyperflagellated swarm cells, which can participate in migrations across surfaces
- Major changes in virulence characteristics during swarming are reported
- Lecithinase negative
- Lipase negative
- Gelatinase positive
- Maltose positive
- Indole negative
- Urease negative
C. septicum is classically associated with GI malignancies and perforations when isolated in blood cultures. While less common than C. perfringens, Clostridium septicum can also cause gas gangrene (as in this case) along with C. novyi, C. histolyticum, C. sporogenes, and C. bifermentans. Importantly, in cases of nontraumatic gas gangrene, C. perfringens is the most common cause and classically presents as a primary perineum or extremity infection following bacterial translocation from the gastrointestinal tract. Patients with immune suppression, specifically neutropenia, are at increased risk of spontaneous gas gangrene due to C. septicum and have a high mortality rate.
To remember the colony morphology of the different Clostridial spp, you can think:
- While C. Septicum Swarms, C. Perfringens Perforates (aka makes a double zone in the agar).
Edited to add: C. septicum is the most common cause of nontraumatic gas gangrene.
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-Nicole Mendelson, MD is a 3rd year Anatomic and Clinical Pathology Resident at the University of Vermont Medical Center.
-Christi Wojewoda, MD, is the Director of Clinical Microbiology at the University of Vermont Medical Center and an Associate Professor at the University of Vermont.