September 2021 – Clinical Microbiology Case 2

A 55-year-old man was admitted with symptoms of septic shock and started on broad spectrum antibiotics, which included piperacillin-tazobactam and vancomycin. A computerized tomography scan showed pneumoperitoneum, metastatic pancreatic adenocarcinoma, and a hepatic abscess that had eroded through the capsule. The abscess was treated by drain placement as the patient was deemed not a surgical candidate.

Blood cultures yielded the organism shown in the image below. The organism grew as β-hemolytic colonies on sheep blood agar. It did not grow on Thiosulfate-citrate-bile salts-sucrose (TCBS) agar. The organism tested oxidase- and indole-positive, and fermented glucose.

Photo Courtesy: Dr. Madiha Fida

Which of the following is the most likely identity of the recovered organism?

  • Listeria species
  • Enterobacteriaceae species 
  • Vibrio species
  • Aeromonas species

The correct answer is ...

The correct answer is: Aeromonas species.

Unlike Vibrio species, Aeromonas species do not grow on TCBS agar, making this agar useful in distinguishing Aeromonas from Vibrio. Enterobacteriaceae species ferments glucose and are indole positive but oxidase negative, which distinguishes it from Aeromonas. Listeria species are gram positive.


  1. Aeromonas infections. (n.d.). Retrieved from
  2. Aeromonas wound infections associated with outdoor activities -- California. (n.d.). Retrieved from
  3. 2Fernández-Bravo, A., & Figueras, M. J. (2020). An update on the genus Aeromonas: Taxonomy, epidemiology, and pathogenicity. Microorganisms, 8(1), 129. doi:10.3390/microorganisms8010129
  4. Freeman, J., & Roberts, S. (n.d.). Approach to Gram stain and culture results in the microbiology laboratory. Retrieved from
  5. 1Janda, J. M., & Abbott, S. L. (2010). The genus Aeromonas: Taxonomy, pathogenicity, and infection. Clinical Microbiology Reviews, 23(1), 35-73. doi:10.1128/cmr.00039-09
  6. Morris, J. G., Jr. (n.d.). Minor Vibrio and Vibrio-like species associated with human disease. Retrieved from

Nadarra Stokes, M.D.

Resident, Anatomic and Clinical Pathology
Mayo Clinic

Audrey Schuetz, M.D.

Consultant, Clinical Microbiology
Mayo Clinic
Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

MCL Education

This post was developed by our Education and Technical Publications Team.