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September 2021 – Clinical Microbiology Case 2

By MCL Education • September 24, 2021

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.

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.

References

Aeromonas infections. (n.d.). Retrieved from https://www.uptodate.com/contents/aeromonas-infections/print
Aeromonas wound infections associated with outdoor activities -- California. (n.d.). Retrieved from https://www.cdc.gov/mmwr/preview/mmwrhtml/00001628.htm
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
Freeman, J., & Roberts, S. (n.d.). Approach to Gram stain and culture results in the microbiology laboratory. Retrieved from http://www.uptodate.com/contents/approach-to-gram-stain-and-culture-results-in-the-microbiology-laboratory
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
Morris, J. G., Jr. (n.d.). Minor Vibrio and Vibrio-like species associated with human disease. Retrieved from http://www.uptodate.com/contents/minor-vibrio-and-vibrio-like-species-associated-with-human-disease