November 2021 – Clinical Microbiology

A 37-year-old woman with a past medical history of ureteral obstruction requires placement of a nephrostomy tube when one of her bilateral ureteral stents is encrusted. She is afebrile and without urinary symptoms. Collected urine is sent to the laboratory in an anaerobic vial for culture and Gram stain. The Gram stain is read as negative for organisms and WBCs. Seven days later, a portion of the anaerobic CDC plate appears as below: no growth on aerobic plates was seen.

Figure 1: PK

Which of the following organisms is most likely to be identified from this culture plate?

  • Proteus mirabilis
  • Mycoplasma hominis
  • Escherichia coli
  • Ureaplasma urealyticum

The correct answer is ...

Mycoplasma hominis

Mycoplasma hominis belongs to a phylogenetic class called 'mollicutes,' which have no cell wall and thus are not seen on a Gram stain. This case illustrates that these bacteria can occasionally grow on routine culture media incubated in anaerobic or 5% CO2 conditions, but they more commonly require specialized media. The pictured colonies are very small, not present on aerobic plates, and have only grown after many (seven) days of incubation.

M. hominis is part of the normal genitourinary microbiota in many adults and typically does not cause any symptoms. The role of M. hominis in many human genitourinary diseases is uncertain for three main reasons:

1. It is often a normal commensal.

2. It is difficult to culture, although molecular assays are available.

3. It is usually isolated with other bacteria.

Because M. hominis lacks a cell wall, it is intrinsically resistant to cell wall-active antimicrobials. Susceptibility testing is standardized by the Clinical & Laboratory Standards Institute and can be performed at specialized laboratories like the Diagnostic Mycoplasma Laboratory at the University of Alabama at Birmingham.

Proteus mirabilisP. mirabilis is a urease-producing organism that can cause calculi primarily made of struvite crystals, which could account for this patient’s stent encrustation. However, P. mirabilis is a gram-negative rod (GNR) that should readily Gram stain. Furthermore, P. mirabilis should present as large GNR colonies, perhaps with swarming motility, on routine culture media (e.g., MacConkey or EMB) earlier than seven days of incubation.

Escherichia coliE. coli is the most common cause of urinary tract infection, but should be readily identified as a gram-negative rod upon Gram staining. Furthermore, E. coli should present as large GNR colonies on routine culture media (e.g., MacConkey or EMB) earlier than seven days of incubation.

Ureaplasma urealyticum: Like M. hominis, U. urealyticum is a mollicute that commonly colonizes the urinary tract and does not Gram stain because it lacks a cell wall. Unlike M. hominisU. urealyticum is unable to grow on routine culture media and instead requires specialized media (e.g., 10B broth and A8 agar) to grow.


  1. Procop, Gary W. et al. Koneman’s Color Atlas and Textbook of Diagnostic Microbiology. Seventh edition. Philadelphia: Wolters Kluwer Health, 2017. Print.
  2. Murray, Patrick R., and Ellen J. Baron. Manual of Clinical Microbiology. Washington, D.C: ASM Press, 2007. Print.
  3. Waites, Ken B., and Namasivayam Ambalavanan. Mycoplasma hominis and Ureaplasma infections. In: UpToDate, Post TW (Ed), UpToDate, Waltham, MA. (Accessed on September 26, 2021.)

Peter Kundert, M.D., Ph.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.