GPR: 2020 - Week 39

From MGH Learn Pathology

Clinical history

A 25-year-old woman with a history of hypogammaglobulinemia presents with persistent fever despite treatment with pip-tazo and meropenem and is found to have a perinephric abscess. Fluid drained from the abscess is cultured on nutrient-rich, slightly acidic (pH 6-6.6) media containing high-dose penicillin G, amphotericin B, dextrose, urea, and phenol red (a pH indicator). Which of the following is most likely growing on the plate?


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Ureaplasma species
  • The media is slightly acidic and thus the phenol red pH indicator initially appears yellow. Ureaplasma species express urease, which catalyzes hydrolysis of urea in the media, resulting in production of ammonia (a base), which increases the local pH on the plate as visualized by the red-appearing phenol red (a pH indicator).
  • Ureaplasma species do not have a peptidoglycan-containing cell wall and thus are insensitive to beta-lactam drugs such as penicillin G .
  • Ureaplasma species do not contain ergosterol and thus are insensitive to the antifungal amphotericin B.
  • Ureaplasma colonies are tiny and not easily visible by eye.



Mycoplasma species

  • Like Ureaplasma species, Mycoplasma species do not have a cell wall and do not contain ergosterol, and thus are also insensitive to both penicillin G and amphotericin B.
  • However, Mycoplasma species do not express urease, and thus even with growth the pH would remain slightly acidic (and the phenol red pH indicator would remain yellow).
  • Like Ureaplasma, Mycoplasma colonies are also tiny and not easily visible by eye.



E. coli

  • Like Mycoplasma species, E. coli also do not express urease and thus the phenol red pH indicator would remain yellow.
  • Furthermore, E. coli contains a cell wall that includes peptidoglycan. While E. coli generally exhibits beta-lactam resistance, the high-dose penicillin concentration in this media would slow the growth of this organism.



Candida sp.

  • Like Mycoplasma species and E. coli, candida species also do not express urease and thus the phenol red pH indicator would remain yellow.
  • Furthermore, Candida species lipid membranes contain ergosterol, which renders most Candida sp sensitive to amphotericin B. However, some Candida isolates exhibit amphotericin B resistance, and this can complicate isolation Ureaplasma or Mycoplasma.
  • If an amphotericin-resistant Candida isolate grew on this plate, the colony would typically be visible by eye (unlike the smaller Ureaplasma or Mycoplasma colonies).
  • Photo credit: Melis Anahtar, MD, PhD @melisanahtar
Melis Anahtar, MD, PhD @melisanahtar
  • Manual of Clinical Microbiology, 11th edition (2015).
  • Mandell Douglas and Bennett's Principles and Practice of Infectious Diseases, 8th Edition (2015)

This week's Gross Pathology Roundup was presented by Jonathan Stefely, MD on Mon, Sep 21, 2020.