Prototheca wickerhamii

This organism was recovered from a tunnelled CVC of a haemtaology patient. I have never heard of it, so I thought we could summarise it:

Microbiology:

• Achlorophyllic algae, spherical, unicellular, 3-30μm in diameter
• Do not process glucosamine (of fungal cells) or muramic acid (of bacteria)
• Ubiquitous in nature, detritus inhabitants
• First human case of infection in 1964
• Primarily SSTIs in the immunocompetent
• Disseminated in the immunocompromised
• Asexual reproduction through free cell formation, via endospores causing lysis of the parent cell
• Specific salt and sugar growth requirements, little data regarding fermentative pressures under anoxic conditions.

Pathogenesis:

• Previously thought to b a skin saprophyte
• Low virulence in the immunocompetent
• Human infection through contact with potential sources or traumatic innoculation
• Poor funtion of PMNs likley to be significant (PMNs ingest and kill P. wickerhamii)
• Neutropaenia per se does not seem to be a significant risk factor. Optimal killing requires fully functional PMNs, specific IgG opsonisation. AIDS & low CD4 counts are also significant risk factors
• Rare opportuinistic pathogens

Clinical Features:

• Hospital acquired cases: surgery and orthopaedics
• Penetrating injury
• Non transmissible
• Environmental pathogens, can affect rural regions
• Cutaneous leisons
  • Vesiculobullous leison with ulceration and crusting, but cause chronic suppurative infections
• Olecranon bursitis
  • Elbow grazing/repeated trauma
  • Serosanginous fluid production
• Disseminated disease
  • Skin, subcut tissues, gut, peritoneum, blood and spleen
  • Most of the time P. wickerhamii is involved
  • Catheter related protothecosis well described (urinary, CVC, Tenchkoff)

Diagnosis:

• Difficult clinically, as not usually suspected
• Large non-budding cells seen in tissue
• The lack of characteristic endospores causes Prototheca to resemble nonsporulating cells  of Blastomyces dermatitidis, Cryptococcus neoformans, Paracoccidioides brasiliensis, and some stages of Coccidioides immitis, Pneumocystis jiroveci, Rhinosporidium seeberi, and the agent causing chromomycosis
• Simple nutritional requirements, but grow best on blood agar, as fungal agars contain cycloheximide which may inhibit growth. Prototheca isolation medium for selective cultivation should contain acombination of flucytosine and potassium hydrogen phthalate inhibits most bacteria and fungi
• Grow at 30^oC for 72 hours, while slow growing strains will need 7 days at 25^oC
• Growth is optimized between 25 and 37°C, and organisms usually proliferate within 48 h as soft, wet, yeastlike, white-to-light-tan colonies. The organism can be either aerobic or microaerophilic
• Sab Plate:

• Microscopy:

• P. wickerhamii: note the symmetrical morula-like structures
• Identify to species level with good discrimination using commercial kit
• Bio-Rad offers the AUXACOLOR™ 2 kit, specially designed for an easy, quick and reliable identification of the 33 yeast species most frequently isolated in human medical mycology. AUXACOLOR™ 2 is an identification system based on the principle of sugar assimilation. The growth of yeasts is visualized by the color change of a pH indicator. The kit also comprises three enzymatic tests, including a test for the detection of the phenoloxidase activity of Cryptococcus neoformans.

• Methods of susceptibility testing: E-test, broth and agar dilution. E-testing has been shown to be accurate and reproducible for this organism
• Susceptibilties of our isolate - MIC (mg/L):
  • Fluconazole >64 (R)
  • Flucytosine >64 (R)
  • Amphotericin 1 (S)
  • Voriconazole 0.25 (S)
  • Caspofungin 8 (?Intermediate)

Management:

• Controversial
• Most experience with azoles and amphotericin (convential & liposomal - best results were achieved with this).
• Medical and surgical approach needed - treatment failure is not uncommon
• Cutaneous:
  • Topical/systemic treatment with local debridement
• Olecranon bursitis:
  • Bursectomy with local instillation of amphotericin B
• Systemic infection:
  • Catheter removal with treatment with amphotericin B
  • IP amphotericin B in PD infections
• Prolonged therapy (months)
• Breakthrough with azole treatment is common

For details, read this.

AMH.