July 2021 – Clinical Microbiology

The correct answer is: Hyphae suggestive of Mucorales and the cellophane tape preparation resembles Rhizopus species.

Mucormycosis, previously called zygomycosis, is a serious but rare invasive fungal infection. A diagnosis of infection with one of the Mucorales is often a medical emergency due to the rapid spread of infection, angioinvasive nature of the mold, and high mortality rate.

Mucorales infections are often seen in individuals with uncontrolled Type 2 diabetes, malignancy, or those on long-term immunosuppression. Mucorales are found ubiquitously in the environment and infection occurs most often by inhalation of the spores or inoculation after a skin injury. The most common types of infections are rhinocerebral, pulmonary, cutaneous, and disseminated. Mucormycosis can be diagnosed from a direct microscopic examination of the clinical specimen stained with KOH-Calcofluor White, as seen in Image 1. The image shows characteristic ribbon-like hyphae fragments that are twisted, folded, and collapsed. The hyphae are large, wide, and pauciseptate. The hyphae are fragile because they lack septations and can often be damaged in processing in the laboratory, yielding low recovery in microbiologic culture. Similar to its rapid spread in the human body, Mucorales also grows quickly in the laboratory as a white, wooly mold filling the entire plate and pushing the lid upwards, giving rise to their nickname as the “lid-lifters”. 

There are multiple genera of Mucorales that infect humans, but the two most common agents causing infection are Rhizopus and Mucor. These two genera can be distinguished by their microscopic morphologies after growth in culture. Image 2 shows Rhizopus species, with nodal rhizoids, occurring directly underneath the sporangiophore or stalk. The round, sac-like object full of spores and responsible for asexual reproduction is the sporangium, which is attached to the sporangiophore. Mucor is an incorrect answer because rhizoids are not present in this species.

Other useful methods for identification of the Mucorales in the microbiology laboratory include MALDI-TOF mass spectrometry and Sanger sequencing. When the laboratory has high suspicion for or confirmation of Mucorales in a specimen, this a critical result and the care team should be immediately notified. Due to the invasive nature of the organism, this will often require additional surgical debridement as the primary patient management strategy. Amphotericin B is the primary antifungal of choice, and either posaconazole or isavuconazole is used in combination and for stepdown therapy.

References

1. www.cdc.gov/fungal/diseases/mucormycosis/health-professionals.html  
2. Ribes JA, Vanover-Sams CL, Baker DJ. 2000. Zygomycetes in Human Disease. Clinical Microbiology Reviews 13:236-301.