A Mold That Grew Overnight in a Neutropenic Patient
A 34-year-old woman with acute myeloid leukemia is day 18 after induction chemotherapy, with an absolute neutrophil count of 80/µL. She has fever and pleuritic chest pain; CT shows a nodule with a surrounding halo of ground-glass opacity. A BAL is cultured on Sabouraud dextrose agar at 30°C and sheep blood agar at 35°C. Within 36 hours, a floccose gray mold grows on both media. Lactophenol cotton blue prep shows broad, ribbon-like, sparsely septate hyphae with irregular branching; sporangiophores terminate in sporangia. The serum galactomannan index is 0.3 (negative). The laboratory is staffed by one technologist tonight, and the plate is on the open bench.
1. Identify the likely organism group and the morphologic features that distinguish it from Aspergillus. What is the significance of the negative galactomannan?
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Likely group: Mucorales, such as Rhizopus, Lichtheimia, or Mucor. The colony is floccose and fast-growing, and the lactophenol cotton blue preparation shows broad, ribbon-like, sparsely septate hyphae with irregular branching. Sporangiophores ending in sporangia are characteristic of the Mucorales.
Contrast with Aspergillus: Aspergillus species have narrower, regularly septate hyphae with acute-angle branching and produce conidiophores with vesicles and conidia, not sporangia. The hyphal morphology and the sporangia are the key distinctions. Do not identify a mold to species from colonial appearance alone; the microscopic morphology and, when needed, sequencing are required.
Galactomannan: A negative galactomannan index does not exclude Mucorales and is consistent with them, because galactomannan is a cell-wall antigen of Aspergillus and is not expected to be positive in mucormycosis. A negative result cannot rule out aspergillosis either, because sensitivity varies, but in this case the morphology points away from Aspergillus. The clinical picture-profound neutropenia, fever, and a halo sign-makes an invasive mold infection urgent.
2. What should the technologist do immediately with the plate and the report? Include biosafety and communication.
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Plate handling: Keep the plate closed and sealed, and move it into a biological safety cabinet. Do not open the plate on the open bench, do not perform a wet mount on the bench, and do not use a loop to disturb the colony in a way that creates airborne spores. Mucorales can produce abundant airborne sporangiophores, and laboratory-acquired exposure is a real risk. Disinfect the bench and discard the original plate according to biohazardous-waste policy.
Report: Call the clinician or the on-call infectious disease provider immediately with a preliminary result: "Mold consistent with Mucorales isolated from BAL; identification pending." Do not wait for species-level identification to communicate the result. Document the notification time and the recipient. Enter a preliminary report in the laboratory information system and add a comment that the morphology is consistent with Mucorales.
Why urgent: Mucormycosis is angioinvasive and can progress within hours in a neutropenic patient. The clinician may need to adjust antifungal therapy and obtain surgical consultation before the final identification is available.
Safety and staffing: If the technologist is alone, do not attempt extensive manipulation of the mold. Secure the plate in the cabinet, notify the supervisor, and complete the minimal documentation needed for the preliminary report. The plate should not be transported through a pneumatic tube.
3. What additional identification or susceptibility testing is appropriate, and how would you judge whether this is a contaminant?
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Additional identification: Perform a careful microscopic examination in the safety cabinet and, if the laboratory has the capability, identify to genus or species by ITS sequencing or another validated molecular method. MALDI-TOF may be useful for some molds but is not universally reliable for Mucorales. Species-level identification is helpful for epidemiology but should not delay the preliminary report.
Susceptibility testing: Routine susceptibility testing is not standardized for all Mucorales, and results may not correlate well with clinical outcome. Do not set up a disk diffusion test or an automated panel designed for yeasts. If testing is available, use a method validated for molds and interpret cautiously. The clinician needs the group-level identification more than a premature susceptibility result.
Contaminant versus pathogen: A single colony in a nonsterile site such as BAL can be a contaminant, but this patient is profoundly neutropenic, has a compatible CT halo sign, and the organism grew rapidly on both media. Those features strongly favor a pathogen rather than a contaminant. The negative galactomannan does not argue against Mucorales. If the culture were from a nonsterile site in an immunocompetent patient with no compatible imaging, repeat sampling and clinical correlation would be more important. In this case, report urgently and let the clinical team integrate the result with imaging and histology.