Blood Culture–Positive Molds: Introduction, Common molds, Comparison, and Keynotes
Blood Culture–Positive Molds
Introduction
Table of Contents
Fig. Blood Culture–Positive Mold -Note: The culture bottle shows cottony to fluffy white mycelial growth at the surface. Growth at 37 °C supports a thermotolerant mold (clinically significant invasive pathogen). This excludes many saprophytes that fail to grow at body temperature.
Blood cultures are the cornerstone for diagnosing fungemia, but while yeasts (Candida spp.) are frequently detected, molds are rarely recovered in blood culture systems. This is because most molds (Aspergillus, Mucorales) cause tissue-invasive disease without sustained fungemia. However, certain molds such as Fusarium spp. and, less commonly, Scedosporium and Lomentospora can be recovered from blood culture bottles, especially in immunocompromised patients. Identifying molds from blood cultures is clinically significant, as it often indicates severe disseminated infection with poor prognosis.
Common Molds Recovered in Blood Cultures
Fusarium spp.
Most frequently isolated mold in blood cultures.
Shows septate hyphae, canoe-shaped macroconidia, and thermotolerance at 37 °C.
Strongly associated with fungemia in neutropenic and transplant patients.
Aspergillus spp.
Rarely grows in blood cultures.
Characterized by acute-angle branching septate hyphae.
Positive blood culture for Aspergillus usually suggests contamination or overwhelming invasive disease.
Scedosporium / Lomentospora spp.
It can mimic Aspergillus microscopically.
Clinically significant in CNS, pulmonary, and disseminated infections.
Notably resistant to amphotericin B.
Mucorales (Rhizopus, Mucor, Lichtheimia)
Broad, aseptate hyphae with right-angle branching.
Rarely isolated from blood culture (more commonly seen in tissue biopsy).
Causes mucormycosis in diabetics and transplant patients.
Comparison of Blood Culture–Positive Molds
Mold
Microscopy (LPCB)
Growth at 37 °C
Blood Culture Yield
Clinical Significance
Fusarium spp.
Septate hyphae, canoe-shaped macroconidia
Good growth
Fungemia, high mortality, and resistance to many antifungals
Fungemia, high mortality, resistant to many antifungals
Aspergillus spp.
Septate hyphae, acute-angle branching, conidiophores with vesicles
Fig. LPCB Microscopy of Blood Culture–Positive Mold (Mag. 1600X)-Note: The LPCB preparation shows septate hyphae with branching. Some arthroconidia/fragmented segments appear along the hyphae. No broad aseptate hyphae (like Mucorales) are seen. Morphology suggests a septate mold, possibly Aspergillus or a related opportunistic mold.
Note: “Blood culture shows cottony fungal growth. LPCB preparation reveals septate hyphae with branching, consistent with Aspergillus species (probable A. fumigatus). Further identification and antifungal susceptibility recommended.”
Keynotes
Fusarium is the leading mold isolated in blood cultures, often representing true fungemia.
Aspergillus and Mucorales rarely yield positive blood cultures; diagnosis usually relies on tissue biopsy and antigen testing.
Scedosporium/Lomentospora are emerging molds in immunocompromised patients and are notable for antifungal resistance.
A positive blood culture for mold usually signifies severe disseminated infection with high mortality.
Final identification requires morphology, MALDI-TOF, or molecular sequencing, along with antifungal susceptibility testing.