- Paecilomyces (and its reclassified relatives Purpureocillium and Isaria) are environmentally common molds widely used as biological insecticides — parasitizing insect pests in agricultural and pest control applications.
- Paecilomyces variotii is the species most commonly encountered in buildings and as an opportunistic human pathogen; it has been isolated from water-damaged materials, HVAC systems, and food products.
- In immunocompromised patients, Paecilomyces/Purpureocillium species can cause serious infections including endophthalmitis, endocarditis, and disseminated disease — notably resistant to many antifungal agents.
- A landmark iatrogenic outbreak — Purpureocillium lilacinum infection linked to contaminated methylprednisolone — caused serious eye infections in recipients across multiple U.S. states in 2012.
- The insect-parasitizing species Beauveria bassiana and Metarhizium anisopliae, historically classified as Paecilomyces relatives, are now major commercial biopesticide products used globally.
Paecilomyces is a genus that has undergone major taxonomic reorganization as molecular tools revealed its polyphyletic nature — the classically defined genus spanning organisms now distributed across multiple genera. Despite taxonomic revision, the organisms collectively encompassed by the historical genus remain ecologically significant as entomopathogenic (insect-killing) fungi, building contaminants, food spoilage organisms, and opportunistic human pathogens.
Taxonomy and Reclassification
The genus Paecilomyces A.H.S. Br. and G. Sm. was established in 1946 and historically included species resembling Penicillium in conidiophore structure but producing conidia in chains from swollen-tipped phialides. Molecular phylogenetics revealed the genus to be polyphyletic — spanning multiple non-sister lineages. Major reclassifications include:
- Paecilomyces lilacinus → Purpureocillium lilacinum (family Ophiocordycipitaceae)
- Paecilomyces fumosoroseus → Isaria fumosorosea
- Paecilomyces farinosus → Isaria farinosa
- Paecilomyces variotii — retained in the genus (now family Thermoascaceae, order Eurotiales)
In clinical and environmental literature, the historical genus name is still frequently used, particularly for P. variotii and P. lilacinum/Purpureocillium lilacinum.
Morphology
Paecilomyces sensu lato shares a broadly similar conidiophore architecture with Penicillium, but distinct features set it apart:
- Phialides: Swollen at the base, abruptly narrowing to a long, needle-like neck — “flask-shaped with an elongated neck,” giving the distinctive silhouette.
- Conidia: Small (2–5 μm), single-celled, in dry, divergent chains that flare outward from the phialide tips rather than remaining tightly parallel as in Penicillium.
- Colony colors: Variable by species — P. variotii produces yellow-brown to tan colonies; Purpureocillium lilacinum produces pale lilac to violet-purple colonies (the basis for both its species name and the color of the reclassified genus name Purpureocillium).
Entomopathogenic Species: Biological Pest Control
The historically Paecilomyces-adjacent entomopathogenic fungi represent one of the most important categories of biological control agents in modern agriculture:
- Beauveria bassiana and Metarhizium anisopliae (now classified in their own families) parasitize a broad range of insects including thrips, aphids, whiteflies, and soil-dwelling grubs. Commercial products based on these fungi are applied to crops as mycoinsecticides — spores contact the insect cuticle, germinate, penetrate through the cuticle, and colonize the insect hemocoel, killing the host within days while producing a white mycelial “bloom” on the insect body.
- Isaria fumosorosea (formerly Paecilomyces fumosoroseus) is similarly used against whitefly, thrips, and other agricultural pests, with products registered in the U.S. and EU.
The 2012 Methylprednisolone Outbreak
A landmark outbreak of fungal meningitis and other infections in 2012 — one of the largest iatrogenic infectious disease outbreaks in U.S. history — was caused not by Paecilomyces itself but by the closely related black mold Exserohilum rostratum from contaminated preservative-free methylprednisolone acetate produced by the New England Compounding Center (NECC). However, Purpureocillium lilacinum (formerly Paecilomyces lilacinus) was implicated in a parallel contaminated drug product causing eye infections — specifically, contaminated triamcinolone acetonide used for intravitreal injection caused P. lilacinum endophthalmitis in multiple patients across several states. This event highlighted the capacity of environmental molds including Paecilomyces-group organisms to cause serious healthcare-associated infections through contaminated injectable preparations.
Human Health Significance
Paecilomyces variotii is the most clinically relevant species in immunocompromised patients:
- Reported infections include sinusitis, pulmonary infection, peritonitis (in peritoneal dialysis patients), and disseminated disease.
- Intrinsic resistance to amphotericin B has been documented in some P. variotii strains — complicating treatment of invasive infection.
- Purpureocillium lilacinum causes a distinct spectrum including onychomycosis, skin infection, ocular infection (keratitis, endophthalmitis), and systemic infection particularly in transplant recipients.
Indoor Building Occurrence
Paecilomyces variotii is a thermotolerant species capable of growth at temperatures up to 50–60°C — giving it unique potential to colonize hot water systems, composting environments, and HVAC components where temperatures exclude most other molds. It has been isolated from soil tracked indoors, water-damaged materials, and food products including bakery goods. It is not classified as a specific water-damage indicator species but may appear in elevated concentrations in damp buildings.
Frequently Asked Questions
What is Paecilomyces and why is it being reclassified?
Paecilomyces is a genus of molds historically defined by shared morphological features (flask-shaped phialides with elongated necks, diverging spore chains). Molecular phylogenetic analysis revealed that organisms grouped under this name belong to multiple unrelated evolutionary lineages — a situation called polyphyly. Major species have been reclassified: P. lilacinus to Purpureocillium lilacinum, and others to Isaria. Paecilomyces variotii retains the genus name. Clinical literature often still uses historical names, so both name systems appear in medical references.
What insects does Paecilomyces kill?
Entomopathogenic fungi related to Paecilomyces — particularly Isaria fumosorosea, Beauveria bassiana, and Metarhizium anisopliae — parasitize a broad range of agricultural pests including whiteflies, thrips, aphids, spider mites, mealybugs, and soil-dwelling beetle grubs. Commercial mycoinsecticide products are registered in the U.S. and EU for use on field crops, greenhouse vegetables, and ornamental plants. These products are considered compatible with integrated pest management (IPM) programs and are approved for organic farming in many certification systems.
Can Paecilomyces cause nail infections?
Yes. Both Paecilomyces variotii and Purpureocillium lilacinum have been documented as causative agents of onychomycosis (nail fungal infection), particularly of the toenails. Like other non-dermatophyte mold nail infections, they may not respond to standard oral antifungals prescribed for dermatophyte nail infections. Culture confirmation and identification to the species level is important because treatment approaches differ between dermatophyte and non-dermatophyte nail infections.
Is Paecilomyces found in buildings?
Paecilomyces variotii is occasionally found in buildings, particularly in association with moist organic material, food storage areas, HVAC components, and water-damaged materials. Its thermotolerance (ability to grow at temperatures up to 50–60°C) distinguishes it from most other indoor molds and gives it access to hot water system biofilms and other warm environments. It is not classified as a specific water-damage indicator species and its presence in typical indoor air samples does not specifically indicate major moisture intrusion.
What antifungals treat Paecilomyces infections?
Antifungal susceptibility varies significantly between species. Paecilomyces variotii is generally susceptible to voriconazole and itraconazole; some strains show reduced susceptibility to amphotericin B. Purpureocillium lilacinum has a different profile — it shows reduced susceptibility to amphotericin B and variable responses to azoles, with voriconazole often showing the best in vitro activity. Because susceptibility is unpredictable and species-specific, culture and antifungal susceptibility testing are important for guiding treatment of any invasive infection in this genus complex.
