Penicillium: From Antibiotic Hero to Indoor Allergen

Penicillium is the most positive press a fungal genus has ever received — Fleming's 1928 observation that P. notatum killed Staphylococcus launched the antibiotic era. The same genus is, in indoor environmental medicine, one of the most clinically significant mould groups. Hundreds of species, several mycotoxin producers, multiple allergen species, and one of the most common moulds recovered from Israeli indoor air samples.

The genus in brief

Penicillium comprises ~350 accepted species. Spore size 2–5 µm — small enough to remain airborne for hours and to penetrate the lower respiratory tract. The colony morphology (blue-green, powdery, growing in concentric rings) is consistent across many species, which is why microscopy alone cannot resolve to species.

Indoor species worth knowing

• P. chrysogenum — the original penicillin producer; in indoor settings, a common water-damage indicator.
• P. brevicompactum — chronic damp indoor environments; produces mycophenolic acid (an immunosuppressant).
• P. expansum — the "blue-green mould" on rotting fruit; produces patulin.
• P. citrinum — indoor air; produces citrinin (a nephrotoxin).
• P. aurantiogriseum — chronic water damage; produces ochratoxin A.
• P. crustosum — common indoors; produces penitrem A, a potent neurotoxin in animal models.

The allergen story

Penicillium is among the top three indoor mould allergens by prick-test positivity in atopic patients (alongside Aspergillus and Cladosporium). The dominant allergen, Pen ch 13, is conserved across species — a Penicillium-allergic patient typically reacts to the genus broadly, not to one species. Cross-reactivity with Aspergillus allergens is partial; Penicillium and Aspergillus are not sensitisation equivalents.

The mycotoxin story

Penicillium species collectively produce more chemically distinct mycotoxins than any other fungal genus — patulin, ochratoxin A, citrinin, mycophenolic acid, penitrem A, verrucosidin, viridicatum toxin, roquefortine. The species-toxin relationship is non-trivial and poorly tracked in commercial reporting. A spore-trap result of "Aspergillus/Penicillium-like 400 spores/m³" tells you nothing about which Penicillium toxins, if any, are being produced.

Where Penicillium grows indoors

• Wallpaper paste on chronically damp exterior walls (P. brevicompactum, P. chrysogenum)
• HVAC drain pans and biofilm (P. citrinum)
• Refrigerator gaskets and forgotten produce (P. expansum, P. crustosum)
• Indoor potted plant soil with persistent over-watering (multiple species)
• Behind tile in chronically wet bathrooms (P. brevicompactum dominant)

Detection strategy

Air sampling reports Penicillium as Asp/Pen-like (combined). Tape lifts give better differentiation. Species-level confirmation requires culture or qPCR. For sensitised patients, qPCR is essential because the relevant allergen exposure is a function of total Penicillium burden across all species, not the dominant species in a single air sample.