What Is Aureobasidium
Aureobasidium is a genus of dark-pigmented fungi that appears as black or dark brown growth on painted surfaces, caulking, window frames, and exterior siding. Unlike true molds that produce spores in visible colonies, Aureobasidium pullulans (the most common species in buildings) produces yeast-like cells alongside occasional spore formation, making it a hybrid organism that behaves differently during inspection and remediation.
Where You Find It
Aureobasidium thrives in high-moisture environments but differs from Aspergillus or Penicillium in that it colonizes exterior and semi-exposed surfaces rather than hidden cavities. You'll encounter it on:
- Silicone and polyurethane caulk around windows and doors, where water bridges the seal
- Painted wood trim and siding with inadequate drainage or condensation issues
- Exterior window frames and the back of gutters
- Bathroom and kitchen tile grout, particularly around tub and shower areas
- Exterior composite decking and pressure-treated lumber
The key difference: Aureobasidium grows where you can see it. If dark growth appears on a caulk line or painted surface, it's worth testing.
Health Effects and Testing
Aureobasidium is a recognized indoor air quality concern. The EPA and CDC do not specify exposure limits for Aureobasidium pullulans, but it appears regularly in indoor air quality assessments. People with mold sensitivities, asthma, or compromised immune systems may experience respiratory irritation, coughing, or allergic reactions to exposure.
Detection requires either tape lift sampling (preferred for visible growth on surfaces) or air quality testing using spore traps. A qualified mold inspector will use AIHA-accredited methods to distinguish Aureobasidium from similar dark-pigmented molds. Tape lifts cost $15 to $50 per sample and take 5 to 7 business days to culture and identify.
Remediation and Prevention
Removing Aureobasidium depends on the scope and location:
- Small surface growth (less than 10 square feet): Clean with a 10 percent bleach solution or commercial fungicide. HEPA vacuum or wet-wipe afterward to remove spores. No containment required if disturbed area is minimal.
- Caulk and sealant: Remove and replace the affected material. Aureobasidium often indicates the original sealant failed. Use mold-resistant caulk formulated with additives that inhibit fungal growth.
- Moisture control: This is the critical step. Identify why water is reaching the surface. Install or repair gutters, improve grading, seal cracks, or add ventilation. Without addressing moisture, Aureobasidium returns in 6 to 12 months.
The EPA's Indoor Air Quality guidelines recommend maintaining relative humidity below 60 percent and ensuring drainage slopes away from foundations.
Common Questions
- Is Aureobasidium dangerous? It's not acutely toxic, but it's a warning sign of moisture problems and can trigger allergic responses. More importantly, where Aureobasidium grows on the surface, moisture is present where it shouldn't be, potentially causing hidden mold in framing or insulation behind walls.
- Can I use bleach to clean it? Yes, but only on non-porous surfaces like tile or painted wood. Bleach kills the visible colony but doesn't penetrate porous materials. If growth is in or under caulk, removal and replacement is the only effective approach.
- How often does it come back? If you address only the visible growth without fixing moisture, expect regrowth within one season. Proper remediation (moisture control plus replacement of sealants) prevents recurrence in most cases.