Mold Biology and Growth Requirements
Mold biology and growth requirements
This domain tests how fungi colonize building materials - the growth equation, ecological succession, and what spore ecology implies for sampling.
The growth equation
Mold needs four things simultaneously: moisture, a food source, suitable temperature and oxygen. Remove any one and growth stops. Moisture is the only practical control.
- Water activity: most building molds require a_w of at least 0.80, roughly 80% equilibrium relative humidity at the surface. Xerophilic species such as Aspergillus penicillioides and Wallemia sebi grow down to about 0.65.
- Food: cellulose in drywall paper, wood and ceiling tile; starch in settled dust; glues, paints and organic films on concrete.
- Temperature: optimum 20–30 °C; growth slows below 10 °C but does not stop, and most species survive freezing.
- Time: spores germinate within 24–48 hours on wet material; visible amplification typically appears in one to two weeks.
Ecological succession
Primary colonizers - Penicillium, Aspergillus, Cladosporium - exploit condensation and dust. Secondary colonizers (Alternaria, Ulocladium, Epicoccum) follow on damp cellulose. Water-indicator species (Stachybotrys chartarum, Chaetomium globosum, Fusarium, Trichoderma) need chronic saturation plus cellulose, and compete poorly against Penicillium under intermittent wetting.
Spore ecology and sampling
Cladosporium and Alternaria release dry spores readily, so air samples detect them. Stachybotrys and Chaetomium hold spores in sticky slime heads that resist aerosolization; air sampling can be negative despite heavy growth, so tape lifts or bulk samples are required.
Worked example
Drywall under a slow roof leak shows dark green-black growth, yet the room's spore-trap sample looks unremarkable. The correct reading: slime-headed spores were not aerosolized, so the air result does not exclude contamination. Take a tape lift of the growth, moisture-map the assembly, and trace the roof defect.
Traps
Believing mold needs darkness or stale air alone; calling all dark growth Stachybotrys; assuming a negative air sample rules out hidden growth.