Wallemia sebi
Wallemia sebi is a xerophilic, salt-tolerant mold that grows below 0.75 water activity. Indoors it is a house-dust and dry-goods organism rather than a wet-building one, which is why it sits at the dry end of the water-damage panel.
What it is
Wallemia sebi belongs to the Wallemiomycetes, a small and evolutionarily isolated class of fungi whose members are defined by tolerance of low water availability. The genus is best known from salted and sugared foods, from salterns, and from dried products where its growth is a spoilage problem.
It is also, consistently, a house-dust organism. That combination — food spoilage and indoor dust — is the clue to how it should be read on a panel. It is recovered from environments that are dry and solute-rich, not from saturated drywall.
The water activity figures
Growth has been reported at water activities as low as 0.69 to 0.75, which places it well below the roughly 0.80 aw that most indoor fungi require. Its optimum sits considerably higher, around 0.92 to 0.97.
What distinguishes it from many xerophiles is that its tolerance is largely independent of which solute is used to lower the water activity. It is both extremely halotolerant and chaotolerant, with reported optimum growth at 4% to 12% sodium chloride. In practical terms, it can colonise materials whose water is chemically tied up as well as materials that are simply dry.
The gap between this species and the wet end of the panel is the point. Stachybotrys chartarum cannot begin below roughly 0.89 aw. Wallemia sebi is growing at water activities where Stachybotrys would be desiccated.
What finding it tells you about a building
It indicates a dust reservoir, and often a pantry, a stored-goods problem, or long-standing settled dust that has never been fully removed. It does not indicate that something in the structure is wet.
It is also worth knowing that this species is rarely recovered by the sampling methods most homeowners encounter first. Surface swabs and short-duration air sampling are poorly suited to an organism that lives in settled dust and sporulates modestly. DNA-based dust panels find it; a spore trap run for ten minutes generally will not. Absence from one kind of sample is not evidence of absence.
On a panel, then, its usefulness is as a counterweight. A result showing Wallemia sebi and Aspergillus penicillioides and nothing above 0.85 aw is telling a coherent story about dust and ambient damp, and that story does not call for opening walls.
Health, and the limit of what this page can say
Wallemia species have been studied in connection with respiratory sensitisation, and the genus has been reviewed as a potential health concern in its own right rather than only as a spoilage organism. That is a statement about a research literature, not about any particular building or person.
Health questions about mold exposure belong with a clinician and, on this site, with the separately reviewed health pages. A species identification establishes what is present; it does not establish exposure, dose or effect.
Sources
- Zalar et al., The genus Wallemia — from contamination of food to health threat (Microorganisms, 2018)
- Zajc & Gunde-Cimerman, Osmoadaptation strategy of the halophilic fungus Wallemia ichthyophaga
- Stevenson et al., Is there a common water-activity limit for the three domains of life? (ISME Journal, 2015)
- WHO Guidelines for Indoor Air Quality: Dampness and Mould (2009)
Questions people ask
- What is Wallemia sebi?
- A xerophilic and highly salt-tolerant mold, well known as a spoilage organism on salted and dried foods and consistently recovered from house dust. It grows at water activities below 0.75, far drier than most indoor fungi tolerate.
- Does Wallemia sebi indicate water damage?
- Not by itself. It sits at the dry end of the water-damage panel and is more consistent with a dust reservoir or stored dry goods than with a leak. The species requiring above 0.90 water activity are the ones that indicate sustained wetting.
- Why does it show on a dust panel but not on a swab?
- Because it lives in settled dust rather than as visible surface growth, and sporulates modestly. DNA-based dust sampling is well suited to it; surface swabs and short air samples frequently miss it, so a negative from those methods says little.
- Is Wallemia sebi toxic?
- The genus has been reviewed in connection with respiratory sensitisation and bioactive compound production. That is a research literature rather than a finding about any particular home, and an identification does not establish exposure or harm.