About Rhizopus stolonifer
Rhizopus stolonifer (commonly called bread mold) is the fast-growing fuzzy gray mold most people recognize from spoiling bread, fruit, and vegetables. It's a Group 2 tertiary colonizer, expected at low levels in most homes. Like Mucor, Rhizopus belongs to the Mucorales and can cause mucormycosis in immunocompromised individuals. Whether the values in this sample are clinically relevant to any specific person is a question for a qualified clinician. Elevated levels are generally associated with wet materials or with spoiling food. A dust result cannot show which, or whether the source is still present.
Ecology and water requirements
Rhizopus stolonifer is classified as a tertiary colonizer in the ecological succession that follows water damage in indoor environments. Primary colonizers (xerophilic Penicillium and Aspergillus species) establish first on damp materials at relatively low water activity; secondary colonizers (Cladosporium) follow as moisture stabilizes; tertiary species like Rhizopus stolonifer require sustained, high water activity (Aw 0.93 or above)[2] and typically appear after weeks to months of continuous wetness. Because of this requirement, its detection in indoor dust samples is one of the more diagnostic available signals of significant, sustained water intrusion[4], distinct from elevated humidity alone. The species cannot establish on materials that dry between wetting events; persistent water contact with cellulose-containing or other organic substrates is required.
Mycotoxin biology
Rhizopus stolonifer produces a range of secondary metabolites with biological activity. The compounds documented in the literature for this species are listed in the chips below. Mycotoxin production is not uniform across all strains and is influenced by substrate, temperature, and water activity[3]. The same isolate may produce substantially different metabolite profiles on drywall paper versus wood versus settled dust[5]. DNA-based detection via the ERMI panel identifies species presence but does not directly measure mycotoxin presence; detection of a mycotoxin-producing species in household dust does not establish exposure to that mycotoxin at any specific concentration.
Detection in dust versus visual inspection
DNA-based detection of Rhizopus stolonifer in settled household dust (the basis of the ERMI panel[1]) integrates spore material from current and recent growth in the home, capturing exposure history over weeks to months. A positive detection in dust does not necessarily indicate currently-active growth at the time of sampling; conversely, an active growth site can produce relatively low dust spore counts if the spores are not being aerosolized (sealed wall cavities, behind vapor barriers, or under flooring). Interpretation requires considering the dust signal alongside building inspection, occupant symptom history, and the broader pattern of species detected on the panel.
Where it’s typically found
Rhizopus stolonifer is documented on the following substrates and locations:
- Damp food items (bread
- fruit
- vegetables)
- wet building materials
- soil
- compost; common in high-humidity areas
Mycotoxins
Compounds documented as produced by Rhizopus stolonifer (presence and quantity vary by chemotype, strain, and substrate):
Mycotoxin production is not uniform across all strains and is influenced by substrate, temperature, and water activity. Detection of a mycotoxin-producing species in household dust does not directly establish exposure to that mycotoxin at any specific concentration.
On your ERMI report
Rhizopus stolonifer is one of the 10 EPA environmental reference species used to compute the SLG2 component of the ERMI score (sometimes labeled "Group 2" in EPA documentation). Group 2 species are commonly found in outdoor air; their levels are subtracted from the indicator-species sum, which means high outdoor-reference detections actually reduce the final ERMI. A low-level detection of Rhizopus stolonifer is common, because these species are also found in outdoor air. Our interpretation report reads its level alongside the indicator species.
Related species
References
- Vesper SJ, McKinstry C, Haugland RA, et al. Development of an Environmental Relative Moldiness Index for U.S. homes. J Occup Environ Med. 2007;49(8):829–833.
- Pitt JI, Hocking AD. Fungi and Food Spoilage. 3rd ed. New York: Springer; 2009.
- Nielsen KF. Mycotoxin production by indoor molds. Fungal Genet Biol. 2003;39(2):103–117.
- Institute of Medicine. Damp Indoor Spaces and Health. Washington, DC: National Academies Press; 2004.
- Miller JD, McMullin DR. Fungal secondary metabolites as harmful indoor air contaminants: 10 years on. Appl Microbiol Biotechnol. 2014;98(24):9953–9966.