home microBEnet Research, MoBE Postdoctoral Fellowship Fungi and bacteria grow in carpet at elevated relative humidity

Fungi and bacteria grow in carpet at elevated relative humidity

Floor dust is an important source of human exposure to microbes due to dust resuspension, especially from carpeted floors. Sources of microbes in floor dust are known to include outdoor air, tracked-in soil, growth on materials, and shedding from occupants or pets, but we wanted to know if growth may also contribute to these microbial communities.

To answer this question, we conducted a study funded through the Microbiology of the Built Environment Postdoctoral Fellowship Program. We embedded dust into a worn residential, medium-pile, nylon carpet from the same home and incubated the carpet coupons at equilibrium relative humidity (ERH) levels between 50-100%. We measured fungal and bacterial communities using qPCR and DNA sequencing of the 16S/ITS regions.

After one week, we identified fungal growth at ≥80% ERH and bacterial growth only at 100% ERH. At 85% ERH, this fungal growth continued until we ended the experiment after 6 weeks. The moisture content was the limiting factor for growth as C, N, P, and S were in excess of stoichiometric requirements. ERH was the dominant factor driving the microbial communities as determined by principal coordinate analysis (PCA). The predominant fungal genera at elevated ERH were Aspergillus, Penicillium, and Wallemia.

We also constructed a rough model to determine the impact that this growth might have on exposure. At these elevated ERH levels, over half of the airborne fungi could originate from growth in the dust after resuspension from the floor by occupants. However, this model needs to be refined and validated in future studies, and does not currently account for differences in dust resuspension that might occur due to relative humidity.

The relative humidity levels included in this study are higher than those found in most typical homes and the recommended level of 30-50%. These levels would be more likely in a hot, humid climate without the use of air conditioning or under other suboptimal conditions. However, this work demonstrates that moisture in the air may be sufficient to support microbial growth in carpet, and this growth has the potential to contribute to human exposure.

Fungal and bacterial growth after (a, b) 1 week and (c, d) 3-6 weeks.
Fungal and bacterial growth after (a, b) 1 week and (c, d) 3-6 weeks. OD = original dust.

 

Manuscript (open-access): Dannemiller, K. C., Weschler, C. J. and Peccia, J. (2016), Fungal and bacterial growth in floor dust at elevated relative humidity levels. Indoor Air. doi:10.1111/ina.12313 Available at: http://onlinelibrary.wiley.com/doi/10.1111/ina.12313/full

Karen Dannemiller

Karen Dannemiller is an Assistant Professor at Ohio State University with a joint position between Civil, Environmental, and Geodetic Engineering and Environmental Health Sciences. Her research interests include fungi in the indoor environment and associations between microbial exposures and childhood asthma.

8 thoughts on “Fungi and bacteria grow in carpet at elevated relative humidity

  1. I am sorry, but this is old news. Reinventing the wheel again! You should have checked old litterature before sending your paper for publication.

  2. I have made a list of publications of that Jordan Peccia would call for Neardenthal knowledge (cfr. last Healthy Buildings Europe). Some of the publications are about what is known about general mould ecology in buildings, including carpets. Worth reading for those working on this. Besides, there are lots of papers on fungal growth on different building substrates at different levels of different abiotic factors in Healthy Buildings and Indoor Air Congresses publications. All building microbiologists should be familiar with these.
    Coppock and Cookson (1951). The effect of humidity on mould growth on construction materials. J. Sci. Food Agric. Pp 534-537.

    Ayers 1969. The effects of moisture and temperature on growth and spore germination in some fungi. J. Stored Products research 5:127-141.

    Smith & Hill. 1982. Influence of temperature and water activity on germination and growth of Aspergillus restrictus and A. versicolor. Trans. British Mycol Soc. 79:558-560.

    Adan & Samson. 1994. Fungal disfigurement of interior finishes. In Singh. J (ed). Building Mycology pp. 130-159. Chapman & Hall, London.

    Beguin & Nolard 1996. Prevalence of fungi in carpeted floor environment: analysis of dust samples from living-rooms, bedrooms, offices and school classrooms. Aerobiologia 12:113-120.

    Korpi et al. 1997. Microbial growth and metabolism in house dust. Int. Biodet. Biodeg. 40:19-27.
    Pasanen et al. 2000. Fungal growth and survival in building materials under fluctuating moisture and temperature conditions. Int. Biodet. Biodeg. 46:117-127.
    Engelhart et al. 2002. Occurrence of Toxigenic Aspergillus versicolor isolates and Sterigmatocystin in Carpet Dust from Damp Indoor Environments. Appl. Env. Microbiol. 68:3886-3890.

    Chao et al. 2001. Dustborne fungi in large office buildings. Mycopathologia 154:93-106.

    James Scott 2001. Ph.D. thesis. Studies on indoor fungi. Toronto 2001.
    Nielsen K.F. 2002. PhD thesis. Mould growth on building materials. Secondary metabolites….. Danmark.

    Nielsen et al. 2004. Mould growth on building materials under low water activities. Influence of humidity and temperature on fungal growth and secondary metabolism. Int. biodet. Biodeg. 54:325-336.
    Yang & Li. 2007. Ecology of fungi in the indoor environment. 191-214. In: Sampling and analysis of indoor microorganisms. Wiley & Sons.

    Johansson et al. 2005. Microbiological growth on building materials. Critical moisture levels. State of the art. SP Swedish Nat. Testing and Res. Inst.

    Johansson et al. 2012. Test method for determining critical moisture level for mould grwth in building mat. PhD thesis: Critical moisture conditions for mould growth on building materials.

    I do not have time to continue with a longer list….

  3. Maria,

    Thank you for the opportunity to discuss how our paper is novel and why we are excited about it. There is a great history of many papers that discuss microbial growth and presence in building materials. There are excellent works in your list here, and many others as well. Before our paper, it had been established that microbial growth occurs on other building materials such as gypsum generally around ~80% relative humidity (as mentioned in the introduction to our paper). It had also been established that microbes are present in house dust and other sources have been identified (also mentioned in our introduction). We previously identified the Korpi et al, 1997 study as a systematic laboratory chamber study of microbial growth in house dust (not carpet), which we cited in our paper. The other papers you mentioned (which make important contributions on their own) fell into two categories that make them distinct from this work: (1) papers that do not include carpet and (2) environmental sampling studies in which fungi are identified but potential growth could not be studied directly, not systematic laboratory chamber studies of relative humidity compared to microbial growth in carpet (see full list divided into categories below). As stated in Yang and Li, 2007 “…it should always be kept in mind that isolation and identification of fungi from carpet or carpet dust does not by itself indicate fungal growth.” Taken together, all these papers provided an excellent rationale to conduct this study. The results in this paper are consistent with this other work in identifying that growth occurs under elevated relative humidity in carpet containing dust around a similar moisture threshold to other materials.

    Our results are novel in providing quantitative growth rates in carpet, refined taxonomic resolution from DNA sequencing for both fungi and bacteria, insights into richness, and a rough resuspension model on how this growth might impact human exposure. Just as any other study, this work needs to be read in context for the additional contribution it makes to the literature to build upon what we already know. If you read the open-access paper itself (instead of only the short blog post) we mention some of these studies to put this paper into context of other work.

    ***Papers that do not include carpet:***
    Coppock and Cookson (1951). The effect of humidity on mould growth on construction materials. J. Sci. Food Agric. Pp 534-537. Materials considered: Brick, wood, painted wood, distempered surfaces

    Ayers 1969. The effects of moisture and temperature on growth and spore germination in some fungi. J. Stored Products research 5:127-141. Materials considered: Agar and cellulose strips

    Smith & Hill. 1982. Influence of temperature and water activity on germination and growth of Aspergillus restrictus and A. versicolor. Trans. British Mycol Soc. 79:558-560. Materials considered: Malt agar

    Adan & Samson. 1994. Fungal disfigurement of interior finishes. In Singh. J (ed). Building Mycology pp. 130-159. Chapman & Hall, London. Materials considered: plaster/gypsum, wallpaper. No full laboratory study of growth vs relative humidity in carpet.

    Korpi et al. 1997. Microbial growth and metabolism in house dust. Int. Biodet. Biodeg. 40:19-27. Cited in our paper and one of the more relevant studies looking at dust (not in carpet). Was done in 1997 (so no sequencing). Growth was based on CO2 levels with some culturing afterward. Compared to our paper, there is much lower taxonomic resolution, no quantitative growth rates at various levels, the nutrient analysis as not as in-depth, and there is no resuspension modeling. This study does do some interesting work looking at the MVOCs from the dust.

    Pasanen et al. 2000. Fungal growth and survival in building materials under fluctuating moisture and temperature conditions. Int. Biodet. Biodeg. 46:117-127. Materials considered: Gypsum board, particle board and wood board

    Nielsen K.F. 2002. PhD thesis. Mould growth on building materials. Secondary metabolites….. Danmark./
    Nielsen et al. 2004. Mould growth on building materials under low water activities. Influence of humidity and temperature on fungal growth and secondary metabolism. Int. biodet. Biodeg. 54:325-336. Great study on 21 other building materials (no carpet), also cited in our paper. Once again, a great study but does not consider carpet.

    Johansson et al. 2005. Microbiological growth on building materials. Critical moisture levels. State of the art. SP Swedish Nat. Testing and Res. Inst. Discusses other materials, no carpet

    Johansson et al. 2012. Test method for determining critical moisture level for mould grwth in building mat. PhD thesis: Critical moisture conditions for mould growth on building materials. Discusses other materials, no carpet

    ***Papers that are environmental sampling studies, not a systematic study of relative humidity vs. growth in laboratory chambers:***
    Beguin & Nolard 1996. Prevalence of fungi in carpeted floor environment: analysis of dust samples from living-rooms, bedrooms, offices and school classrooms. Aerobiologia 12:113-120. This study sampled carpet dust from 100 carpets and speculated on growth but did not directly study growth compared to other sources (tracked-in soil, etc.). Not a systematic laboratory chamber study of relative humidity vs. growth in laboratory chambers.

    Engelhart et al. 2002. Occurrence of Toxigenic Aspergillus versicolor isolates and Sterigmatocystin in Carpet Dust from Damp Indoor Environments. Appl. Env. Microbiol. 68:3886-3890. This study looks at 11 carpet dust samples from homes with mold problems, but is not a systematic study of relative humidity vs. growth in laboratory chambers.

    Chao et al. 2001. Dustborne fungi in large office buildings. Mycopathologia 154:93-106. Also an environmental study, this one measured water activity in the carpet but did not present results about water activity vs. growth and also was not able to identify the origin of fungi based on growth or other sources (outdoor air, etc.). Study states specifically that it could not distinguish if findings were the “…result of fungal growth in carpet or was due to contamination from a different origin.” Not a systematic study of relative humidity vs. growth in laboratory chambers.

    James Scott 2001. Ph.D. thesis. Studies on indoor fungi. Toronto 2001. Another great work looking at fungi in house dust of 369 homes. Really great data here, but not a laboratory-based chamber study of fungal growth in carpet.

    Yang & Li. 2007. Ecology of fungi in the indoor environment. 191-214. In: Sampling and analysis of indoor microorganisms. Wiley & Sons. Not a systematic study of relative humidity vs. microbial growth in carpet. Cites some of the other work listed here and specifically states on page 202 “However, it should always be kept in mind that isolation and identification of fungi from carpet or carpet dust does not by itself indicate fungal growth.”

  4. Karen,
    Your paper is called “Fungal and bacteria growth in floor dust”. That is, carpet dust, not carpet. Your carpet is made of nylon, which probably acts as a mechanic substrate to sustain the dust. Nylon has been used as a “recipient”, for example, nylon mesh- bags containing soil to study ectomycorrhizal growth in the lab (http://onlinelibrary.wiley.com/doi/10.1046/j.0028-646x.2001.00199.x/full). You have not demonstrated that the nylon has been degraded by the fungi/bacteria. You mention that there are enough nutrients in the floor dust, and this is true. On the other side, all soiled/dusty building substrates (including minerals, synthetic fibers, and of course vegetal and animal fibers) can be overgrown by fungi and bacteria (general knowledge, no need for ref., I hope). There are systematic laboratory studies of everything used in buildings that can be hung in lab chambers containing salt solution throughout the literature, also in recent literature because of the “green wave”.

    You make carpet, which is usually a sink of organisms, to a source by exposing it to what Jeff Siegel calls for “extreme conditions” in the laboratory. I think you should have included the “in vitro” aspect of your work in your title. Many people read only the title and think that because they have carpets at home, they have also microbial growth. Scary.
    You also speak about “refined taxonomic resolution from DNA sequencing for both fungi and bacteria. I would say that Aspergillus, Penicillium and Wallemia for fungi, or Phyla for bacteria, is low taxonomic resolution. You have just got the primary colonizers, in other words, the generalists. Working with OUTS is not refined taxonomy. As Wertenbaker said, “A science can scarcely be said to exist before its material, whether it be species or elements or geologic and geographic features, has been organized and classified”.
    As and additional contribution, you say that “our results demonstrate that ERH is an important variable for defining the rates and community dynamics of microbial growth in floor dust, 80% for fungi and 100% in bacteria”. We knew this.

  5. Detailed experimental protocols and results are presented in the full version of the paper, available at the link above. Please feel free to contact me directly for additional comments/questions.

  6. As an outsider traipsing through the field of microbes, I read the above conversation with curiosity. (I am active online posting as a communicator/educator). I would just like to compliment all participants for a truly informative and rational exchange of ideas. Would that other discussions were handled in such a dignified way! To come across people addressing challenges to their personal statements (and career research) with restraint and courtesy is a find! What a perfect classroom example for academics to share with current grad students. Reading this as a guide to conducting professional conversations online in a respectful manner would be of lasting benefit to your students—So nice to take in, like a breath of fresh air!

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