The microbes on surfaces could be considered inputs if they lead to indoor emissions, or they could be losses resulting from deposition . Different building materials and environmental conditions can create different selective pressures for microorganisms if varied over wide ranges, which can result in differential survival and persistence rates. However, much of the previous work investigating the impact of environmental conditions on microorganism survival has focused on infectious organisms. For the vast majority of building operating conditions, more recent evidence suggests that the majority of bacteria and fungi found on surfaces are not actually growing in what are mostly inhospitable environments. However, it is likely that many of the microbes identified in areas of the home with periodic water exposure are alive; of course many cleaning events also introduce water, but they also introduce chemicals that are designed to remove or reduce microbes. Surprisingly, while studies have shown the impact of cleaning products on specific microbial groups such as fecal coliforms, no published studies have characterized how they impact diversity or community structure within buildings. Approaches for studying the active portion of microbial assemblages while still culture independent are beginning to be applied to indoor environments, and future work is likely to inform the extent of microbial activity and persistence in the indoor environment. Importantly, while it is likely that most microbes deposited onto surfaces become inactive or die,drying rack for weed these microbes may remain possible sources of allergens.The effects of moisture problems on the growth of indoor microorganisms have long been examined due to associations between indoor dampness and ill health outcomes. Moisture is the limiting factor for microbial growth in the indoor environment, and fungi are more tolerant of low-moisture conditions than bacteria.
Aside from direct input of bulk-phase water, either intentionally or unintentionally, levels of adsorbed water may be sufficient to support growth. For instance, growth has been observed on wood at an air relative humidity of 78%, on gypsum board at 86%, and in floor dust at 80%. While water availability is generally thought to be the limiting growth factor, critical surface moisture levels are challenging to define. Growth can occur directly on a wide range of building materials, such as insulation, concrete, paper, paints, and glues, and some building materials may come pre-contaminated with degrading fungi. Interestingly, while high relative humidity can support microbial growth, experiments indicate that spore release for some fungi can be higher under lower relative humidity. Often saprophytic fungi that are also abundant as aerosols are commonly found on damp building materials. The most common genera in moisture-damaged buildings include Aspergillus, Penicillium, Cladosporium, Eurotium, and Chaetomium, among others. Historically, most research has relied on culture-dependent, microscopic, and biochemical assays of microbial presence in buildings, while new DNA sequence-based approaches are beginning to be applied . Regardless of the methodological tool, there are analytical issues that persist independent of the specific approach when studying aerosols, namely identifying an indoor source of microbial contamination rather than simply detecting the presence of a microbe indoors. For aerosols, two approaches have typically been taken. In one approach, the microbial composition of aerosols in moldy homes is compared to dry homes; in another, indoor and outdoor concentrations of taxa are compared. The two approaches have also been used simultaneously.The former formed the basis for the Environmental Relative Moldiness Index , which sought to identify fungal species that may be informative for determining the mold-burden of a building. For building materials, the taxonomic identification of growing organisms, versus merely present, relies on direct culture and microscopic examinations of tape lifts. While it is expected that unintended water intrusion would lead to greater microbial growth and detectable microbial biomass when compared to “dry” homes, this pattern is not generalizable. In some studies of floor dust, an increase in moisture in the building is associated with an increase in fungal richness , while other studies conducted at the site of fungal growth have demonstrated dominance of a small number of species with increased moisture, and thus an apparent decrease in richness.
Therefore, the increased overall richness seen in homes with increased moisture may be due to contributions from growth at multiple locations. For composition , it might be predicted that moldy homes would have a distinct microbial makeup, as they would support the growth and persistence of certain taxa that would not thrive in a dry home. A recent study of the 2013 flood in Boulder, Colorado demonstrated the lasting effects of moisture in a home. After remediation had been completed, previously flooded homes still retained different microbial communities when compared to non-flooded controls. In particular, fungal concentrations were three times higher in flooded compared to non-flooded homes, and flooded homes had higher concentrations of Penicillium, Pseudomonadaceae, and Enterobacteriaceae.Indoor chemistry may be affected when fungi, bacteria, and other microbes produce chemical metabolites, especially on wetted building materials. Microbial volatile organic compounds have been isolated by measuring emissions from microbecolonized materials, often in laboratory chambers. Common indoor MVOCs are summarized in Table 2. Frequently observed chemical classes include alcohols, carbonyls, furans, terpenes and terpene alcohols, and sulfides. Semivolatile toxins are also produced by mold growing on building materials.However, the actual impact of microbes on indoor chemistry may be weak, since MVOCs may only be slightly elevated even in moldy versus non-moldy spaces, if at all, and the concentrations may not be that high compared to other VOCs typically present indoors. Also, MVOCs from microbial emissions are difficult to isolate, because no MVOCs are exclusively emitted from any particular species or genera, or even from microbes only. That said, the prevalence of sick building syndrome symptoms have been previously associated with MVOCs, including 1-octen-3-ol, 2-pentanol, 2-hexanone, 2-pentylfuran, and formaldehyde . Beyond the microbial influence on indoor chemistry, chemical compounds and physicochemical states could also influence the indoor micro-biome. Microbes growing on building materials may be influenced by adsorbed water or organic films, as well as compounds from the nearby air.
Though little is known about how these variables impact microbial communities, certain inferences may be drawn. Adsorbed water may be a few monolayers thick, and more than that if the surface is wetted. Most microbes prefer neutral pH ranges, and Corsi et al. proposed that changes in the concentration of carbon dioxide, ammonia, or other compounds indoors might lead to pH changes in these surface water films in such a way as to influence microbial growth or diversity. Though organic surface films may resemble each other among surface types across different indoor spaces, some films could become more toxic over time due to absorption of harmful semivolatiles, such as pesticides. Furthermore,vertical cannabis airborne chemicals could influence microbes. Russell et al. demonstrated that bacteria on roots of plants, and this effect could conceivably occur with microbes in indoor environments. Microbes might also be inactivated by direct oxidation from hydroxyl radical or ozone on surfaces. Finally, a related focus of indoor micro-biome research and chemical interactions has been on whether different building materials harbor microbial communities of differing composition. Studies with wetted materials do indicate some differences in the microbial composition and metabolite production based on growth substrate. For example, wooden materials show greater fungal diversity than plasterboard or ceramics , and cellulose based materials are more sensitive to contamination by fungal growth than inorganic materials such as gypsum, mortar, and concrete. However, field studies in non-wetted buildings have challenged the viewpoint that substrate composition drives microbial community structure by showing that source strength dominates instead, e.g. Refs.. Most recently, a study in offices assessed the impacts of geography, material type, location in a room, seasonal variation, and indoor and micro-environmental parameters on bacterial communities of standardized surface materials. Bacterial communities did not depend on the surface material itself, but they did depend on geography and location in the room. Specifically, floor samples of all surface materials showed richer microbial assemblages than other locations within the rooms, a finding also observed in a recent study of public restrooms.Many previous studies of the indoor micro-biome relied on culture-based methods, microscopic identification, or biochemical assays, such as measuring ergosterol or ATP. More recently, the use of high-throughput DNA sequencing has allowed for a more thorough characterization of microbial communities. Analysis can involve targeted sequencing of specific genes, sometimes called amplicon sequencing or “barcoding” because it uses a common region to identify the microbes present, or metagenomics, which aims to sequence randomly from all of the genetic material found in a given environmental sample. Sequence-based approaches offer several advances over culture- or microscopy-based techniques in identifying microbes in buildings. In addition to the increased efficiency by which microbes can be detected compared to these previous methods, DNA-based detection often facilitates the refined identification of species. Moreover, culture-based analysis may not detect organisms in a “viable but not culturable” state. On the other hand, sequence-based approaches cannot differentiate the DNA of viable and non-viable organisms or other fragments. A complementary approach would be to combine existing biochemical assays with emerging DNA based approaches to provide a fuller view of microbial activity and diversity. Ironically, the detection of many additional species can result in greater analytical challenges, increasing the difficulty of separating out the “signal” from the “noise.” The vast amount of data generated with high-throughput sequencing can require the use of additional statistical tools such as methods to control for many comparisons in an analysis, and these may be borrowed from other genetic methods.
The same sample is not typically analyzed by different methods , often because of logistical issues surrounding the processing, but studies that have used a combination of approaches have shown that they offer different but complementary views of the indoor micro-biome. Quantitative Polymerase Chain Reaction provides quantitative information on the abundance of a specific taxonomic group of interest. The use of qPCR with universal fungal or bacterial primers can provide a general estimate of total bacterial genomes or fungal spore equivalents in a sample, although these determinations of biomass based on universal primers are estimates of concentration due to differences in gene copy number and amplification bias across different species. Despite potential biases, qPCR analyses may be done in conjunction with DNA sequencing to improve understanding of microbial exposure and to yield quantitative estimates of the concentrations of individual species. Using these new techniques, the most significant contribution to the literature has arguably been the acknowledgement of the sheer diversity of microorganisms in buildings. Often, hundreds to thousands of OTUs are identified by any given study . Since it remains unclear whether overall microbial diversity itself or individual microbial groups are more important to human and building health, current techniques that better capture overall microbial diversity may be positioned to answer long-standing questions in the field. Moreover, there are opportunities for further expansion to broader taxonomic groups, including viruses, and to analyze different targets, such the RNA transcripts and proteins to more fully characterize microbial gene expression and proteins of interest in the indoor environment.Perhaps the most practical question while investigating the micro-biome of buildings is the choice of sampling methodology. It would be ideal if common practices were used to facilitate understanding and comparison across studies. There are many biological sampling methods available, each with distinct advantages and disadvantages. Most require sample collection followed by offline analysis, although several newer on-line techniques are also available. While there is at present no “gold standard” method that meets all requirements for sampling and subsequent analysis for all purposes , below we summarize many commonly used methods for biological sampling in indoor environments and discuss considerations on spatial and temporal resolution.Moistened sterile swabs are widely used for biological sampling directly from surfaces, although it can be difficult to obtain adequate biomass from some locations. Settled dust samples are also collected using wipes or vacuum filter devices, as they represent an integrated record of microbial communities in a space. It is important to consider the size cutoff of the filter for vacuum collection, since larger particles may dominate the composition analysis but are not likely to contribute significantly to indoor exposure due to rapid settling after resuspension. More traditional approaches include tape lifts and contact plates for microscopy and culturing.