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Bacteria and the Indoor Environment | Fungus of the month: Trichoderma

Bacteria and the Indoor Environment

By Tharanga Abeysekera and Dr. Harriet Burge

Bacterial populations can be cultured from air samples (culture plates, impingers, etc), from surface samples (swabs, contact plates, etc), and from bulk samples (water samples, pieces of solid material, dust, etc).

In situations where populations of unknown bacteria are being studied, especially indoor environmental bacteria, analysis is necessarily based on culture. Although many bacteria can be identified by PCR-based techniques (which compare unknown DNA fragments to known DNA probes), most of these are human pathogens that are unlikely to be present in environmental samples. If one is tracking a specific human pathogen in the environment, PCR is probably the best approach.

Identification of bacteria by cultural analysis is based on morphology (e.g., spherical, rod-shaped, etc), by staining reactions (e.g., Gram-positive or negative, acid fast), and by the pattern of results from a series of physiological tests.

Bacteria are of importance to humanity because they process dead organic material in the environment; they cause infections and, less commonly, allergic reactions; they contaminate food, sometimes producing some extremely potent toxins; and, finally, are responsible for the production of some common food products (e.g., vinegar).

Bacteria are always present in all indoor environments and are often the most abundant microorganisms present. These include primarily:

Staphylococcus sp.
Micrococcus sp.
Bacillus sp.
Gram-negative non-fermenters

Most of these bacteria are shed from human skin surfaces. It is not surprising to find hundreds of thousands of bacteria per gram of dust in carpets. As long as the bacterial types are a mixture of those listed above, there is generally no cause for concern. Among the Staphylococcus species that are commonly found indoors is Staphylococcus aureus, which is an important pathogen in hospital environments. Should we be concerned if Staphylococcus aureus is found on air or surface samples in other indoor spaces? Probably not unless it is the predominating colony on the plate (e.g., covering 80% or more of the plate).

Bacteria may also enter with outdoor air or floodwater, and may also grow in indoor environmental reservoirs. Common indoor reservoirs are water systems (including drinking water), humidifiers, fish tanks, very wet organic material, and spoiled food. Understanding the populations of bacteria likely to develop in each of these reservoirs is crucial to interpreting sampling data.

Formal guidelines for interpreting bacterial populations in sampling data have not been established. We consider that a mix of skin-surface bacteria in indoor air, surface, bulk and dust samples is normal, even if levels are relatively high. High levels of these organisms are generally indicative of human activity during sampling. It is very important to remember that people doing sampling also shed these bacteria and this bacterial cloud (which is not intrinsic to the environment being studied) will appear as part of the data.

If there has been a sewage spill or flood, then Gram-negative enteric bacteria are to be expected. These are shed from the digestive systems of people and animals. Food should not be contaminated with these organisms, and such environments should be thoroughly cleaned. This is one area where a disinfectant is appropriate, especially if children are present. The enteric organisms are generally not hazardous once dead. Samples can be collected and analyzed specifically for these enteric bacteria. If collecting air samples on culture media (for example, to track whether or not these bacteria are traveling into occupied spaces from an identified reservoir) MacConkey's agar should be used. For bulk or swab samples, the analyst will also use this agar, on which indicator enteric bacteria can be counted directly. Note that PCR methods will indicate the presence of these organisms whether they are dead or alive.

Humidifiers are an important source for bacterial exposure that may lead to allergic type disease. Humidifier water and scale scrapings should always be sampled where chest tightness, cough, and fever are associated with a particular indoor environment. One group of bacteria that is often associated with these symptoms is the thermophilic actinomycetes. These organisms require temperatures in excess of 50°C for growth, and are not detected when plates are incubated at the usual 37°C. The presence of thermophilic actinomycetes in air or in any reservoir in occupied environments is of concern.

Is it necessary to have an outside comparison for bacterial air sampling? Many people do collect these samples, however, it is not uncommon to find inside counts higher than those outdoors. Unless some outdoor activity is occurring that is likely to produce concentrated bacterial aerosols (e.g., farming activities), outdoor air samples rarely contribute to the interpretation of indoor air samples. Since bacteria can be found in all environments, to help assist in the interpretation of data, it is best to also collect a sample from a "non-impacted" area. This is especially true for surface and bulk samples.


Fungus of the Month: Trichoderma

By Eric Schile

The genus Trichoderma contains about 40 species. The taxonomy of this genus has been, and continues to be chaotic. One of the difficulties that exists in species level identification with this genus, is that many species are very similar microscopically and macroscopically in morphology. Trichoderma species are present in nearly all soils worldwide. Three species, T. viride, T. harzianum, and T. koningii, are usually found in indoor environments on building materials such as wallpaper, tiles, wallboard, and wood that is rich in cellulose.

Generally, Trichoderma species require relatively higher water activity than some other indoor molds such as Penicillium or Aspergillus. Like Stachybotrys, Trichoderma species produce their spores in a sticky matrix which means aerosolization of spores occurs less frequently than, for example, Penicillium. Trichoderma spores appear similar in shape and size to Penicillium and Aspergillus, but form in sticky clumps with a distinctive green pigment rather than in chains. Because of these distinctive morphological features, Trichoderma can be readily identified on tape lifts. It can also be readily identified on spore traps when clumps of spores are present. Trichoderma spores generally can disseminate through rain, insects, water splash, and wind when dried out.

Usually, Trichoderma growth on surfaces appears greenish in color and has a fuzzy appearance, similar to some Penicillim and Aspergillus species. Certain Trichoderma species can grow very quickly (i.e. 24 to 48 hours). Because of this rapid growth, Trichoderma can easily compete with other fungal colonies in culture and can overgrow the culture media before other fungi can really get started.

Some species produce distinct odors. Trichoderma viride produces a coconut odor in culture. This odor might not be unique to T. viride and other species of Trichoderma may produce a similar odor. The presence of such odor is a good indication that Trichoderma is present in a sample.

Many species of Trichoderma have been shown to be effective for controlling a wide range of plant pathogens. Their effectiveness is due to an ability to grow toward the hyphae of other fungi, coil about them, and degrade the cell walls of the target fungi. This process, called mycoparastitism, limits activity and growth of plant pathogenic fungi. The antifungal properties of this fungus have been known since the 1930s and have recently been utilized for commercial purposes. One successful application has been for Botrytis rot control on apple and strawberry crops.


This article was originally published on June 2005.