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Bacteria and the
Indoor Environment
By: Tharanga Abeysekera and Dr. Harriet Burge
Bacteria
(prokaryotes) are microscopic organisms that differ from animal cells (eukaryotes) because they have no
organized nucleus and no membrane systems in the cells. They do have DNA, as all living cells do however,
the DNA exists in the cytoplasm and not in a membrane bound nucleus as seen in the animal cells. Nearly all
bacteria are free-living and can be cultured in the laboratory. A few require living cells for reproduction
or are difficult to grow on standard laboratory media.
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.
Coryneforms
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.
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