Home > Mold, Microbes > Bacteria in Indoor Environment

Bacteria are microscopic organisms found in indoor environments typically come from human sources (skin and respiration) or from the outdoors. Like mold, most of the bacteria found in the air in buildings are saprobes meaning they grow on dead organic matter. As far as building envelopes are concerned the primary concern is about bacteria colonies that may grow in damp areas. Most of the 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 below, there is generally no cause for concern.

The most abundant bacteria present include:

Micrococcus sp: Micrococcus species are human shed bacteria and are caused by the normal shed of skin. It is found in areas of higher occupant density and/or inadequate ventilation. Micrococcus species are generally regarded as being harmless bacteria. Normally, these bacteria are removed through ventilation systems or cleaning procedures such as mopping or vacuuming.

Bacillus sp: Bacillus sp mainly associated with soil and dust. Appropriate temperature and moisture with deposited dust on hard surfaces allow for ideal growing conditions. Most are not serious pathogens.

Staphylococcus sp: Staphylococcus sp. is an inhabitant and shed from of the skin surfaces. Among the Staphylococcus species that are commonly found indoors is Staphylococcus aureus, which is an important pathogen in hospital environments. We shouldn’t be concerned unless it is the predominating colony found on air or surface samples in indoor environment.

Gram positive rod: Gram positive rod bacteria mainly associated with soil and dust. Appropriate temperature and moisture allow for ideal growing conditions on carpet, wall, furniture’s etc. Most are not serious pathogens. These bacteria can be removed by good house keeping practice and adequate ventilation systems.

Gram-negative rod: These organisms are rare in indoor environments, if they found in higher concentration may be related to the bio aerosol of contaminated water or other contamination of wet/moist surfaces or materials, or possibly AHU systems in which they are proliferating. Some Gram-negative bacteria (or endotoxin extracted from their walls) have been shown to provoke symptoms of fever. Occasionally, growth in Air Handling Units has been great enough for aerosols to be generated which contained sufficient allergenic cells to have caused the acute pneumonia-like symptoms. If there has been a sewage spill or flood, then Gram-negative bacteria are to be expected and such environments should be thoroughly cleaned with disinfectant.

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.

Bacteria may also enter with outdoor air or floodwater and grow in indoor environmental reservoirs. Common indoor reservoirs are water systems, air handling unit and wet organic material. Inadequately maintained air handling system is an important source for bacterial exposure that may lead to allergic type disease. Air handling system must be check for the contaminated water where chest tightness, cough, and fever are associated with a particular indoor environment.

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) and by the pattern of results from a series of biochemical tests.

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., Gardening 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.

Photo shows Different types of Bacteria sampled from the air

Photo shows Bacillus sp. under the Microscope