What Bacteria are Really in Indoor Environments?

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Television commercials, literature, and popular belief tell us that there are bacteria floating around everywhere, sitting on every surface, just waiting to jump up and infect us. But what is really there? You may be surprised to find out.

While it is true that you will not find a surface without bacteria on it, the majority of bacteria that you’ll find are common, non-dangerous species. Studies have shown that of the bacteria found in indoor air, the most common four are: Micrococcus, Staphylococcus, Bacillus, and Pseudomonas.

Micrococcus is a sphere-shaped (coccus/cocci generally means spherical), relatively harmless bacterium. It is very common on skin, and it can also be found in soil, water, and meat products. It is generally a saprophyte (meaning it feeds on dead and decomposing materials), and can cause spoilage of fish. This organism can also be responsible for causing human sweat to smell badly. In immunocompromised people, it can be an opportunistic pathogen. Some common species include M. luteus, M. roseus, and M. varians. Micrococcus requires oxygen to grow and reproduce.

Staphylococcus is another sphere-shaped bacterium. It is much more well-known than Micrococcus, especially in the context of hospitals. When the medical profession refers to MRSA, they mean a particularly drug-resistant strain of this bacterium. However, Staphylococcus is found virtually everywhere, and usually does not result in infection. It is very common on skin, and can also be found in the nasal passages, throats, and hair of 50% of healthy individuals. Food poisoning and skin infections, as well as toxic shock syndrome, are among the illnesses caused by Staphylococcus. Unlike Micrococcus, Staphylococcus is able to grow both with and without oxygen.

Bacillus is a rod-shaped bacterium (“bacillus” means “rod”). Bacillus is a very hardy bacterium, as it has the ability to produce endospores – small, tough structures that can survive adverse conditions. For the most part, Bacillus is a harmless saprophyte, and it can be found in soil, water, dust, and sometimes within the human digestive system. Some species of Bacillus can cause food poisoning, and some can cause illness or infection. An interesting side-note about this genus is that it contained the mystery organism isolated from a 250-million-year-old salt crystal, potentially the oldest living cell discovered (the organism was later reclassified to Virgibacillus).

Pseudomonas is another rod-shaped bacterium. It can be found in soil and water, and on plants. It is an opportunistic pathogen, and generally considered a nosocomial infection (gained while in the hospital), as the organism tends only to attack individuals that are immunocompromised. Along with infection, it also has the ability to produce exotoxins. As a general rule, the bacterium will not infect a healthy individual.


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Suggested Reading

Gorny, R.L, Dutkiewicz, J, Krysinska-Traczyk, E. 1999. Size distribution of bacterial and fungal bioaerosols in indoor air. Ann Agric Environ Med. 6:105–113.

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About the Author

Meagan Klub is a Microbiologist with a particular interest in pathogenic bacteria (disease-causing bacteria). She graduated from the University of Guelph in Guelph, Ontario, Canada with a Bachelor's degree in Microbiology.
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