Bacteria

Bacteria are among the earliest known life forms. They are usually microscopic and single-celled organisms with a surrounding semipermeable membrane. The DNA in bacteria and archaebacteria is not surrounded by a membrane inside the cells, which separates them from eukayotes. Bacteria are ubiquitous and abundant in outdoor air, soil, water as well as on the skin and in the digestive system of humans without causing harm. Many bacteria are helpful digesting food or protecting from other, more dangerous bacteria.

Bacterial Pathogens

Some pathogenic bacteria cause disease in humans, animals or plants. Examples for human diseases include whooping cough, some pneumonias, anthrax, cholera, and tuberculosis. True pathogens typically only live and survive in a host and are transmitted from one person to another. Other bacteria can grow in the environment and cause infections. Some of those are opportunistic pathogens and only cause infections when they reach certain organs and/or when the host’s immune system is weakened. A good example for an opportunistic pathogen is Legionella. Less common adverse health effects are caused by hypersensitivity reactions to bacterial cells.  The most important of these is hypersensitivity which involves exposure to a group of bacteria called actinomycetes.

Classification of Bacteria

Bacteria can be identified and classified in several ways including DNA analysis, fatty-acid profiling and substrate utilization. Another common way to group bacteria is by visual shape and color of colonies and microscopic characteristics of the cells as well as staining characteristics. Gram staining is the most commonly used staining process. All bacteria may be classified as one of three basic shapes: spheres (cocci), rods (bacilli), and spirals or helixes (spirochetes). Bacteria are also classified by whether they need oxygen to live and grow. Those that need oxygen are called aerobes. Those that have trouble living or growing when oxygen is present are called anaerobes.

Airborne Bacteria

1) What are the Risks of Exposure to Airborne Bacteria?

Airborne bacteria to which people are typically exposed in residential or office indoor environments rarely cause human illness, although some types of bacteria or high concentrations have been associated with hypersensitivity, infectious or inflammatory disease.  Many bacteria are crucial for the environment and for the normal function of the human body.  They are typically found in high numbers on the skin and in intestines. It has been shown that bacteria found in indoor environments are often shed by building occupants or entered with outdoor supply air and that the risk of illness from environmental bacteria can increase when bacteria multiply indoors (Otten and Burge, 1999).

2) How do I Test for Bacteria in Air?

Sampling for airborne bacteria on culture plates generally underestimates human exposure because bacteria that don’t grow under the conditions of the culture test are not detected. However, sampling of bacteria on agar plates is widely used to assess indoor air quality. Generally, air samples for bacteria are collected on tryptic soy agar (TSA) using single stage, multiple-hole, agar impactors (e.g. SAS, N6, Andersen). The standard flow rate for sampling in homes or offices is 28.3 l/min and the sampling pump should be calibrated before use. Another application for air testing is in sterile environments (e.g. USP<797> sterile compounding) where higher flow rates are beneficial to shorten the sampling time and reduce the risk of media desiccation. Standardization of the sampling and testing procedures is essential to put the measurements into context and for the proper interpretation of the results.

3) What do the Results Mean?

In 1994 – 1998, the US Environmental Protection Agency conducted a study to assess typical bacteria concentrations in 100 large office building (Tsai and Macher, 2005). Bacterial air concentrations were compared by incubation temperature, location, season, and climate zone. A summary of the results is shown in Table 1.

_{Table 1: Comparison of average aggregated concentrations of airborne cultured bacteria (sum of meso- and thermophilic bacteria in CFU/m3) by sampling location and season (n = 100 buildings; summer: n = 52 buildings; winter: n = 48 buildings)}

The data from the Building Assessment Survey and Evaluation (BASE) study provides a reference point for large offices and other indoor environments. Significant changes in the composition or concentration of bacteria can be the result of moisture intrusion or other changes that may indicate increased health concerns for occupants.

Bacteria Testing for Special Applications

Eurofins Built Environment Testing is a leader in microbial environmental testing. Some of the specific environmental testing applications for bacteria include but are not limited to

1. Legionella testing
2. Cleanroom testing
3. Floods and Sewage Spills

References:

_{Otten JA, Burge HA. Bacteria. In Macher J, Ammann H, Burge H, Milton D, Morey P. Bioaerosols: Guidelines and Control,. ACGIH, Cincinnati 1999.}

_{Tsai, F.C.; Macher, J.M. 2005. Concentrations of airborne culturable bacteria in 100 large U.S. office buildings from the BASE study. Indoor Air 15(Suppl 9):71-81.}