Water for Microbiological Media

Water is used in many aspects of microbiological experiments. This section focuses on bacterial growth, hence water used for the preparation of culture media. For information about microbiology related techniques, such as PCR, nucleic acid and protein electrophoresis, Southern, Northern, and Western blotting, please refer to the corresponding sections.

The impact of water quality on microbiological growth is well documented in the text on Figure 1 below.

In summary, since 99% of the culture media is water, water quality is critical. It should be:

free of nutrients
free of toxic substances
resistivity > 0.3 MΩ.cm
low concentration of chlorine
low concentration of heavy metals
low microbial count

Figure 1. Extract of recommendations of the Bureau of Microbial Investigation on water quality for media preparation. This text serves as a basis for the ISO 11133 highlighting the issues linked to poor water quality.

Water storage should be done under very controlled conditions to avoid contamination of the water by the reservoir itself or by contact with air.

Water quality defined as pure water or Type 2 water grade by water purification system manufacturers is recommended for media preparation for microbiology. This is prepared by a combination of technologies including activated carbon, reverse osmosis and electrodeionization. A 0.2 µm filter may be added at the point of delivery to ensure low microbial count. The combination of these technologies ensures:

the removal of chlorine or similar oxidant present in tap water
a resistivity > 5 MΩ.cm
a low microbial count
a constant water quality
low organic content (< 30 ppb or µg/L)

Storage of pure water is typically done in opaque plastic containers (PP or PE), equipped with germicidal UV lamps and automatic draining systems to ensure a reduction of the microbial proliferation during the storage.


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