Why you need good quality water for microbiological media preparation
Water is the main ingredient used in the preparation of microbiological media. Therefore water used must be purified and/or deionized water free from any nutritive and/or toxic substances so that there is no inhibition of the traget micro-organims. Purified water shall have a resistivity of at least 300 000 Wcm and the conductivity should be less than 10 mS (microSiemens).
Tap or potable water must not be used. In some areas tap water may be contaminated and may contain relatively high amounts of heavy metals and /or chlorine. Even in very low levels, these can cause precipitation problems and may inhibit the growth of microorganisms.
If the distilled water is prepared from chlorinated water, it is necessary to neutralize the chlorine prior to distillation. This is achieved by adding sodium thiosulphate.
The distilled water can be stored in containers. These should be produced from inert materials (e.g. neutral glass, polyethylene etc.). The containers must be free of any inhibitory substances prior to their initial use. If, during storage, no precautions are taken, atmospheric CO2 will dissolve, making the water acidic. Also algae may grow quickly in water tanks and their metabolites can inhibit growth of microorganisms.
In some cases it may be necessary to use freshly prepared water, free of dissolved carbon dioxide.
Water processed through an ion exchanger (de-ionized), may have high microorganism content. De-ionized water should not be used without verifying that it does not contain microor-ganisms. Filtering the water is not enough as water may contain substances inhibitory to the growth of particularly fastidious microorganisms.
In Standard Methods for the Examination of Water and Wastewater a test for the bacteriological suitability of laboratory water is described.
