Detection of Microorganisms in Water According to UNE-EN ISO 8199 

UNE-EN ISO 8199 provides general requirements and guidance for performing the microbiological analysis of water using culture-based techniques. It covers the common operations involved in these methods, including sample preparation, culture media and laboratory equipment, as well as the techniques available for detecting and enumerating microorganisms.

This framework can be applied to the analysis of drinking water, water from industrial processes, wastewater and recreational water whenever it is necessary to identify or quantify microorganisms such as bacteria, viruses and fungi.

Preparation and microbiological analysis of water samples

An example of a procedure for preparing and analysing samples includes the following steps:

• Collect the water samples in sterile containers.
• Filter the sample to remove large particles when necessary.
• Transfer a volume of between 100 and 500 mL to a test tube and centrifuge it at a speed of between 3,000 and 5,000 rpm for 10–15 minutes.
• Resuspend the resulting sediment in a small amount of sterile water.
• Perform the corresponding microbiological analyses, such as inoculation onto a culture medium, incubation and subsequent colony enumeration.
• Analyse and interpret the results obtained.

Sample collection and preparation must be carried out under conditions that minimise the risk of external contamination and preserve the representativeness of the water being analysed. After concentrating the microorganisms, the resuspended sediment can be used in the microbiological techniques selected according to the type of sample and the analytical objective.

Centrifugation of water samples

The centrifugation of water samples can be used to concentrate sedimentable microorganisms present in a limited volume. This operation is particularly useful when the initial microbial concentration is low and may make subsequent detection or quantification using microbiological methods more difficult.

During the process, particles and microorganisms capable of sedimenting under the applied centrifugation conditions move towards the bottom of the container. After removing the supernatant and resuspending the sediment, a concentrated fraction is obtained that can be used in subsequent operations, such as inoculation onto culture media, incubation and colony counting.

Centrifugation also allows some of the suspended solids present in the sample to be separated. However, certain microorganisms may be associated with these particles and sediment together with them. For this reason, the centrifugation conditions must be selected according to the type of microorganism, the nature of the sample and the microbiological method being applied.

For this application, the Ortoalresa Digtor 22 centrifuge offers a maximum capacity of 4 × 750 mL, depending on the rotor and adapters installed. This configuration makes it possible to work with the sample volumes of 100 to 500 mL described in the procedure, facilitating the concentration of microorganisms present in the water before microbiological analysis.