Bacteriological Sampling

INTRODUCTION

The objective of sampling is to obtain a representative volume of the material and to store and transport that sample in such a manner as to limit any changes that may occur in the sample before it is analysed; this is especially true for bacteriological testing, where viable organisms are counted. [1] To achieve an accurate result the sampling process must not introduce additional organisms into the sample and the storage and transportation of the sample must maintain, but not promote the growth of viable organisms. [2] To meet these aims, bacteriological samples are usually collected in sterile containers, these bottles contain a small amount of a solution designed to neutralise the effects of chemicals added to drinking water to inhibit bacterial growth. Prior to sampling, upon inspection of the bottle, it may appear that the bottle has been contaminated, but this solution should not be washed out of the bottle before the sample is collected. Your guarantee that the container is suitable for use is the integrity of the cap seal: if this seal has been broken discard the bottle.

SAMPLING POINTS

Samples can be taken from a variety of sampling points, including tap, well, spring, storage tank, borehole or surface water. In domestic premises samples should be taken at a tap used to supply water for drinking or cooking purposes. In food production premises samples should be taken at a point immediately before the point where the supply is used for food production and after any treatment given to the water before use. In bulk or surface waters sample below the surface of the water, if possible at a depth <0.5 metres.

SAMPLING OF WATER

1. Principle

Samples are taken in such a manner that they are:

• representative of the water body
• not contaminated by any material which could affect any future chemical or bacteriological test

2. Health & Safety

All relevant health and safety codes should be observed at all times when undertaking sampling activities.

3. Procedure

Taps should be in good repair. If they leak between the spindle and the gland when the tap is turned on, ideally, they should not be used. Samples required from a particular sample point must be taken in a specific order. Bacteriological samples are taken following chemical sampling.

Bacteriological examination is an extremely sensitive technique and can easily be ruined by contamination. Please observe the following:

• Never hold the bottle by the neck.
• Never flush out the bacteriological bottle prior to taking the sample. Ensure that a pre-sterilised bottle is used as indicated by the tape on the bottle being intact. Ensure that the bottle is within its expiry date. If the sterile tape indicator or the expiry date is not satisfactory DO NOT USE the bottle.
• Never lay the bottle down on the ground, on a fence post or on any other area that animals may have been in contact with as this may cause cross contamination of bacteriological samples.

Sampling from taps

1. Reduce the flow if necessary to achieve an even flow before sampling.
2. Remove the top from the bacteriological bottle.
3. Do not lay the top down. Hold the top firmly between the fingers of one hand with the open end down. Do not allow the fingers to touch the inner surface of the top and do not turn the top over.
4. Without changing the water pressure place the bottle in the stream of water from the tap and fill to the base of the neck leaving a small gap at the top to allow mixing in the laboratory. Do not allow the bottle to overflow. (If the bottle does overflow or the top of lip of the bottle touches either your hand or the tap, discard this bottle and take a new bottle.)
5. Carefully replace the top.
6. Turn off the tap.
7. Label all the bottles, at the time of sampling, with their appropriate pre-printed labels or hand written label with (as a minimum) site, purpose, time and date of sampling.
8. Store the bacteriological bottle upright in a cool place, protected from light and deliver to the laboratory as soon as possible.

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