TEST METHOD: MICROBIOLOGICAL SCREENING OF ENVIRONMENTAL SAMPLES

Revised: 19 FMarch 2011 (addition of information on fecal coliform testing and Avenda steriliser)

OVERVIEW

Surfaces and waters may require testing for microbial contamination levels. This is typically the case for bathing waters when contamination is suspected. Surface testing may be occasionally required to demonstate sterile procedures are being met. Rapid screening includes a Colilert tray test for total coliforms, fecal coliforms, enterococci and E.coli, and total viable counts for aerobic bacteria, yeasts and moulds. Plate count tests are also available for salmonella and other bacteria.

TASK SAFETY REQUIREMENTS

Safety boots are mandatory

Please use disposable gloves

Observe the requirements of the Chemical Hygiene Plan

SPECIFIC JOB STEPS RISK ASSESSMENT RISK CONTROL
Collection of samples for Colilert & Enterolert testing.

Sterile IDEXX sample bottles marked with a line to denote 100mL are used to collect samples for Colilert testing.

 

Sample may degrade prior to testing Sample should be stored in esky or fridge. Samples for public health testing may be no more than 6 hours old when tested. Samples for environmental assessment may be tested up to 24 hours after collection.
Colilert & Enterolert testing (Total coliforms, E.coli, Fecal coliforms, enterobacteria)

1:1 samples have the bacterial media added directly to the sample bottles. Dilutions are conducted in measuring cylinders using sterile de-ionised water as a dilutant, prior to the addition of media.

100mL of sample with well mixed media is poured into the counting tray (Quantitray). Media is selected according to the sample and the organism of interest (see IDEXX literature).

The trays are run through the sealer and placed in the incubator for the period of time and at the temperature specified for the media. Note that Colilert 18 is now USEPA approved for fecal coliform testing as well as Total coliform and E.coli testing. The temperature is different, however

The trays are evaluated against the reference materials provided by IDEXX. Results are recorded as organisms/100mL.

 

Selection of Quantitray type

Some tests require different temperatures

Quantitray type (low or high count) is selected according to best estimate of the cleanliness status of the water. Beach or swimming area sampling usually uses the more precise chambers, contaminated watercources would normally require the larger trays.

There are 2 incubators (up to 50oC) and an environmental chamber (up to 30oC) available for tests requiring assorted temperatures. P/A bottles can be used in all, IDEXX Quantitrays need to be curved slightly to fit into the smallest incubator.

Preparation of equipment/materials packs for general microbial work.  

Items may be sterilised prior to use using the Avenda sterilisation unit in the microwave. Instructions for use of the unit are marked on the cover. Once sterilised, the items may stay inside the sealed container for up to 48 hours until required for use

 

More than 48 hours has passed since sterilisation Resterilise
Preparation of equipment/materials packs for membrane filtration counts (MF pack).

Place 1 filtration unit, 1 x 0.45 m m membrane filter, 1 watchglass, 1 medicine glass, a pair forceps and 1 pasteur pipette in a Clavies bag (autoclave bag). Fold the top of the bag over twice and place in the autoclave. Sterilise for 15 min at 121oC. Once the pack is cool, tape the top and place in cupboard until required.

 

   

Media

Select the appropriate test method and media after referring to the Merck microbiological manual that is located in the laboratory bookcase.

 

   
Disposal  

Autoclave all samples for 20 mins prior to disposal unless in Quantitrays. These may be sterilised by exposing to bright sunlight for 2 days

 

 

Incubated samples may have high bacterial levels

 

Both the autoclave and UV rays in sunlight will kill bacteria/spores.

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