1. Unscrew the tube and withdraw the combined cap and 'tongue' unit without touching the agar coated surfaces.
2. Take a sample of the fluid to be tested in a container and dip the dipslide tongue into it, or expose the slide to the running fluid. Both agar surfaces must be fully wetted.
3. Remove the slide from the fluid and allow excess fluid to drain off the slide. Blot the lower edge of the slide on clean absorbent paper.
4. Screw the dipslide tightly back into the tube.
5. Fill in the label and affix it to the tube.
Incubation: Place the tube upright, in a warm place, or incubator at a temperature of 27°C to 30°C, or at an approximate temperature equal to that of the fluid being tested. Temperatures above 30°C may dehydrate the agar and give inconsistent results.
The incubation period for bacteria is usually 24-48 hours. For yeasts and moulds an incubation time of up to 24-72 hours may be required to achieve quantifiable microbial growth. If the incubation temperature is considerably cooler than the fluid being tested, microbial growth may be slowed and continued incubation for a further few days may be necessary to detect the presence of organisms.
Interpretation of results:
Bacteria will grow on the paler straw coloured agar. Most bacterial colonies will appear red in colour, but can on occasions be colourless.
The density or numbers of colonies, both red and colourless, should be compared with the chart supplied with Combi dip slides. If the fluid to be tested has a severe infection (over 107/ml), a confluent growth can appear on the surface of the slide, rather than distinct colonies. If this growth is colourless, it could be misinterpreted as a negative result. To prevent this, all apparently clear dipslides should be examined against a reflected light or compared with a sterile, unused dip slide.
Fungi and yeasts will grow on the pink rose bengal agar side of the slide. Most yeasts will appear as smooth round colonies, white or pink in colour.
Occasionally red colonies may appear. These are usually bacterial growth. The density or number of yeast colonies, of all colours, can be quantitatively evaluated using the chart.
Other fungi or moulds are characterised by the furry nature of their colonies, which spread rapidly across the slide making quantitative evaluation difficult. However, if the slide is examined at an early stage of growth, distinct colonies will be observed and the chart will give an indication of the level of infection.
It is not possible to give any universally valid limits, however the following general guidelines may be used:
104 slight infection - probably acceptable.
105 or more moderate infection - take immediate action to clean / sterilise system.
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