How to use Dip Slides

Instructions for use

  1. Unscrew the dip slide paddle from the test tube, holding it by the cap to avoid touching the culture medium.
  2. Either completely immerse the slide or expose it to a spray or running stream of the fluid to be tested so that both agar surfaces are covered for 10 seconds.
  3. Remove the slide from the fluid and allow it to drain for a few seconds.
  4. Replace the dip slide into the test tube and tighten the cap. Using the labels provided, mark the tube with the date and test location.
  5. Keep the exposed dip slides in an upright position and incubate at the same or similar temperature as the typical operating temperature of the fluid being tested i.e. at a temperature similar to that at which the suspected infection has arisen.
    Approximate examples:
    Process waters            25-30˚C
    Machine tool coolants   30˚C
    Rolling mill coolants      30˚C

    When testing for bacteria incubate the dip slide for 2-3 days. Microbiological growth will be more rapid if it is the same or very near to the typical operating temperature of the fluid being tested.

    When testing for yeast and mould incubate the dip slide for 3-4 days. If there is no growth after this time, a further incubation period of 48 hours is advisable to detect the presence of organisms. The optimum temperature for yeast and mould is 27-30˚C.

    If you do not have an incubator and are leaving your samples at room temperature, bacteria will need 3-4 days to develop and yeasts and moulds 4-5 days.

    If you would like to purchase an incubator, please contact us for further information.

Interpretation of results

Most aerobic bacteria grow on the paler TTC culture medium. The TTC dye stains most colonies red for easy analysis. Bacterial colonies usually appear as red spots but can also appear colourless. Compare the density of both red and colourless colonies with the comparison chart overleaf to estimate the number of organisms in the fluid being tested.

If the bacterial content of the fluid is high, a confluent growth can appear on the surface of the slide rather than distinct colonies. If this confluent growth is colourless, it could be misinterpreted as ‘no growth’. For accurate results, it is recommended that the slide is examined using a reflected light against a sterile unused dip slide for comparison.

Yeast and mould contamination can be determined from the darker malt agar culture medium. Yeasts usually appear as smooth round colonies often coloured white, beige or sometimes red. Yeast colonies can be evaluated quantitatively like bacteria using the comparison chart overleaf. The quantitative evaluation of moulds is a more difficult task. Moulds are characterised as ‘fur’ colonies that tend to spread across the surface of the dip slide, joining together as one mass. For measurable results, examine the dip slide at an early stage of growth to observe distinct colonies, these can then be subjectively described as slight, moderate or heavy using the comparison chart overleaf.

Contamination Scale:
10² and 10³ = Slight
104 and 105 = Moderate
106 and 107 = Heavy

REMEMBER counts should never exceed 10³ colony forming units per ml at any time.

Storage

Dip slides should be stored in a cool, dry environment between 10˚C and 20˚C. Do not refrigerate or freeze. Variations in storage temperature can create condensation inside the test tubes. This will not impact the performance of the dip slide.

Faulty dip slides
A dip slide is not suitable for use if either culture medium is:

  -   contaminated
  -   detached from the paddle chamber
  -   dry, withered and not sitting proud of the chamber edges

Disposal

Used dip slides should be burnt, autoclaved or immersed in a suitable disinfectant overnight.