3
Preparation
14
Nutridisks are the option most commonly
used with the Wagtech Test kits. Nutridisks
are larger than the standard aluminium
petri dishes supplied in the kit but are still
designed to fit into the standard petri dish
rack. This allows up to 7 Nutridisks to be
incubated at the same time.
To use the Nutridisks they are moistened
with 3.0-3.5ml of sterile distilled water.
They are then ready to use immediately.
An excess ring of water surrounding the
pad should be visible.
All Nutridisk types are supplied with
the appropriate membrane filters, which
are also pre-sterilised and individually
packaged. The membrane filters are
tailored to meet the special requirements
of microbial detection and are available
with 47mm or 50mm diameters.
See Appendix 4: Reagents & Consumables
Useful to Know
• Growth or culture media is a
substance designed to support the
growth of microorganisms (bacteria).
The media is a vital part of the
microbiological water quality test
• There are different types of media for
growing different types of bacteria.
The Wagtech kits use Membrane
Lauryl Sulphate Broth (MLSB) as
the growth media
•
MLSB is a differential media which means
it can distinguish one microorganism
type from another growing on the
same media. It uses the biochemical
characteristics of a microorganism
growing in the presence of specific
nutrients or indicators (such as Phenol
Red) added to the medium to visibly
indicate the defining characteristics
of a specific microorganism
• MLSB is the growth medium for Coliform
bacteria and Escherichia coli (E.coli). It
feeds coliform bacteria but inhibits the
growth of any non-target organisms
that may be present in the sample
• The MLSB in the kit is supplied as a
fine powder. This increases its shelf
life and makes transport and shipping
easier. Preferably it should be stored
in a dark environment away from
extremes of heat and moisture
• Typically in its powder form the shelf
life of the MLSB is 12 months
•
In order to be used in the microbiological
test the MLSB has to be prepared
in liquid form. When water is added
to powdered MLSB a deep red liquid
is formed
• Coliforms are identified by their ability
to cause a colour change in the growth
media when incubated. MLSB contains
lactose as the major carbon source,
which during incubation is degraded
to acid by E. coli and coliform bacteria;
this is indicated by a change of the
colony colour from red/pink to yellow
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