7.1 Enumeration Procedure
Following incubation, remove the petri dishes
from the incubator. Take note of the temperature
at which the samples were incubated.
Remove the petri dishes from the rack
and place on the clean work-surface.
Remove the lids and, using the hand-lens
if necessary, count all yellow colonies.
• If the incubation temperature was 37ºC
then the yellow colonies represent a
count for Total Coliforms
• If the incubation temperature was 44ºC
then the yellow colonies represent a
count for Thermotolerant Coliforms
7.2 Disposal of Used Materials
Before disposal, any materials used
in the microbiological analysis must
be made safe.
Potentially contaminated materials include
Absorbent Pads and Filter Membranes.
Used pads and membranes cannot simply
be thrown away after use as they represent
a significant potential risk to public health.
Components can be made safe by sterilising
the petri dishes and their contents. Ideally this
should be carried out using an autoclave at
121ºC for 15 minutes. Alternatively, a pressure
cooker may be used. Once sterilised, the used
pads and membranes can be incinerated.
The petri dishes should be washed and
re-sterilised, ready for the next use.
29
7
Microbiological Results
7.0 Microbiological Results
Bacteria in water are generally not present individually but as clumps or in association
with particulate matter.
When enumerating bacteria in water it is not the number of individual bacteria present
which are counted but the number of clumps of bacteria or the particles and their
associated bacteria. Each clump or particle may have many bacteria associated with it.
Membrane filtration and colony count techniques assume that each bacterium, clump of
bacteria, or particle with bacteria attached will give rise to a single visible colony. Each of these
clumps or particles is therefore a Colony Forming Unit (CFU) and the results are expressed as
colony forming units per unit volume. For standard volumes of sample this would be CFU/100ml.
This may vary depending on the type of water being tested and the volume of sample water
actually filtered.
Useful to Know
• Count the colonies within a few
minutes, as the colours are liable
to change on cooling and standing
• Always try to have more than
one person counting
• Try to count in the best available
natural light - avoiding direct sunlight
• Count yellow colonies that
are >1mm diameter
• DO NOT count colonies that are CLEAR,
RED or ANY OTHER COLOUR - these
bacteria do not ferment lactose and
are not Thermotolerant Coliforms
• Gridded membrane filters permit
easier counting when large numbers
of colonies are visible
• Count colonies systematically,
column by column in the grid
• Where there are too many colonies to
count or it is difficult to see individual
colonies clearly, mark the result as
“Too Numerous To Count” (TNTC)
To Next Page
Loading...
Need help?
General