You’re in charge of the Membrane Filtration Station for
microbiology for the DMRQA test this year.
It’s a big responsibility, and
demands your fullest attention.
Maybe you’re a new technician, or
maybe the person who ,”always does it” isn’t around anymore. What if, you don’t
pass, and wouldn’t it be great to have another guide?
This short informative report will help give you some
guidance to assure you’re on the road to success when it comes to testing using
membrane filtration.
ONE:
THE 5 “P’S” – LET’S START
THERE!
You’ve heard it before:
“Proper preparation prevents poor
performance.” Sometimes with colorful language added…but nevertheless it’s a
phrase that is worth repeating!
When we talk about being properly prepared in membrane
filtration, it begins at the sink. You want to make sure that the filter
funnels are nice and clean and free from any debris. They should also be
checked periodically for high or low pH walls with a pH indicator solution like
bromothymol blue. Make sure all filter funnels, magnetic bases, and manifold
are scrupulously clean.
TWO:
FUNNELS,
FLUIDS, AND BBQ BACTERIA!
After thoroughly cleaning,
inspect your polysulfone or glass filter funnel for any deep nicks or
scratches. This can harbor bacteria and skew your count or ruin your sterile
control.
Sterilize the funnel(s) initially in an autoclave, being sure not
to touch the inside walls of the funnel when handling.
TIP: Beware - The funnels come out of the
autoclave HOT!
Let them cool, or dispense some
sterile dilution water down the interior walls of the funnel prior to filtering
the sample so as not to cause any flash thermal exposure (B-B-Q!) to the
bacteria, especially if filtering small sample volumes.
Be sure to rinse the interior walls of the funnel after
sample filtration with sterile, and properly buffered dilution water. Typically
it will be slightly basic.
THREE:
INCUBATORS:
“THIS ONE’S TOO HOT, THIS ONE’S TOO COLD, THIS ONE’S JUST RIGHT!”
Different bacteria
require specific temperatures for optimum growth. Total Coliform are fine at 35
degrees Celsius, while E. coli – a part of the Coliform Group are
thermotolerant and like it hot – at 44.5. degrees C.
When incubating bacteria, because it’s your job to
grow them, be sure that the incubator is at the correct temperature not only
during the thermometer check times, but use a continuously monitoring device,
(plugs into your laptop), so you know for sure that the incubator held
temperature during the whole incubation period.
Use a
thermometer that has been calibrated against a NIST traceable thermometer.
When laying your membrane filter on the prepared media
plated or other media source, place one portion of the filter at an angle onto
the media surface – then while applying a very slightly downward pressure
simply roll the rest of the filter onto the media surface using your sterile
forceps or clips.
FOUR:
TIP: Do at least 5 serial
dilutions, more if you have the sample material.
Be sure to
incubate your cultures upside down – media side up, to keep the moisture level
of the media up.
Following these simple rules will go a very long way in
assuring you get great results with your DMRQA test in membrane filtration!
Good Luck!
“Membrane Filtration is used the world around – and for very
good reason… It works!”
Check
out MIDSCI’s pre-poured plates
