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Prepared Media Plates - Sterilization Methods

Updated on March 14, 2015

Sterile Prepared Media Plate

Sterile media plates: Modified m TEC, m-FC, and MEI for Enterococci
Sterile media plates: Modified m TEC, m-FC, and MEI for Enterococci | Source

Sterilization: The Absence of All Living Things

Sometimes folks get the words "sterile" confused with disinfected, clean, or even purified.

The truth is: those other terms are not the same as the term, Sterile.

Sterility means, "The absence of all living things."

How many times when you were removing a sliver, and someone (probably your mom), put a needle over an open flame for a few seconds to "sterilize" the needle.

Well, an open flame does go a long ways towards removing pathogenic, (disease causing) bacteria and viruses - so we'll give your mom credit for that one! An open flame treatment does not, in fact create a sterile device, but a disinfected one - which is great for removing slivers!

Hand sanitizers reduce the normal flora (bacteria) on your hands, but they do not eradicate all bacteria from your skin. You would eventually get quite sick if that happened from an imbalance in skin pH among other problems.

When we refer to a sterile plate in the laboratory, even technicians have no way of telling whether or not for sure the prepared media plate or petri dish they are using is truly sterile. That's why we run a sterile control with each batch of bacteria we cultivate to give us a reasonable assurance that the conditions we were working with - plates, media, rinse water, funnels, etc., were all sterilized properly before filtration and analysis.

So, to review...we can "clean" our dog or our car...

We can "sanitize" our hands or face...

We can "Pasteurize" our milk...

But we need to "Sterilize" our prepared media plates before use!

OK - that should help!

Fruits and Veggies can be sterilized with gamma radiation
Fruits and Veggies can be sterilized with gamma radiation

Zapped!!! with GAMMA Rays!

Zapped!!! with  GAMMA Rays!
Zapped!!! with GAMMA Rays!

Typical Filter Manifold Set Up

Filter Manifold with Polysulfone Filter Funnels
Filter Manifold with Polysulfone Filter Funnels

Prepared Media Plates and Colony Counting

Membrane Filtration Sterilization Tips

Of course you're going to sterilize your filter funnels prior to sample filtration. But, if they are sterilized via autoclaving, MAKE SURE the funnels have cooled sufficiently before introducing your sample. Not that big of a deal if you are filtering a full 100 mls., but if you are doing serial dilutions, and some of your aliquots are in the 2-5 milliliter range, you ring the risk of B-B-Qing the bacteria in your sample from the very hot walls of the filter funnels.

Don't do that! Simply rinse the funnel walls first with cool, sterile buffered dilution water. There - problem solved! Maybe that's why you didn't get any growth on that small sample last week!

Prepared Media Plates and Positive Controls

Fruits under Gamma Radiation

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