Australia's Science Forum

I need to make those agar plates that are filled right to the top. We want to use them to press agains surfaces, so that the actual real ration of organisms can be viewed. The surface presses involve benches & desk, aprons and lab coats(fabrics), phone mouthpiece and food cutting boards.
So we're not streaking, spotting or lawn culturing with a wire or swab (as in a representative sample)
its a petri dish filled to the very top with nutrient agar. And then the plate is lightly brought into full contact with the surface to be tested.
So far so good.
But what i cant remember is, how do I keep the lid of the petri dish from "setting" or sticking to the agar when its cooling?

I need them to be in the fridge for 3 days before use, so cant use straight away. i thought of glad wrap, but how to sterelise glad wrap without affecting the integretity of the agar its laid against? will the glad wrap itself cause changes in the agar over the 3 days? its not like I can whizz the glad wrap over the bunsen to sterelise it before wrapping?

ANY ideas tips that you can offer would be greatly appreciated

Can't say I've done or seen what you hope to do but putting gladwrap over would probably be ok. I imagine it should be sterile, but I guess you might have to trial it first. Only thing then is what you do with the lid in the meantime, cause if they are plastic you would have to disinfect with a liquid perhaps. Have you checked on Google or Youtube? Sorry I can't help more.

I've never done this either, however I have a UV Sterilisation cabinet which would allow me to leave the lids off while the gel is setting and not allow contamination. I often place regular nutrient plates in there to ensure sterile plates.

yes Dime
been Googling and uTubing but with no success. Thats why I turned to you guys for help. At the moment I have some glad wrap 'sterelising' between two layers of paper towel soaked in Metho.

I have no time to tria re the microbial growth- the teacher will be using them before I get to see them again-straight from fridge, so they must be clean, not sticking to the glad wrap or to the top of the plate.. I will be trialling the 'stickability' today though

Please post any other ideas thoughts etc...

david, I LOVE the idea of a UV stereliser! Too late for today, but for future , Where from, what size? cost? Please

Had mine for about 10 years I guess. It isn't very big and probably isn't designed for plates, but I do it anyway

I use mine for dissecting equip and safety glasses, sometimes vials when they do pracs on factors that make food rot (it eliminates bacteria that was originally in the vial etc as a source of decomposition).

Hope I'm allowed to tell you where it came from without getting in trouble for advertising Info can be found at this site:

http://www.omegascientific.com.au/starkeys_page.htm

current price about $600

There would be other suppliers.

That looks perfect for many tasks.

David, is it possible to make the moltena gan and pour into the perti dishes, and then once they are set, to "sterelise' them by Nuking them in the UV? or wouldnt it work this way? My thoughts are that this would eliminate the "working aseptically" step, witht he bunsen. firing the neck of the flask etc

Another option is a product called Petrifilm from 3M (you can buy them from Southern Biological & other suppliers)
They're a paper sheet with dye impregnated to stain for specific bacteria, & you can pour a thin layer of sterile agar onto the surface, then peel back the cover sheet, touch onto any surface to pick up bacteria, then incubate. We put ours in sealed zip-lock bags for safety. They're not cheap at around $139 for a bag of 100, but when you factor in the cost of petri dishes, nutrient agar, & the time it takes to make & sterilise large batches of agar & pour plates, there's probably not much difference in overall cost. Plus they take up almost no space, will keep in the freezer for 12 months, & because there's a grid on them the kids can do a proper count of colonies.
There are different ones that stain for the presence of aerobic bacteria, E.coli & coliforms, & yeasts & moulds.

I use the principle that it does work, I've had no problems with contamination.

You need to keep the lids off though (UV treat them separately) as not all plastics allow UV through to the gel. Same with glass plates - glass is good for filtering UV and the gel surface doesn't end up being treated.

Even though I briefly studied Microbiology (25yrs ago.....) I would hardly call myself an expert andf I'm sure my lecturers / prac instructors would shudder at the things I do. I don't bother with following aseptic technique too strictly. I just get the agar powder mixed, heat until it's just boiling and the 'solution' becomes translucent (otherwise it doesn't pour or set well). Then I pour into plastic plates. Once set, remove lid and place in UV cabinet for 10-mins.

Not very scientific, but it works OK for school level studies.

Bindibadgi that is an amazing product.
It certainly would save a lot of time, effort and fussing.
I LIKE it ! a LOT

Hi Bindibadgi,
I used to use those Petrifilm type swabs when I worked at Heinz. They are very good, only we just used a bit of distilled water to dampen them for the swab.
Very easy to count colonies, and finally to dispose of.

I was thinking about this over the weekend.
Since the bottom is deeper than the top what about if you filled the top and than used the bottom as the lid. just put it in revese.