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No growth on dipslides despite purposeful contamination - a mystery
Hi everyone, I made up a batch of lotion and thought that before I get expensive professional testing done, I would test it using dipslides. Figuring that if it doesn’t pass that test, then it’s not worth sending it in at all and I’d need to go back to the drawing board.
Having read on this forum that there is often a problem with growth on the slides even without any product on them (easy contamination of slides), I was not expecting the following to happen.
I smeared a thin even layer of the lotion on the slide and included a control slide (no product). Nothing grew after 48 hours (bacterial) or 72 hours (yeasts/fungi), and for good measure I left them in the incubator for another few days extra.
Feeling happy that nothing grew, I then decided to contaminate my lotion and see whether it was capable of reducing the contamination. In one small sample, I smeared my finger all through it, well and truly, about 10 times. The whole small sample came in contact with my finger multiple times. I put some of that on the dipslide. I then set that sample aside, open, to re-test a week later.
In another small sample, I added 10% spit (yes I read that here on the forum, and thought why not give it a go). I smeared this on a second slide, and then set that aside to re-test a week later.
These 2 samples have now been incubating for 2 full days, and all that has grown are 2 tiny colonies on the sample with the finger contamination. The spit sample doesn’t even have anything on it.
I am at a loss what has happened, as I was expecting a lot of growth on both slides. Now I can’t even re-test after a week, to see whether my lotion has enough preservation to reduce the contamination over time.
Specs
- I’m using a laboratory incubator, no fan, 30 degrees. I’ve double-checked the temperature with a thermometer and it’s running perfectly on temperature.
- I’m using Lovibond TTC/Rose Bengal displides. Here is the link for the displide;
https://www.lovibond.com/en/PM/Water-Testing/Products/Microbiology/Dipslides/D003-TTCRBS-Dipslides-Rose-Bengal-pack-of-10Before using the slides, I had the supplier double-check with the manufacturer that these were indeed the right dipslides for cosmetic testing.
My gut says maybe it’s a problem with the dipslides, as I really contaminated my samples a lot.
Anyone have any clue what’s happening here?
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9 Replies
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No mystery - the medium includes Rose Bengal that inhibits bacterial growth. It’s intended to culture only fungi - yeast and mold. You’ll likely find no growth unless you have thrush.
Please do not use dip slides for cosmetics. They do not provide valid data either way - growth or not.
If they manufacturer recommended these for cosmetics - they are clearly ignorant and irresponsible. -
These dipslides have 2 sides - one side is Rose Bengal for fungi, and the other side is TTC for bacteria.
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What is your preservation system?
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Thanks - and they still suck.
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I’m going to order the Mikrocount
kits from Schulke and repeat
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Dipslides from anyone are a poor for anything but water.
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We have been using Mikrocount
kits from Schulke for our water and product microbial testing for years. We have test results validated by external labs periodically.In our experience, the results from the Schulke Mikrocount
kits are OK. -
Water is prob ok - product not so sure.
Undiluted product would be the worst. Assume you’ve diluted product. Would have to satisfy suitability for each product. In that context might be ok for some - but even there, question the volume actually exposed. -
@chemicalmatt
Much as I think these are gimmicks for cosmetic testing, also know you’re solid in your work. Can you describe your application?
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