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Quicker way to perform serial dilutions


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#1 SC34

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Posted 08 June 2016 - 08:50 PM

Hi,

 

I am currently working at a food testing lab, some of the samples we process require us to perform serial dilutions to -6 which is quite time consuming and we use a lot of media compared to 'clean' samples which don't require any dilutions - my question is, is there a quicker/more cost effective way than performing serial dilutions? Has anyone had experience with alternative methods?

 

Thank you



#2 Charles.C

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Posted 09 June 2016 - 03:24 AM

Hi,

 

I am currently working at a food testing lab, some of the samples we process require us to perform serial dilutions to -6 which is quite time consuming and we use a lot of media compared to 'clean' samples which don't require any dilutions - my question is, is there a quicker/more cost effective way than performing serial dilutions? Has anyone had experience with alternative methods?

 

Thank you

 

Hi SC,

 

May i ask why you need such high dilutions as 10(-6) ? (a typical count of 150 colonies on a plate would imply a result of 150 million cfu/gram ?)

 

What actual measurements / food types are involved ?

 

Spread plates can be more "economical" than pour plates. Pros and cons.


Kind Regards,

 

Charles.C


#3 SC34

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Posted 09 June 2016 - 03:10 PM

Hi Charles,

 

We routinely test raw meat/fish which we have to go to -4 on, when we perform shelf life testing on these products we often go to -6, we also perform shelf life testing on milk samples which again as the shelf life progresses we go to -6. We already use a combination of prepoured and pour plates depending on the organism we are testing for.

 

 



#4 Charles.C

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Posted 09 June 2016 - 03:22 PM

Hi Charles,

 

We routinely test raw meat/fish which we have to go to -4 on, when we perform shelf life testing on these products we often go to -6, we also perform shelf life testing on milk samples which again as the shelf life progresses we go to -6. We already use a combination of prepoured and pour plates depending on the organism we are testing for.

 

Hi SC,

 

Thanks for reply.

 

I deduce this is some form of ASLT.

 

Sounds like the "normal" "M" value may be in the millions. Not impossible for raw meat/fish. Seems improbable for milk though.


Kind Regards,

 

Charles.C


#5 Charles.C

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Posted 09 June 2016 - 04:10 PM

addendum

 

I do agree with you about the time in doing dilutions.

Shortcuts are possible if some relaxation of accuracy is permitted plus experience in predicting likely levels is available.

 

eg

(1) 1ml of original (-1)  > 1000ml = (-4) > plate

(2) another 1ml > 100ml = (-6) > plate

 

Option (1) is not unheard of IMEX for routine use. Some people further reduce the 1ml volume onto the plate. And no duplicates. :smile:


Kind Regards,

 

Charles.C


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#6 Dr.Des

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Posted 16 June 2016 - 08:44 AM

As someone who has done countless serial dilutions over that past 35 years, I would love to find an answer to that question!

A spiral plater is a possible answer, but they're not great for certain food samples, and the capital costs take a long time to be recouped.

If as you say it's only some of the samples that need the extra few dilutions, I don't think equipment is worth the investment.

(My experience with spiral platers has not been positive!)

 

Performing a serial dilution from a stomacher bag to -6 should take less than a minute per sample (not including plating out). In our research projects we regularly have samples where we dilute to -10 or -11 so if there were shortcuts I suspect we would have found them by now.

I would not be a fan of the shortcuts mentioned, plate counting techniques have enough inaccuracies/uncertainty without deliberately adding more!

 

In terms of media cost,  diluent is cheap so it's the extra agar plates that increase your costs - do you plate out all of the dilutions or just some of them?

 

 

 

 



#7 SC34

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Posted 22 June 2016 - 10:26 PM

Thank you for your input Dr.Des.

 

I have googled spiral platers and it would seem that if you had a liquid sample it wouldn't be too costly (aside from the initial set up cost) but any solid sample would require being weighed into a filter bag which adds to the cost, apart from that they appear to be a good investment - one plate for the equivalent of 3 dilutions sounds great! but I've never used or seen one in use - can I ask what in particular you didn't like about them?

 

Rest assured we wouldn't implement any shortcut that could add any inaccuracies or uncertainties to our results - aside from the fact that the managers wouldn't allow it the company I work for prides itself on good customer service - giving them inaccurate results would somewhat undermine that!

 

And yes we do currently plate out all of the dilutions although after you asking if we did that I am thinking about suggesting we don't plate out -1 -2 if we are expecting to be reading on -5 -6 not sure they would go for not plating the -3 -4 as well - no harm in suggesting it though :-)

 

Thank you



#8 Dr.Des

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Posted 23 June 2016 - 08:17 AM

My dislike of the spiral platers was mainly due to the fact that I have historically always tested more solid products, meat especially, than liquid ones.

The lab I was in at the time purchased it without really researching how solid samples would block the tip, they over-estimated the efficacy of filter bags!

For counting colonies in overnight cultures and liquid or water samples it would be excellent.

 

If our food samples are complete unknowns in that we've no history of testing that type of sample, then we'd plate all the dilutions.

Once we have some idea of the likely count, we plate out 3 dilutions, one on either side of the expected value.

That comes with experience and knowing your product/process really well though, so we cant always do it. 

If you know for sure that the counts will be in the -5 -6 range, then I'd be plating out -4 -5 -6, it seems pointless doing -1 to -3, except to cover that 1 in 1000 chance that one of your samples has <1000 cfu/g.

Go through your results history and figure out how often that happens and you can assess if there is an acceptable level of risk in not plating the lower dilutions.



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