# Micro counting methods - from two different plate dilutions

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### #1 Dom

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Posted 31 May 2011 - 11:10 AM

Hi All

i just have a query regarding the counting of micro plates and how to get a count from too different plate dilutions, does one simply average two dilution counts? or is there a method to be used.

For example: a dil of 10(-1) = value of 15 colonies and a dil of 10(-2) of 1, does one simply add the 150 and 200 to get 175cfu/g?

And if your second dilution is 0 colonies, is this dilution still taken into account or does one just take thego with the first dilutions colony count into account?

Thanks B x

### #2 Charles.C

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Posted 01 June 2011 - 06:47 PM

Dear Dom,

One immediate comment is that the colony counts per plate you mention would not normally be considered acceptable due too low for accuracy but if there is no option…..
Not quite sure where you calculated 200 from but yr principle is, I think, similar to some reference methods.

Generally, there exist a few slightly different viewpoints on the calculation details for yr type of question which can yield somewhat different answers although the differences are probably unlikely to be significant within the typical accuracy of such measurements.

One popular source is the US-FDA-BAM procedures/formulae as per these links –

http://www.fda.gov/F...M/ucm063335.htm

http://www.fda.gov/F...alBAM/UCM063346

However another source which I personally prefer (simpler arithmetic) for plate counts is as per the attachment below which details rules/examples for most of the variations which commonly occur in practice –

(The classic example similar to yr query (although using [preferred] duplicate plates) is in table1, No.1111)

Ignoring the objections to the quality of yr posted data, and hoping for no arithmetic error (always possible) I think a simple arithmetic average of yr data gives 125 cfu/gram assuming that unspecified experimental conditions are typical ones.

Rgds / Charles.C

Kind Regards,

Charles.C

### #3 SaniS

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Posted 25 August 2011 - 08:19 PM

Dear All,

Its old and pretty interesting discussion!!

I need your help for my query.

For a conventional aerobic plate (USFDA- BAM) method can we use spread plate method over pour plate method (0.1 ml from each dilution to duplicate plates)

Is there any changes we have to take care when calculating the results?

FYI my product is frozen meat

Regards
San

### #4 Dr.Des

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Posted 26 August 2011 - 02:27 PM

Hi SaniS
I think you could use the spread plate technique, but there are one or two things that are worth noting.
In general with spread plates you would pipette 0.1 or 0.5ml aliquots onto the plate and this volume must be taken into account when calculating results.
The smaller volume also means that your limit of detection is affected - for pour plates the lowest count you could have would be 10 cfu/g since you plate out 1ml of a 10-1 dilution.
If you plate out 0.1ml or 0.5ml, your limit of detection rises to 100 or 20 cfu/g respectively.
thus if you detect no bacteria, you report your result as <100 or <20 cfu/g.

Another thing to remember is that pre-poured agar plates are prone to aerial contamination so need care in preparation and storage.
Pre-poured plates may also need to be dried before use.

Des

Dear All,

Its old and pretty interesting discussion!!

I need your help for my query.

For a conventional aerobic plate (USFDA- BAM) method can we use spread plate method over pour plate method (0.1 ml from each dilution to duplicate plates)

Is there any changes we have to take care when calculating the results?

FYI my product is frozen meat

Regards
San

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