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What Dilution do You Consider it...?

environmental samples plating Dilutions

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#1 VickieLew

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Posted 26 March 2019 - 04:14 PM

Hello,

I have been asked the following question countless times during my 30+ year career;

 

When plating environmental surface samples taken using a pre-moistened sponge or swab, what dilution do you consider 1ml of liquid pulled directly from the sample (sponge/swab) to be? 

 

I have asked many colleagues this same question over the years and as recently as last week.  I get a variety of "sympathetic" answers; Good question!, Let me know when you find out, etc.  Some answers are more definitive; "It's a 0 dilution" or "It's a -1".  And of course, everybody's favorite answer; "Well, it depends..."!

 

I'd like to poll this community; what dilution do you consider it, and...WHY.  If anyone has any reliable/credible references, please provide links or attachments.

Don't be shy, my experience asking this question seems to indicate that there isn't a right or wrong answer! 

I'll provide my answer and reasoning after the polling, I don't want to influence any input members may have.

 

I think this will be interesting, hopefully educational, and maybe even fun (if you're a micro-geek like me!).

Vickie



#2 Charles.C

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Posted 26 March 2019 - 08:35 PM

Hello,

I have been asked the following question countless times during my 30+ year career;

 

When plating environmental surface samples taken using a pre-moistened sponge or swab, what dilution do you consider 1ml of liquid pulled directly from the sample (sponge/swab) to be? 

 

I have asked many colleagues this same question over the years and as recently as last week.  I get a variety of "sympathetic" answers; Good question!, Let me know when you find out, etc.  Some answers are more definitive; "It's a 0 dilution" or "It's a -1".  And of course, everybody's favorite answer; "Well, it depends..."!

 

I'd like to poll this community; what dilution do you consider it, and...WHY.  If anyone has any reliable/credible references, please provide links or attachments.

Don't be shy, my experience asking this question seems to indicate that there isn't a right or wrong answer! 

I'll provide my answer and reasoning after the polling, I don't want to influence any input members may have.

 

I think this will be interesting, hopefully educational, and maybe even fun (if you're a micro-geek like me!).

Vickie

 

Hi Vickie,

 

I'm not too sure as to the significance of the question.

 

Presumably one assumes (rightly or wrongly) that the total liquid volume (X ml) extractable from the swab is equivalent to the area swabbed.

 

So the plate count/(Area swabbed) is presumably X times the result for 1ml ? (assuming the plate has a countable level of bacteria)

 

Personally I have always avoided explicit use of  textbook formulae with "dilution numbers" in Plate Count calculations.

 

(Based on previous threads here, one occasional problem is that external labs often return a plate count result without specifying what it refers to).


Kind Regards,

 

Charles.C


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#3 EagleEye

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Posted 27 March 2019 - 06:44 AM

Hi Vickie,

 

I'm not too sure as to the significance of the question.

 

Presumably one assumes (rightly or wrongly) that the total liquid volume (X ml) extractable from the swab is equivalent to the area swabbed.

 

So the plate count/(Area swabbed) is presumably X times the result for 1ml ? (assuming the plate has a countable level of bacteria)

 

Personally I have always avoided explicit use of  textbook formulae with "dilution numbers" in Plate Count calculations.

 

(Based on previous threads here, one occasional problem is that external labs often return a plate count result without specifying what it refers to.)

 

Mr. Charlse said it..

 

I couldn't agree with you more..



#4 Bo16

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Posted 03 April 2019 - 02:06 PM

This depends on what you define in your procedure: 1st define area to be swabbed (location and size of swabbing for that area)  Then define dilution or no dilution and reporting requirements.

Some examples:  (same swabbing area)

Moistened swabs - swab surface then swab plates:  Colonies per swab;

Moistened swabs - inserted into 10 ml of buffer, vortex, 1ml plated:  1:10 dilution but still per swab.

Rinse water is different:  per ml, per 100 ml - depends on the volume of water your test since there is not usually a dilution involved.

 

Use science and define the use of a dilution or no dilution (factor of 1, no such thing as a factor of 0) for your method.

 

Since determination of in or out of specification comes from sanitation validations, historical values or regulations (sterile) as long as you are consistent and defined your process you should be OK.

 

 



#5 Charles.C

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Posted 04 April 2019 - 12:35 AM

This depends on what you define in your procedure: 1st define area to be swabbed (location and size of swabbing for that area)  Then define dilution or no dilution and reporting requirements.

Some examples:  (same swabbing area)

Moistened swabs - swab surface then swab plates:  Colonies per swab;

Moistened swabs - inserted into 10 ml of buffer, vortex, 1ml plated:  1:10 dilution but still per swab.

Rinse water is different:  per ml, per 100 ml - depends on the volume of water your test since there is not usually a dilution involved.

 

Use science and define the use of a dilution or no dilution (factor of 1, no such thing as a factor of 0) for your method.

 

Since determination of in or out of specification comes from sanitation validations, historical values or regulations (sterile) as long as you are consistent and defined your process you should be OK.

 

Hi Bo,

 

The ideal safety objective is to estimate the density/existence of relevant pathogens on a (usually) extended target surface.

 

More commonly "indicators" are used such as APC, generic E.coli.

 

The specific product/process situation is likely to relate to the "natural" level of surface bacterial "contamination". And perhaps the target level of "cleanliness".

 

Typically one indicator measurement per "location" is made and assumed representative.

 

There are no generally accepted limits for APC values on facility food contact surfaces.  Literature values regarding desirable bacterial limits per unit area  for a satisfactorily just cleaned surface illustrate the considerable variations in opinion.

 

The practical (direct) estimation of bacterial density on the target surface is reliant on obtaining an appropriate (min/max) number of (visually countable) colonies on the plate. Dilution is an ancillary factor IMO.


Kind Regards,

 

Charles.C






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