Environmental Swabs - Tesco Requirements

Has anyone had experience of Tesco requiring testing of environmental swabs for aerobic colony count and enterobacteriaceae with limits of <10 and <1 respectively?

Most swabs for enumeration are cotton bud type, inside a tube containing 10ml neutralising buffer, which is required to quench residual cleaning agents. I believe 1ml is then used for each test, using pour plate method, therefore it is not possible to state that enterobacteriaceae could be <1 cfu/swab. No growth on a plate would equate <10 cfu/swab.

Is this an over-zealous auditor or are Tesco really expecting this?

Hi Poppysnoss. I have experience in this area.

In short - your auditor is not being over-zealous (at least not in the application of Tesco requirements). I believe there is a Tesco COP on the issue so what you are being asked for is consistent with other suppliers to Tesco I have worked with in the past.

As to whether the specific requirements are over-zealous in and of themselves - well that would make an interesting topic for another discussion.


George Howlett.

Hi Poppysnoss. I have experience in this area.

In short - your auditor is not being over-zealous (at least not in the application of Tesco requirements). I believe there is a Tesco COP on the issue so what you are being asked for is consistent with other suppliers to Tesco I have worked with in the past.

As to whether the specific requirements are over-zealous in and of themselves - well that would make an interesting topic for another discussion.

George Howlett.

Hi George.

Thanks for the reply. I should clarify more what I meant - it isn't possible to obtain a <1 cfu/swab result when using a swab with 10ml neutralising buffer solution, as this is how they are provided. Therefore, it seems a bit strange to me to implement a specification of <1 cfu/swab, when it can't be achieved.

Or am I missing something.....?

Pops

Dear poppy,

a specification of <1 cfu/swab, when it can't be achieved

Maybe a few, very big plates (extension of the ISO, Staph.aureus plate method )

or 10 normal size plates (all -ve).

or (more likely ), MPN method and someone's given you the wrong procedure.

Rgds / Charles.C

Dear Poppy and Charles
It is a rare result to found this low limit in the environmental swabs in any food plants neither for aerobic colony count nor enterobacteriaceae. Would you please can inform me about the "Tesco requiring standard" and what is the kind of your food organization under the inspection?
Any way the MPN technique is the method of choice for the expected very low count in any sample.
Hoping that is my reply is not too late for this discussion.
Regards
Youssef

While the limits that have been set are quite tight, it is not necessarily incorrect to have limits of <1 cfu/swab.
It depends on the methodology used, but in my experience the liquid that the swab is suspended in is not counted as a dilution, it is considered just the 'initial suspension.
So 1ml of the liquid = 1ml of sample.
When that 1ml is put onto a petri dish,any resulting colonies give the count/ml.
Therefore if you get 1 colony, your result is 1cfu/swab, 15 colonies = 15cfu/swab.
Similarly if you get no colonies, your result is <1 cfu/swab.

Des

Has anyone had experience of Tesco requiring testing of environmental swabs for aerobic colony count and enterobacteriaceae with limits of <10 and <1 respectively?

Most swabs for enumeration are cotton bud type, inside a tube containing 10ml neutralising buffer, which is required to quench residual cleaning agents. I believe 1ml is then used for each test, using pour plate method, therefore it is not possible to state that enterobacteriaceae could be <1 cfu/swab. No growth on a plate would equate <10 cfu/swab.

Is this an over-zealous auditor or are Tesco really expecting this?

...It depends on the methodology used, but in my experience the liquid that the swab is suspended in is not counted as a dilution, it is considered just the 'initial suspension...

(quoting an excerpt of the original message)

I'm just a layman, so pardon my ignorance. How is it possible to not count the liquid in which it is suspended as dilution? What if it is suspended in 100ml, or 1000ml? But I think what you said, 'depends on the methodology used' is key. That is, whether you count the buffer or liquid the swab is suspended in depends upon sample size, amount of buffer, and how it is reported. Do I have that right?

Thanks!

Yep, methodology is key here. Labs, Producers and buyers may all have their own methodology based on ISO methods, historical experience or just convenience!

If you are asked for to meet a target given in terms of cfu/swab, then using the initial suspension as the neat sample is fine. You could then demonstrate that you can meet a target of <1cfu/swab. The result of cfu/swab is not a very quantitative or objective measure as it doesnt specify what are has been swabbed, and of course doesnt take into account the volume of suspension liquid. Calculations are simple and quick though.

If you wish to adopt a more quantitative approach, then you would swab a fixed area eg 20cm² and suspend the swab in 40 or 100ml suspension medium. You could then calculate the cfu/cm² of surface by taking into account the area swabed, the number of colonies counted and the dilution at which the colonies were counted.

ISO18593 covers this area in more detail.

Des

Dear All,

Another conclusion could be that Tesco are causing considerable confusion by using non-standard "specifications".

Rgds / Charles.C

(quoting an excerpt of the original message)

I'm just a layman, so pardon my ignorance. How is it possible to not count the liquid in which it is suspended as dilution? What if it is suspended in 100ml, or 1000ml? But I think what you said, 'depends on the methodology used' is key. That is, whether you count the buffer or liquid the swab is suspended in depends upon sample size, amount of buffer, and how it is reported. Do I have that right?

Thanks!

Hi All.

Haven't logged on for a while as in the middle of exam hell...

Anyway, I would always count the liquid that the swab is suspended in. If a cotton tipped swab is immersed in 10ml, and nothing grows on the plate, then in my experience the result is reported as <10 cfu/swab - because the 10ml suspension is still diluting the swab. However, I have heard in the past that this is reported slightly different in Ireland and the initial liquid is not taken into account, but not sure why?

Oh, meant to report that my original query was solved by switching to contact plate method for surfaces.

Thanks for all the replies.

Oh, meant to report that my original query was solved by switching to contact plate method for surfaces.

Thanks for all the replies.

Dear poppy,

Very cryptic.

Are you saying that you concluded that the Tesco requirement/specification (as initially interpreted by you/here) was nonsense so you simply answered their requirement in a different way.? If so, how so, please ?

Rgds / Charles.C

The official criteria in Ireland say that labs should report results as cfu/cm^2, thus ensuring that the volume of liquid used in suspending the swab is taken into account as well as the area examined. I think that some labs just historically used the old method of cfu/swab, or in the case of satisfying the specifications set by a buyer, they are obliged to work that way.

Hi All.

Haven't logged on for a while as in the middle of exam hell...

Anyway, I would always count the liquid that the swab is suspended in. If a cotton tipped swab is immersed in 10ml, and nothing grows on the plate, then in my experience the result is reported as <10 cfu/swab - because the 10ml suspension is still diluting the swab. However, I have heard in the past that this is reported slightly different in Ireland and the initial liquid is not taken into account, but not sure why?

Dear Des Walsh,

Thks for yr input on this topic. I was not previously aware of the procedural ISO standard you mentioned. Various (primarily APC) numerical swab guidelines exist as frequently discussed on this forum. IMEX these are usually “keyed” to swabbed areas. Even so, the numbers given vary widely. However I hv also seen arbitrary reference “units” utilised in some industries for convenience, eg for rating the internal cleanliness of standard milk vessels.

I suspect that in this specific thread, as already suggested, different “people” are simply using different starting assumptions / enumeration methods.

I had a look at ISO 18593 which is not the easiest read I hv ever met . The extracts below summarise IMO the numerical aspects of their swab option using a swab stick / pour plate procedure.

3.1 Because these methods are not quantitatively reliable or reproducible, results should only be used in a “trend analysis”.

3.3 Using the swab method, a specified area of the surface to be examined is marked (e.g. using a template)
and then wiped. The swab sticks are broken into a tube or bottle containing a sterile dilution fluid or
neutralizing fluid and mixed by hand.

If high numbers of microorganisms are expected, prepare further decimal dilutions in peptone water diluent to obtain countable number of colonies (see ISO 6887-1).

Inoculate duplicate plates of media with initial suspension, using 1 ml of inoculum for pour plates. Treat any further dilutions in the same manner. Invert the dishes and incubate the plates for the appropriate time and temperature.

9.2.1 Calculate the number of colony-forming units (CFU) per millilitre of initial suspension, as described in the relevant International Standard.
9.2.2 Calculate the number of CFU per square centimetre of surface investigated, N_S, using the formula
N_S = N x F x D / A
where
N is the number of CFU in 1 ml of dilution liquid (or neutralizing liquid);
F is the amount, in millilitres, of dilution fluid (or neutralizing liquid) in the tube
A is the surface investigated, in square centimetres;
D is the reciprocal of the dilution used.

9.2.4 If the area swabbed was not defined, to calculate the number CFU per swab, N_SW, use the formula
N_Sw = N x F x D

Comments

Para. 3.1 above is worth noting.
I assume the “initial suspension” in 9.2.1 refers to the holding fluid/swab as introduced in 3.3 above.
The preferred calculation is seemingly to measure the area swabbed and calculate the result as per the equation given, eg as CFU/cm².
If the swabbed area is unknown/not readily measurable, equation 9.2.4 is permitted. I guess this may have potential for defined but irregular shaped objects. Hands ?
For either N_S or N_Sw, the initial volume of suspension fluid (F) seems to be used in the calculation.
If Tesco were using the above basis, Poppy’s original criticism that “cfu/swab is <1 is illogical” seems well justified to me (the intriguing aspect is that Tesco provide different "nil" limits for APC and Enterobacteriaceae).

@Poppy - I deduce from yr post(s) that Tesco did not (procedurally) explain fully their requirements.?

Rgds / Charles.C

PS If anyone is interested in the detailed meaning of "dilution theory" a nice explanation IMO with examples is here -

http://www.jlindquis...ro/102dil1.html
http://www.jlindquis...cro/102P1S.html