This article walks you through the steps of product inspection, testing and the environmental monitoring programme. The following information complies with:

BRCGS Food Safety Issue 8 5.6.1.2 & 5.6.2.5 Understanding results
BRCGS Packaging Issue 6 4.8.5 Understanding results
BRCGS Agents & Brokers Issue 2 4.4.5 Understanding results
Storage & Distribution Issue 4 No specific references
FSSC22000 Version 5.1 No specific references
IFS Food Version 7 5.6.4 Analysis of results
SQF Edition 9 No specific references

Please note, that although some of the above standards don’t mention specifically that you must understand the results of testing, they all do expect corrective and preventive actions to be taken. Which indirectly means, you must understand the results – in order to apply the right type of action.

Important: This article is written for the food industry. We get a lot of questions about pharmaceuticals, which we will try to answer but as we are not specialists in this field. The reference standards are different for pharmaceuticals and are therefore not something we’re familiar with.

How to read a micro lab report

Unless you’ve worked in a lab or had to manage one, being faced with a micro lab report can be daunting.

Unfortunately, there’s no simple training courses you can go on where they teach you how to read a lab report. Or at least there wasn’t until now. In this article we’re going to take you through all the key points you need to know, so you can read micro lab reports with ease.

There are five main types of results that you get on a micro lab report, which are:

  1. Detected or Not Detected
  2. A specific result (e.g. 100cfu/g)
  3. A less than result (e.g. <10cfu/g)
  4. A log result (e.g. 105cfu/g)
  5. A greater than result (e.g. >105cfu/g)

We’ll go through each type of result, so you understand each one.

1. Detected or Not Detected

This type of result is given to pathogenic bacteria that only need a small quantity of bacteria to be present in the food, to cause food poisoning – such as Listeria or Salmonella.

Therefore, the lab will report that they have either detected the bacteria (reported as ‘D’) or not detected the bacteria (reported as ‘ND’).

2. A specific result (e.g. 100cfu/g)

Results like this are either provided in cfu/g or cfu/ml. cfu stands for colony-forming unit. This means that cfu/g is colony-forming unit per gram and cfu/ml is colony-forming unit per millilitre.

A colony-forming unit is where a colony of microbes grow on a petri dish, from one single microbe. On the petri dish image below, each one of these would be a colony-forming unit.

When the lab carries out your test, they count of the number of colony-forming units on the petri dish. They give you the results of the number of colony-forming units, for the number of grams or millilitres of test material that they put on the petri dish.

So, in the most simplistic terms this would be, for example:

  • 100 colony-forming units for one gram put on the petri dish.
  • This would be written as: 100cfu/g.
Colony-forming units

3. A less than result (e.g. <10cfu/g)

Sometimes you get results that say: <10cfu/g or <100cfu/g. But if you think about it, you can’t get less than 1 colony-forming unit on a petri dish – can you? It’s either there or it’s not.

What this means is, that the lab hasn’t seen any colony-forming units on the petri dish at all. But they can’t say that the result is ‘0’ (zero), as every test has an inaccuracy. By reporting <10cfu/g, the lab is saying that there’s an inaccuracy of 10 in that test, but essentially that the result was 0, because they couldn’t find anything on the plate.

4. A log result (e.g. 105cfu/g)

When a result is high, the result is condensed using what are called logarithms. The logarithm is the superscript number to the right of the 10.

To be able to read logarithms, you need understand how they work.

Understanding logarithms

A micro logarithm is generally known as a ‘log.’ The log is a value of how many cfu (colonies) there are. Each log is worth a multiple of 10, as follows:

Logs start at 1. Each log increase, increases the cfu by a multiple of 10. One easy way of remembering this is that you add the log number of zeros to 1 to create the log.

So, for 1 log it’s 1 and a zero after it (10), 2 log is 1 with 2 zeros after it (100)

Table 1

Log cfu number
1 10
2 100
3 1,000
4 10,000
5 100,000

Table 2

Log cfu number
1 10 to 99
2 100 to 999
3 1,000 to 9,999
4 10,000 to 99,999
5 100,000 and so on

There’s a range of numbers in every log, shown in the table 2.

This means that if you had a result of 1,600cfu, this would be 3 log.

As it would sit within the 3 log range of between 1,000 and 9,999.

Calculating logs

To work out the result from a lab report, you need to multiply the result before the 10log by the log value.

Like this:

  • Result x 10log = cfu count/ml or g

For example.

  • 1.5 x 105cfu/g

Here the log is 5. If you refer table 1 you can see a log of 5 is 100,000. So, the calculation would be:

  • 1.5 x 100,000 = 150,000cfu/g

Let’s try another example.

  • 5.7 x 103cfu/g

Here the log is 3. Referring to table 1 we can see that a log of 3 is 1,000. So, the calculation would be:

  • 5.7 x 1,000 = 5,700cfu/g

Common mistakes

Let’s understand why you’d look at the two results below and think the 9.8 x 103cfu/g was the highest result.

  • 9.8 x 103 cfu/g
  • 1.1 x 105 cfu/g

People often just look at the first number in the results – 9.8 is higher than 1.1, so they presume it’s the higher result. But if we work them out, you can see that it’s not.

  • 9.8 x 103 cfu/g = 9.8 x 1,000 = 9,800cfu/g
  • 1.1 x 105 cfu/g = 1.1 x 100,00 = 110,000cfu/g

5. A greater than result (e.g. >105cfu/g)

You’ll get a greater than result, if the results are really high. Let’s take an example to explain.

Let’s say, you were to get a result of 100,000 colony-forming units. Imagine what the petri dish would look like. How would the lab count 100,000 colony-forming units on the plate? The answer is they don’t. You wouldn’t be able to physically do it – the plate would be too full.

When this happens and the plate is too full to count, you get a greater than result. This means, that the lab can only tell you the maximum number of colony-forming units that they would have been able to count on the plate and say that it was greater than that.

The problem with this is you don’t know what the true result would’ve been. A result like this isn’t good. What should’ve happened, is the lab should’ve diluted the sample before it was plated. But they’d need to know that they need to dilute it, so you would need to communicate with them for this to happen.

Dilution

To ensure that you get a result that you can use, where the cfu is expected to be high, the lab needs to dilute the sample.

If the lab knows the results are going to be high, they won’t put 1 gram of that product on a petri dish. Instead, they’ll take for example, 1 gram of the product and dilute it with water. If they dilute it with 99 grams of water – this would be 1 in 100 dilution.

They would then add 1 gram of the 1 in 100 dilution to the plate.

If when they then did the test, they found 14 colony-forming units on the plate, they’d then multiply the 14 back up by the dilution factor of 100 – making the total number of colony-forming units (14 x 100) 1,400.

The higher the expected result of the test, the more the lab will dilute the sample, in order to give you a good result – as in an actual reading, rather than a greater than result.

Swab results (e.g. 103cfu/swab)

If you send swabs away for testing, you may find that your lab reports the results per swab or an area.

If the lab has specified the surface area to swab, they ‘ll give the results by area.

If they haven’t specified a surface area to swab, then they’ll give the result by swab. You then need to calculate the results by area. This is so that the results are comparable.

For example: If you take a swab of an area that’s 10cm x 10cm2. The total area you’ve swabbed is 100cm2.  When you receive the result it might be 1,400cfu/swab so you’d divide this by your area and you’d get 14cfu/cm2.

Swab results

Log reduction for micro validation

You may have heard of the term ‘log reduction’. This term is typically used when validating food safety controls.

When you cook raw meat, you’re essentially carrying out log reduction. This is because you’re reducing the number of cfu in the meat, to a safe level. Also, when you clean a surface with disinfectant, you’re achieving a log reduction.

When you carry out a chlorine wash of raw salad or vegetables, typically you try to achieve a log reduction of 2. This means you need to understand from your reports if you’ve achieved a log reduction.

Let’s look at this as an example of how this is done.

If the cfu of the unwashed salad was 12,000cfu/g, using our log table 2 we could establish that this value sits in the 4 log range.

To reduce the micro load of the salad, by 2 log, you’d need to remove enough microbes in the washing process, to achieve a log of 2.

This means the result of the washed produce needs to sit between 100 to 999cfu/g which is shown on table 2 above.

Frequently asked questions

What does log cfu/g mean?

A log is a multiple of 10, that’s used for reporting micro. cfu/g means colony-forming unit per gram. It’s basically, the number of colonies counted on a petri dish.

What is ‘D’ and ‘ND’ on my lab report?

The lab has reported that they have either detected the bacteria (reported as ‘D’) or not detected the bacteria (reported as ‘ND’).

What’s a less than result mean?

What this means is, that the lab has not seen any colony-forming units on the petri dish at all. But they can’t say that the result is ‘0’ (zero), as every test has an inaccuracy. By reporting <10cfu/g, the lab is saying that there’s an inaccuracy of 10 in that test, but essentially that the result was 0, because they couldn’t find anything on the plate.

What’s a greater than result mean?

You’ll get a greater than result, if the results are really high. When this happens and the plate is too full to count. This means, that the lab can only tell you the maximum number of colony-forming units that they would have been able to count on the plate and say that it was greater than that.

What do I need to do if my sample is expected to have a high count?

Ask the lab to dilute your sample so you can get an accurate count.

What size area should I swab if the lab hasn’t specified this?

Any size is OK as long as you record the size of the area that you’ve swabbed so you can interpret the result when you get it back.

Why are logarithms used by the lab when they send the results back to me?

The count on the plate you sent was high, so they’ve shown the result using a log count to reduce the number of zero’s shown in the result.

Have your say…

11 thoughts on “The meaning of log cfu and how to read micro lab reports

  1. Handy briefing article!

    We’ve last month issued (with FSS) “Guidance for Food Business Operators: Getting the Best from Third Party Laboratories”.
    This Guidance aims to raise FBOs’ awareness of the need to use analytical laboratories with the right expertise, accreditations, using appropriate methods and facilitate development of partnerships between such third-party laboratories and their customers in the food industry, moving away from purely transactional arrangements.

    The focus of this first edition is on microbiological analytical services provided by a third party to a FBO.

    The Guidance is a free download (https://www.chilledfood.org/guidance-for-food-business-operators-getting-the-best-from-third-party-laboratories) and includes:

    Checklist
    Fitness for Purpose – Laboratories and Methods
    Provision of Samples to Laboratories
    Reporting Results
    Complaints Procedure
    Selecting a laboratory through tender
    Special measures for laboratories
    ContractsAppendices
    1. Terminology
    2. Microbiology
    2.1 Legally Recognised Methods
    2.2 What other Microbiological Tests are Relevant for Various Food Materials
    Tables
    1: Industrywide Continuous Improvement Indicators for Laboratories
    2: Findings, Laboratory Action and Communication of Results
    3: Microbiological Methods – Specified by EU Regulation 2073/2005
    4: Typical Expected Turnaround of Microbiological Tests if Compliant with Standard Methods

    1. MPN isn’t a result that would ordinarily be provided on a certificate for the related tests we do in the food industry, such as tests on food or environmental. It certainly isn’t a measure I have come across.

      MPN, or Most Proabable Number, is a calculation based on liquid broth growth using 10-fold dilutions of the sample. It works, in essence, in a similar format to counting cfu on a petri dish.

      If you’re expecting high results, and you have asked the lab to dilute the sample accordingly, then MPN may be reported instead of cfu. However, it would have to be a very specific type of product for this.

      This article I found online gives some interesting information on the use of MPN: https://microbeonline.com/probable-number-mpn-test-principle-procedure-results/

  2. This is a great article, thank you for this. It is so easy to forget how to interpret the results at times, but even harder to actually explain to people what the results mean and why. This article clears that up. I’ve already sent the link to my Technical Team and asked them to read it in preparation for an on-site workshop on the subject.

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