WHAT DOES CFU STAND FOR?

Colonies growing on the filter surface should counted as individual organisms. The count of these colonies gives you your CFU count.

For most samples, the count per ml is the generally accepted system for recording your results. Therefore, for non-diluted samples, the number of colonies observed on the filter will be the number recorded (as sample count/ml).

For diluted samples, you should multiply the count by the dilution factor.

In recording your count with the blue Tester for coliforms, count only the blue colonies. Coliform and faecal coliforms (blue Testers) are always reported as the number per 100 ml of liquid sample and the count should be multiplied by 100.

Some populations of microorganisms grow faster than others.

When a micro-organism is trapped from the environment where it was living to be incubated on a test device, it will need to adapt its metabolism to the new conditions (change in temperature, nutrients, ..) before it starts multiplying.  Moreover, if the microorganisms are stressed by a heat or chemical treatment or starvation, the lag phase before they multiply may last up to a couple of days.

When testing a new sample we don't know how long it will take for the microbes on the filter to appear visibly.

To get a more accurate count, it's a good idea to take an extra reading two thirds of the way through the incubation time, and another one-third of the recommended incubation time after you take the final reading.

Thanks to these extra readings you will be able to:

• better count slow-growing populations whose colonies are difficult to see and count after a normal incubation period
• reduce unsure counts resulting from the fact that some fast-growing colonies overlap, join or merge with others.

As you can see, at the end of the the recommended incubation period, a mould has grown quickly and overgrown a nearby bacteria colony, which might have been missed had we not taken the first reading.

In addition, in the second reading, a small colony has appeared on a grid-line which might have been overlooked had it not been for the third reading.

After three separate readings we know that the final count is 8, even though 8 individual colonies are not easy to observe on any of the individual readings. It can be a good idea to use a marker pen and put a dot over each colony on the tester chamber to facilitate counting over multiple readings.

Once you know the optimum incubation duration for a given temperature, you may reduce your counts to just one or two observations.

Learn more about how using nomad testers as part of a microbial monitoring program can help your business.