May 23, 2002 —
The Wageningen researcher Christine Bunthof has developed a direct
method for counting bacteria in dairy products. The method not only
distinguishes viable and non-viable bacteria but also semi-viable
bacteria. These are too weak to divide, but still exhibit activity. The
semi-viable bacteria play an important role in cheese ripening and
therefore influence the taste.
the new counting method, developers of probiotic dairy products can
also investigate how active the added bacteria are when they enter the
stained the bacteria with two fluorescent substances. One substance
ensures that the bacteria emit a green colour if they are active. The
substance itself is not fluorescent but is converted by bacterial
enzymes into a green fluorescent dye. The second substance stains
bacteria with a damaged membrane. This substance emits red but only
when it is bound to the bacteria`s DNA.
The method distinguishes
viable from non-viable types of bacteria. It also detects semi-viable
bacteria as, although these can no longer divide, they are still active
and therefore emit green. Only non-viable bacteria emit red. Christine
Bunthof used a flow cytometer to count the bacteria. In this device,
the bacteria are transported one by one past a laser which lights up
the bacteria. A detector counts the number of red and green bacteria.
time immemorial the diary industry has always counted bacteria which
can still divide. When cultured on a nutrient medium, these bacteria
form visible colonies. However, this old method fails to detect
semi-viable bacteria, as active bacteria which do not multiply remain
invisible. Yet this intermediate group constitute a considerable
proportion of the active bacteria in ripening cheese.
counting method is not only useful for following the ripening of cheese
and yoghurt. It is also important for the development of probiotic
dairy products. These products contain bacteria with a healthy effect.
However, some bacteria do not survive the acidic gastric juices and the
bile in the small intestine. Using this new method, producers can test
how well bacteria can reach the intestines in an active form.
Source : NWO (Netherlands Organization for Scientific Research)