Count semiviable bacteria in cheese.

Wageningen University researcher Christine Bunthof has developed a direct method for counting bacteria in dairy products. The technique not only distinguishes viable and nonviable bacteria, but also semiviable bacteria. These are too weak to divide, but they still exhibit some activity.

Semiviable bacteria play an important role in cheese ripening by influencing the product's taste. With the new counting method, developers of probiotic dairy products can also investigate how active the added bacteria are when they enter the gastrointestinal tract.

This new counting technique also is applicable to yogurt. Some bacteria from these and other products do not survive the acidic gastric juices and the bile in the small intestine. Using this new method, producers can test how well bacteria reach the intestines in an active form.

The microbiologist from Wageningen stained the bacteria with two fluorescent substances. One substance ensures that the bacteria emit a green color if they are active. The substance itself is not fluorescent, but it is converted by bacterial enzymes into a green fluorescent dye. The second substance stains bacteria with a damaged membrane. This substance emits a red color, but only when it is bound to the bacteria's DNA.

The process distinguishes viable from nonviable types of bacteria. It also detects semiviable bacteria since, although these can no longer divide, they are still active and emit a green color. Only nonviable bacteria emit red. Investigators use a flow cytometer to count the bacteria. In this device, the bacteria are transported one-by-one past a laser which illuminates the bacteria. A detector counts the number of red and green bacteria.

Since time immemorial, the dairy industry has always counted bacteria that can still divide. When cultured on a nutrient medium, these bacteria form visible colonies. However, this old method fails to detect semiviable bacteria, since active bacteria that do not multiply remain invisible. Yet this intermediate group constitutes a considerable proportion of the active bacteria found in ripening cheese.

Further information. Christine Bunthof, Laboratory of Food Microbiology, Wageningen University, Postbus 8129, 6700 EV Wageningen, The Netherlands; phone: [phone]; fax: [phone]; email: [e-mail].