Application of the Conductance Measurement Technique for Detection of Streptococcus salivarius ssp. thermophilus Phages

¹ Istituto Sperimentale Lattiero Caseario, 20075 Lodi, Italy

Conductance measurement for the detection of Streptococcus salivarius ssp. thermophilus phages was carried out using the Malthus growth analyzer system. Phage activity on a sensitive strain is revealed by a delay in the conductance detection time or a change in the trend of the conductance m e. Analysis of the conductance curve leads to the detection of phage-resistant secondary cultures. The inverse proportion existing between the number of phages and the conductance values recorded at a given time permit evaluation of phage numbers in a sample.

Conductance measurement highlights active phages against one of the lactic species present in yogurt. The conductance measurement proved to be an effective alternative to the traditional resazurin method for the detection of S. salivarius ssp. thermophilus phages. The sensitivity of the instrument, time savings, and the ability to analyze a large number of samples simultaneously make the conductance method useful for phage detection.

Key Words: Streptococcus salivarius ssp. thermophilus • phage detection • conductance measurements

Submitted on June 28, 1990
Accepted on October 26, 1990