Conductimetric Analyses of Bacteriophage Infection of Two Groups of Bacteria in DL-Lactococcal Starter Cultures

¹ Swedish Dairies' Association, Ideon ß3, S-223 70 Lund, Sweden

Two conductimetric methods were used for simple and rapid detection of bacteriophages against the two major groups of bacteria in DL-lactococcal, mixed-strain starter cultures. Bacteriophages against the acid-producing bacteria Lactococcus lactis ssp. lactis and Lactococcus lactis ssp. cremoris in a starter were analyzed in a Malthus® instrument using reconstituted skim milk as the medium. The presence of phages against Lactococcus lactis ssp. lactis biovar. diacetylactis strains in a starter could be analyzed by measurement of CO₂ production with an indirect conductance method in the Rabit® instrument. For both methods, the presence of phages against bacteria in a starter increased the detection time. At 30°C and .l% inoculum, a very sensitive assay was obtained that detected phages against acid-producing strains in a starter within 3 to 4 h. The presence of 40 to 1000 infectious phages was sufficient for detection with a mixed-strain starter culture. Bacteriophages against CO₂- producing bacteria were detected after 7 to 9 h with a .1% inoculum at 30°C. Bacteriophage infection of single strains of bacteria reduced total change in conductance. For pure strains of an acid- or a CO₂-producing strain, the detection limits were 1 and 100 pfu, respectively.

Key Words: bacteriophage analysis • conductance • indirect conductance method • starter culture activity

Submitted on March 10, 1994
Accepted on July 26, 1994