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

Abstract

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 .1% inoculum, a very sensitive assay was obtained that detected phages against acid-producing strains in a starter within 3 to 4h. 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 9h 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.