Comparison of the Lactococcus lactis Differential Medium (DCL) and SDS-PAGE of Whole-cell Proteins for the Identification of Lactococci to Subspecies Level

Abstract

In order to evaluate the practical usefulness of the DCL differential medium and highly standardised SDS-PAGE fingerprinting of whole-cell proteins for the identification of Lactococcus strains to subspecies level, a total of 99 Lactococcus lactis subsp. lactis and subsp. cremoris strains were analysed by both techniques. The SDS-PAGE protein patterns were compared to reference strains of lactic acid bacteria of the genera Aerococcus, Carnobacterium, Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Pediococcus, Tetragenococcus , and Vagococcus . Numerical analysis of the SDS-PAGE protein patterns clearly discriminated all species investigated. Seventy two Lactococcus lactis strains were assigned to their respective Lactococcus lactis subspecies. Twelve strains were originally incorrectly identified to subspecies level and two strains were identified as Lactococcus garvieae . Four Lactococcus lactis strains did not belong to the genus Lactococcus ; one strain remained unidentified. For eight Lactococcus lactis strains identification to subspecies level was not possible by SDS-PAGE. Identification of Lactococcus lactis subsp. lactis var. diacetilactis strains could not be achieved by SDS-PAGE. One strain received as Lactobacillus sp. and two Lactococcus sp. strains were also identified as Lactococcus lactis . Due to a very high electrophoretic similarity observed between Lactococcus lactis subsp. lactis and Lactococcus lactis subsp. cremoris strains, subspecies discrimination required a high reproducibility level of the electrophoretic procedures used. The DCL differential medium failed for four Lactococcus lactis subsp. cremoris strains on solid medium but was correct when tested in broth. For three Lactococcus lactis subsp. lactis strains, including the type strain, the reaction was not conclusive. For 80 out of the 85 Lactococcus lactis strains identified to subspecies level by SDS-PAGE, the DCL differential medium was correct. One Lactococcus lactis strain, on repetition, displayed DCL test results contradictory to the SDS-PAGE identification. For four Lactococcus lactis strains no conclusion could be drawn from the DCL test results, while a good reaction was obtained (identified as Lactococcus lactis subsp. lactis ) for four non-lactococcal strains as well as for two Lactococcus garvieae strains. The test results for one unidentified isolate were not conclusive. Ten strains were identified as Lactococcus lactis subsp. lactis var. diacetilactis.