[Enumeration and precise identification techniques of Lactococcus in food]

To explore qualitative and quantitative detection method for Lactococcus in food samples.

Lactococcus were enumerated and isolated from commercially available food samples, systematic identification of isolates was achieved by combining techniques of colony morphology on plates, microscopic examination, biochemical reactions, metabolic substances, 16S rRNA gene sequencing, and matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) analysis.

A total of 47 food samples including yogurt and cheese, that containing Lactococcus were collected in this study. The result indicated that the highest isolation rate of Lactococcus was observed in yogurt samples at 87.5%(7/8), followed by cheese at 58.3%(14/24). The Lactococcus content in both yoghurt and lactic acid beverage was higher than 1×107 CFU/mL. In cheese, 64.3％(9/14)contained Lactococcus between 1×106 and 1×107 CFU/g, while the remaining 35.7％(5/14)were within the range of 1×103 to 1×105 CFU/g. In biscuit, it was exceeded 1×106 CFU/g, while butter and sour cream contained approximately 1×104 CFU/g.184 strains were isolated from the aforementioned samples.59 isolates were identified as L. lactis subsp. lactis, 13 were L. lactis subsp lactis biovar diacetylactis and 3 were L. cremoris.

The Lactococcus detection method established in this study enables the enumeration and identification of Lactococcus in food. The combination of traditional microbiological identification method with molecular biology and mass spectrometry techniques enables precise differentiation of Lactococcus species, subspecies, and even biological types in samples.