Antimicrobial Effect of Lactococcus lactis subsp. lactis LDTM 6804 on Psychrotrophic Spoilage and Pathogenic Bacteria in Dairy Products

Abstract

In dairy industry, microbiological spoilage related to the quality and safety of milk and milk products is still an important issue for consumers. The growing demand for safer dairy products has led to consider the use of biopreservatives, mainly lactic acid bacteria (LAB). LAB exert inhibitory effects on the growth of undesirable microorganisms or pathogenic bacteria producing a variety of antimirobial compounds including organic acids, hydrogen peroxide and bacteriocins. Therefore, the purpose of this study was to evaluate a Lactococcus strain isolated from raw milk as a potential bioprotective culture using the dairy product models. Lactococcus lactis subsp. lactis LDTM6804 (L. lactis LDTM6804), due to the highest antimicrobial activities against psychrotrophic spoilage and pathogenic bacteria, was selected among 306 isolates from raw milk. Furthermore, L. lactis LDTM6804 was characterized and its whole genome sequencing was performed. Gene cluster involved in nisin Z biosynthesis was identified in chromosome of L. lactis LDTM6804. Protective effects of L. lactis LDTM6804 was examined with 10 % skim milk and cheese model. When L. lactis LDTM6804 (6 log CFU/ml) was added to skim milk, viable cell numbers in Pseudomonas fluorescens, Burkholderia cepacia and Listeria monocytogenes were decreased by 9.29, 8.84, 4.93 log units, respectively after 14 d of storage at 15 ℃ compared to skim milk with reference strain L. lactis IL1403. Cheeses were manufactured and infected by L. monocytogenes, Staphylococcus aureus and Escherichia coli, respectively at approximately 5 log CFU/g. In addition, L. lactis LDTM6804 at 1 % was inoculated to cheese as an adjunct to the starter culture. After 25 d of storage, L. lactis LDTM6804 reduced the viable cell numbers of L.monocytogenes, S. aureus and E. coli by 0.42, 0.57 and 0.74 log units, respectively compared to control cheese without L. lactis LDTM6804. During fermentation of 10 % skim milk and ripening process of cheese, nisin activity throughout storage was assessed using quantitative real time PCR (qRT-PCR). An electronic nose apparatus was employed to investigate changes of flavour compounds by L. lactis LDTM6804. Our results demonstrate that L. lactis LDTM6804 exerts strong protective effects against psychrotrophic spoilage and pathogenic bacteria without organoleptic quality changes in dairy products. Further investigations on the optimum level of addition of the protective culture and inhibitory effect of fermentates or purified nisin of L.lactis LDTM6804 were required.