Hydroxypropyl ß-cyclodextrin improving multiple stresses tolerance of Lactococcus lactis subsp. lactis.

L. lactis is known as industrial starter in the fermentation of dairy and meat products, and it plays an important role in human health as an edible probiotic. During industrial production, L. lactis often experiences different stresses that delay the growth and decrease the survival in some serious conditions. In this study, the protective effects of hydroxypropyl ß-cyclodextrin (HP ß-CD) on L. lactis under multiple stresses were investigated. The microbial cells were treated with different stresses including heat, NaCl, cold, and H2 O2 stresses, and the results were showed by measuring the OD600 or spot plating method. The growth and tolerance were improved when HP ß-CD was added during different stress conditions, better than that of trehalose. Besides, the scanning electron microscopic and fluorescence spectrum studies showed that HP ß-CD could combine with L. lactis to protect the cell structure, suggesting that HP ß-CD may act as a protective agent of L. lactis. Therefore, HP ß-CD could be considered as a potential protective agent to be applied in food industry, and its protective mechanism on L. lactis still needs further investigation.

Short communication: Global transcriptome analysis of Lactococcus lactis ssp. lactis in response to gradient freezing.

Lactic acid bacteria are often preserved as starter cultures by freezing to extend shelf stability as well as maintain cell viability and acidification activity. Previous studies showed that the endocyte extracted from gradient-freezing pretreated cells could act as lyoprotectant in the lyophilization process of Lactococcus lactis ssp. lactis. In this study, the molecular mechanisms of L. lactis in response to gradient freezing exposure are described using high-throughput sequencing. Nineteen of 56 genes were upregulated after gradient freezing, whereas 37 genes were downregulated. Further validation results of quantitative real-time PCR experiments were consistent with the RNA sequencing. Gene Ontology (http://www.geneontology.org/) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG; https://www.genome.jp/kegg/) pathway were used to analyze the differentially expressed genes. Several pathways, such as glutathione metabolism, ATP-binding cassette transport, metabolism of cell wall and cell membrane components, and stress response-related pathways, were affected by gradient freezing. Six genes relevant to freezing stress response were selected for quantitative real-time PCR, including 3 upregulated genes (hisK, eutD, dukA) and 3 downregulated genes (als, yedF, pepN). The Gene Ontology enrichment and KEGG pathway analyses showed these genes may influence stress response-related pathways, improving the survival of the L. lactis under freezing stress. The identification of these genes deepened an understanding about their response under freezing stress, helping us find potential genes or pathways related to gradient freezing for further research on lyoprotectants.

Protective effects of soybean protein and egg white protein on the antibacterial activity of nisin in the presence of trypsin.

The using of nisin to prevent foodborne pathogens (Staphylococcus aureus and Listeria monocytogenes) from contamination has received broad attentions during meat processing. However, the application of nisin has been limited because its antibacterial activity may be inhibited by trypsin. In this study, the protective effects of soybean protein and egg white protein on antibacterial activity of nisin were evaluated. It could be concluded that exogenous trypsin decreased the antibacterial activity of nisin, soybean protein and egg white protein could keep the nisin activity from enzymolysis of trypsin. Trypsin inhibitors in soybean protein and egg white protein could protect the antibacterial activity of nisin. Nisin with soybean protein or egg white protein in cooked meat product presented better quality preservation effects than nisin alone in the presence of trypsin. The total viable counts (TVC) and total volatile basic nitrogen (TVB-N) of nisin-treated group were significantly higher than these in nisin-soybean protein-treated and nisin-egg white protein-treated groups with trypsin. This study showed the potential of using soybean protein and egg white protein to stabilize the antibacterial activity of nisin under high trypsin conditions.