Predominant lactic acid bacteria in mukeunji, a long-term-aged kimchi, for different aging periods

Pediococcus inopinatus with a well-developed CRISPR-Cas system dominates in long-term fermented kimchi, Mukeunji

Kimchi is produced through a low-temperature fermentation without pre-sterilization, resulting in a heterogeneous microbial community. As fermentation progresses, dominant lactic acid bacteria (LAB) species emerge and undergo a transition process. In this study, LAB were isolated from Mukeunji, a long-term fermented kimchi that is in the final stage of kimchi fermentation process. It was confirmed, through culture-dependent and independent analysis, as well as metagenome analysis, that Pediococcus inopinatus are generally dominant in long-term fermented kimchi. Comparative analysis of the de novo assembled whole genome of P. inopinatus with other kimchi LAB revealed that this species has a well-developed clustered regularly interspaced short palindromic repeats (CRISPR) system. The CRISPR system of P. inopinatus has an additional copy of the csa3 gene, a transcription factor for cas genes. Indeed, this species not only highly expresses cas1 and cas2, which induce spacer acquisition, but also has many diverse spacers that are actively expressed. These findings indicate that the well-developed CRISPR-Cas system is enabling P. inopinatus to dominate in long-fermented kimchi. Overall, this study revealed that LAB with a robust defense system dominate in the final stage of kimchi fermentation and presented a model for the succession mechanism of kimchi LAB.

Biosorption of silver cations onto Lactococcus lactis and Lactobacillus casei isolated from dairy products

The current work deals with the phenomenon of silver cations uptake by two kinds of bacteria isolated from dairy products. The mechanism of sorption of silver cations by Lactococcus lactis and Lactobacillus casei bacteria was investigated. Inductively coupled plasma–mass spectrometry (ICP-MS) was used for determination of silver concentration sorbed by bacteria. Analysis of charge distribution was conducted by diffraction light scattering method. Changes in the ultrastructure of Lactococcus lactis and Lactobacillus casei cells after treatment with silver cations were investigated using transmission electron microscopy observation. Molecular spectroscopy methods, namely Fourier transform-infrared spectroscopy (FT-IR) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) were employed for description of the sorption mechanism. Moreover, an analysis of volatile organic compounds (VOCs) extracted from bacterial cells was performed.

Characterization of macrophage-activating lactic acid bacteria isolated from Mukeunji

Lactic acid bacteria (LAB) were reported to comprise the majority of the bacterial population in mukeunji, a long-fermented kimchi. This current study investigated the probiotic abilities of LAB isolated from mukeunji. Forty bacterial strains from mukeunji were identified by SDS-PAGE gel patterns, which were further tested for acid and bile tolerance. An assessment of the immuneboosting effects of these strains in RAW264.7 cells was also carried out by measurement of secreted proinflammatory cytokines. Heat-killed LAB were also tested for their contribution to cytokine production because certain LAB are not durable in the digestive tract. Immune-boosting strains were further characterized by SDS-PAGE of whole-cell protein and 16S rRNA gene sequencing, resulting in the identification of Lactobacillus plantarum, L. sakei, Weissella cibaria, and Pediococcus parvulus. These data and the fact that mukeunji is highly consumed in Korea, together, highlight the need of detailed epidemiological and animal feeding studies.