Heme-Dependent Cytochrome Formation in Lactobacillus maltaromicus

The influence of low-temperature resistant lactic acid bacteria on the enhancement of quality and the microbial community in winter Jerusalem Artichoke (Helianthus tuberosus L.) silage on the Qinghai-Tibet Plateau

Jerusalem Artichoke (Helianthus tuberosus L.), an emerging "food and fodder" economic crop on the Qinghai-Tibet Plateau. To tackle problems such as incomplete fermentation and nutrient loss occurring during the low-temperature ensilage of Jerusalem Artichokes in the plateau's winter, this study inoculated two strains of low-temperature resistant lactic acid bacteria, Lactobacillus plantarum (GN02) and Lactobacillus brevis (XN25), along with their mixed components, into Jerusalem Artichoke silage material. We investigated how low-temperature resistant lactic acid bacteria enhance the quality of low-temperature silage fermentation for Jerusalem Artichokes and clarify its mutual feedback effect with microorganisms. Results indicated that inoculating low-temperature resistant lactic acid bacteria significantly reduces the potential of hydrogen and water-soluble carbohydrates content of silage, while increasing lactic acid and acetic acid levels, reducing propionic acid, and preserving additional dry matter. Inoculating the L. plantarum group during fermentation lowers pH and propionic acid levels, increases lactic acid content, and maintains a dry matter content similar to the original material. Bacterial community diversity exhibited more pronounced changes than fungal diversity, with inoculation having a minor effect on fungal community diversity. Within the bacteria, Lactobacillus remains consistently abundant (>85%) in the inoculated L. plantarum group. At the fungal phylum and genus levels, no significant changes were observed following fermentation, and dominant fungal genera in all groups did not differ significantly from those in the raw material. L. plantarum exhibited a positive correlation with lactic acid and negative correlations with pH and propionic acid. In summary, the inoculation of L. plantarum GN02 facilitated the fermentation process, preserved an acidic silage environment, and ensured high fermentation quality; it is a suitable inoculant for low-temperature silage in the Qinghai-Tibet Plateau.

Complementary Antibacterial Effects of Bacteriocins and Organic Acids as Revealed by Comparative Analysis of Carnobacterium spp. from Meat

Carnobacterium maltaromaticum and Carnobacterium divergens are often predominant in the microbiota of vacuum-packaged (VP) meats after prolonged storage at chiller temperatures, and more so in recent studies. We investigated the antibacterial activities of C. maltaromaticum and C. divergens (n = 31) from VP meats by phenotypic characterization and genomic analysis. Five strains showed antibacterial activities against Gram-positive bacteria in a spot-lawn assay, with C. maltaromaticum strains having an intergeneric and C. divergens strains an intrageneric inhibition spectrum. This inhibitory activity is correlated with the production of predicted bacteriocins, including carnobacteriocin B2 and carnolysin for C. maltaromaticum and divergicin A for C. divergens The supernatants of both species cultured in meat juice medium under anaerobic conditions retarded the growth of most Gram-positive and Gram-negative bacteria in broth assay in a strain-dependent manner. C. maltaromaticum and C. divergens produced formate and acetate but not lactate under VP meat-relevant conditions. The relative inhibitory activity by Carnobacterium strains was significantly correlated (P < 0.05) to the production of both acids. Genomic analysis revealed the presence of genes required for respiration in both species. In addition, two clusters of C. divergens have an average nucleotide identity below the cutoff value for species delineation and thus should be considered to be two subspecies. In conclusion, both bacteriocins and organic acids are factors contributing significantly to the antibacterial activity of C. maltaromaticum and C. divergens under VP meat-relevant conditions. A few Carnobacterium strains can be explored as protective cultures to extend the shelf life and improve the safety of VP meats.IMPORTANCE The results of this study demonstrated that both bacteriocins and organic acids are important factors contributing to the antibacterial activities of Carnobacterium from vacuum-packaged (VP) meats. This study demonstrated that formate and acetate are the key organic acids produced by Carnobacterium and demonstrated their association with the inhibitory activity of carnobacteria under VP meat-relevant storage conditions. The role of lactate, on the other hand, may not be as important as previously believed in the antimicrobial activities of Carnobacterium spp. on chilled VP meats. These findings advance our understanding of the physiology of Carnobacterium spp. to better explore their biopreservative properties for chilled VP meats.

Cake layer bacterial communities during different biofouling stages in full-scale membrane bioreactors

A detailed understanding of the bacterial communities in the cake layers formed on the membrane surface is required to control biofouling in a membrane bioreactor (MBR). This study aimed to investigate the dynamics of the cake layer bacterial communities in full-scale MBRs operated in a wastewater treatment plant in Japan and to identify the key bacteria responsible for cake layer formation. The bacterial communities in the cake layer and the activated sludge were analyzed using 16S rRNA gene amplicon sequencing when biofouling occurred under different fouling conditions. The most dominant phyla in activated sludge were almost always Proteobacteria and Bacteroidetes. By contrast, when the cake layer had unique bacterial communities distinguishable from those in the activated sludge, members of Firmicutes were highly dominant in the cake layer, irrespective of the fouling conditions. This study reported for the first time that Firmicutes play an important role throughout the biofouling process.

Comparative Genomic Analysis Reveals Ecological Differentiation in the Genus Carnobacterium

Lactic acid bacteria (LAB) differ in their ability to colonize food and animal-associated habitats: while some species are specialized and colonize a limited number of habitats, other are generalist and are able to colonize multiple animal-linked habitats. In the current study, Carnobacterium was used as a model genus to elucidate the genetic basis of these colonization differences. Analyses of 16S rRNA gene meta-barcoding data showed that C. maltaromaticum followed by C. divergens are the most prevalent species in foods derived from animals (meat, fish, dairy products), and in the gut. According to phylogenetic analyses, these two animal-adapted species belong to one of two deeply branched lineages. The second lineage contains species isolated from habitats where contact with animal is rare. Genome analyses revealed that members of the animal-adapted lineage harbor a larger secretome than members of the other lineage. The predicted cell-surface proteome is highly diversified in C. maltaromaticum and C. divergens with genes involved in adaptation to the animal milieu such as those encoding biopolymer hydrolytic enzymes, a heme uptake system, and biopolymer-binding adhesins. These species also exhibit genes for gut adaptation and respiration. In contrast, Carnobacterium species belonging to the second lineage encode a poorly diversified cell-surface proteome, lack genes for gut adaptation and are unable to respire. These results shed light on the important genomics traits required for adaptation to animal-linked habitats in generalist Carnobacterium.

Heme proteins in lactic acid bacteria

Lactic acid bacteria (LAB) are of profound importance in food production and infection medicine. LAB do not rely on heme (protoheme IX) for growth and are unable to synthesize this cofactor but are generally able to assemble a small repertoire of heme-containing proteins if heme is provided from an exogenous source. These features are in contrast to other bacteria, which synthesize their heme or depend on heme for growth. We here present the cellular function of heme proteins so far identified in LAB and discuss their biogenesis as well as applications of the extraordinary heme physiology of LAB.

Carnobacterium: positive and negative effects in the environment and in foods

The genus Carnobacterium contains nine species, but only C. divergens and C. maltaromaticum are frequently isolated from natural environments and foods. They are tolerant to freezing/thawing and high pressure and able to grow at low temperatures, anaerobically and with increased CO₂ concentrations. They metabolize arginine and various carbohydrates, including chitin, and this may improve their survival in the environment. Carnobacterium divergens and C. maltaromaticum have been extensively studied as protective cultures in order to inhibit growth of Listeria monocytogenes in fish and meat products. Several carnobacterial bacteriocins are known, and parameters that affect their production have been described. Currently, however, no isolates are commercially applied as protective cultures. Carnobacteria can spoil chilled foods, but spoilage activity shows intraspecies and interspecies variation. The responsible spoilage metabolites are not well characterized, but branched alcohols and aldehydes play a partial role. Their production of tyramine in foods is critical for susceptible individuals, but carnobacteria are not otherwise human pathogens. Carnobacterium maltaromaticum can be a fish pathogen, although carnobacteria are also suggested as probiotic cultures for use in aquaculture. Representative genome sequences are not yet available, but would be valuable to answer questions associated with fundamental and applied aspects of this important genus.

Sequencing of variable regions of the 16S rRNA gene for identification of lactic acid bacteria isolated from the intestinal microbiota of healthy salmonids

The aim of this study was to identify lactic acid bacteria (LAB) using polymerase chain reaction (PCR) amplification of variable regions of the 16S rRNA gene. Thirteen LAB strains were isolated from the intestinal microbiota of healthy salmonids. A approximately 500-bp region of the highly conserved 16S rRNA gene was PCR-amplified and following this, a portion of the amplicon (272-bp) including the V1 and V2 variable regions was sequenced. The sequence containing both the V1 and V2 region provided strong evidence for the identification of LAB. The LAB strains were identified as Carnobacterium maltaromaticum, Lactobacillus curvatus, Lactobacillus sakei, Lactobacillus plantarum, Lactococcus lactis subsp. cremoris, Lactococcus lactis subsp. lactis, and Leuconostoc mesenteroides. The method described was found to be a very simple, rapid, specific, and low-cost tool for the identification of unknown strains of LAB.

Taxonomy and important features of probiotic microorganisms in food and nutrition

Lactic acid bacteria are among the most important probiotic microorganisms typically associated with the human gastrointestinal tract. Traditionally, lactic acid bacteria have been classified on the basis of phenotypic properties, eg, morphology, mode of glucose fermentation, growth at different temperatures, lactic acid configuration, and fermentation of various carbohydrates. Studies based on comparative 16S ribosomal RNA sequencing analysis, however, showed that some taxa generated on the basis of phenotypic features do not correspond with the suggested phylogenetic relations. Thus, some species are not readily distinguishable by phenotypic characteristics. This is especially true for the so-called Lactobacillus acidophilus group, the Lactobacillus casei and Lactobacillus paracasei group, and some bifidobacteria, strains of which have been introduced in many probiotic foods, eg, the novel yogurt-like commodities. Consequently, modern molecular techniques, including polymerase chain reaction-based and other genotyping methods, have become increasingly important for species identification or for the differentiation of probiotic strains. Probiotic strains are selected for potential application on the basis of particular physiologic and functional properties, some of which may be determined in vitro. The classification and identification of a probiotic strain may give a strong indication of its typical habitat and origin. The species, or even genus name, may also indicate the strain's safety and technical applicability for use in probiotic products. Molecular typing methods such as pulsed-field gel electrophoresis, repetitive polymerase chain reaction, and restriction fragment length polymorphism are extremely valuable for specific characterization and detection of such strains selected for application as probiotics.