Species: Weissella koreensis
Weissella koreensis is the predominant lactic acid bacteria (LAB) isolated from kimchi, a traditional Korean fermented-vegetable food . Collins et al. first proposed this species in 1993 based on 16S rRNA gene phylogenetic analysis . Members of the genus Weissella are Gram-positive, facultative anaerobic, non-motile, heterofermentative, non-spore-forming and acid tolerant . The cells are either irregular, short rods with rounded to tapered ends or coccoid . As a psychrophilic bacterium, W. koreensis is the dominating species in fermenting kimchi at -1ºC, and it significantly contributes to kimchi’s taste and flavoring .
Studies have shown that the LAB isolated during kimchi fermentation exhibit antimicrobial activities by producing compounds such as organic acids, fatty acids and hydrogen peroxide . W. koreensis also shows anti-obesity effects by producing the non-protein amino acid ornithine . W. koreensis contributes to sourdough fermentation to improve the quality of bread by producing D (-)-lactic acid and metabolites from glucose  . Acetic acid, being one of the products of fermentation, not only improves the flavor of the sourdough, but also increases the shelf life . It has also been found that W. koreensis inhibits spore germination of the target microorganism during food fermentation .
Strain KCTC 3621 of W.koreensis possesses one chromosome with 1672 coding sequences, 61 tRNA genes and 5 rRNA loci . The chromosome has 1.7Mb and a low GC- content (Guanine-Cytosine content) ratio (35.5%), typical of a low GC Gram-positive phylum Firmicutes . A relatively small percentage of the open reading frames (23.4% ) matches with the potential protein sequence in a public database while 7.7% of the genes are specific in KCTC 3621 but not in other Weissella species’ gene sequences . The genome of strain KACC 15510 of W.koreensis is most closely related to strain KCTC 3621, sharing 98.76% and 99.48% similarity values which further confirms that both strains belong to the same species . Upon studying the genome of strain KCTC 3621, six genes related to ornithine metabolism, five genes related to arginine metabolism and six genes related to antibiotic resistance are found .
Lactic Acid Bacteria (LAB) Metabolism
Heterofermentative LAB use carbohydrates as both a carbon and an energy source. . The metabolism of heterofermentative pathway, as compared to homofermentative pathway, results in a diverse set of end products (ethanol, CO2) besides lactic acid  (Figure 1). As a heterofermentative LAB, W.koreensis uses phosphoketolase pathway when fermenting hexose to lactate, CO2 and ethanol/acetate . Glucose is first dehydrogenated to form 6-phosphogluconate, followed by decarboxylation yielding CO2 . Then phosphoketolase cleaves the resulting pentose-5-phosphate into glyceraldehyde phosphate (GAP) and acetyl phosphate . GAP is further metabolized the same way as in the glycolytic pathway, producing lactic acid . Alternatively, if no additional electron acceptor is available (O2), acetyl phosphate is reduced to form ethanol under anaerobic condition .
Based on morphological, physiological and chemotaxonomic characteristics, strains S-5623 and S-5673 of W. koreensis sp. are considered to represent the novel species of genus Weissella . Studies have shown that these two strains are catalase-negative facultative anaerobes . The optimal growth temperature and pH are 25ºC and 6.0 respectively , with a sub-zero low-temperature range suggesting a psychrotrophic lifestyle. No growth has been shown in 8 or 10% NaCl . Both strains hydrolyze arginine and form dextran from sucrose . Acid has been produced from L-arabinose, ribose and xylose . Negative results are shown for aesculin hydrolysis and formation of acid from cellobiose, galactose, maltose, melibiose, raffinose, sucrose and trehalose . Both strains produce D (-)-lactic acid and gas from glucose .
W. koreensis, found in kimchi, is associated with L-Ornithine production from arginine . L-Ornithine is a medicinal, non-protein amino acid which has the potential to combat obesity by promoting hormone release and accelerating basal metabolism rate . W. koreensis strain OK1-6 significantly reduces mRNA expression of C/EBP- α , a potential central regulator for adipocyte development, correlated to lipid accumulation . The decreased expression level of C/EBP- α down-regulates the expression of adipogenic specific genes and results in the reduced accumulation of lipid in cell lines .
Lactic acid bacteria (LAB) are characterized by the production of lactic acid as their metabolic end-product from glucose . They are often found in food fermentation environments where there is a rich supply of sugars . In addition, LAB, being acid-tolerant and psychrophilic, inhibit the growth of spoilage bacteria and prevent the growth of pathogenic microorganisms during food fermentation when the temperature and pH are low . Thus, the use of LAB during food fermentation has the potential to reduce the use of chemical preservatives .
Kimchi fermentation is carried out by dynamic populations of Lactobacillus, Leuconostoc and Weissella species . It has been found that at 10ºC growth environment, Leuconostoc species (Lc.) predominate in the first stage of fermentation (pH >4.6) while W. koreensis predominate after 30 days of fermentation when the environment becomes acidic (pH 4.6)  (Figure 3). As a lactic acid bacterium, W.koreensis has been shown to possess the ability to tolerate low pH environments and this allows W.koreensis to outcompete other bacteria in an acidic fermentation environment . In a stressful environment, for an example, -1ºC, and <pH 4.3, W.koreensis is also able to grow and outcompete other species .
Several sugars such as fructose, glucose, and sucrose are present in kimchi . Unlike Leuconostoc species, W.koreensis is incapable of fermenting sucrose though it does use a small amount of sucrose for dextran synthesis . It is likely that W.koreensis ferments kimchi at -1ºC via heterofermentative reaction in which large quantities of acetate is produced from reduction of fructose to mannitol .
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Page authored by Yue Hu, microbiology and immunology student at the University of British Columbia.