Impact of essential and optional ingredients on microbial and metabolic profiles of kimchi.

This study aimed to examine the impacts of essential and optional ingredients on the microbial and metabolic profiles of kimchi during 100 days of fermentation, using a mix-omics approach. Kimchi manufactured without essential ingredients (e.g., red pepper, garlic, ginger, green onion, and radish) had lower lactic acid content. The absence of garlic was associated with a higher proportion of Latilactobacillus and Lactococcus, while the absence of red pepper was associated with a greater proportion of Leuconostoc than the control group. In addition, red pepper and garlic served as primary determinants of the levels of organic acids and biogenic amines. Sugar was positively correlated with the levels of melibiose, and anchovy sauce was positively correlated with the levels of amino acids such as methionine, leucine, and glycine. These findings contribute to a fundamental understanding of how ingredients influence kimchi fermentation, offering valuable insights for optimizing kimchi production to meet various preferences.

Peptostreptococcus equinus sp. nov., isolated from horse faeces.

A Gram-stain-positive, non-spore-forming, and obligate anaerobic bacteria designated strain CBA3647T was isolated from a horse faecal sample in Jeju, Republic of Korea. Phylogenetic analysis based on 16S rRNA gene sequences showed that strain CBA3647T formed a distinct phyletic lineage from closely related species within the genus Peptostreptococcus. Based on comparative analysis of 16S rRNA gene sequences, Peptostreptococcus anaerobius ATCC 27337T is most closely related to strain CBA3647T with a 16S rRNA gene similarity of 98.31 %, while similarity to other type strains is below 98.0 %. The genomic DNA G+C content of strain CBA3647T was 30.0 mol%. The digital DNA-DNA hybridization values between strain CBA3647T and the six Peptostreptococcus species were equal to or less than 24 %. Cells were non-motile and oval-shaped cocci with catalase-positive and oxidase-negative activities. Growth occurred at 20-40 °C (optimum, 35 °C), pH 6-8 (optimum, pH 7), and in the presence of 0-2 % (w/v) NaCl (optimum, 1 %). Strain CBA3647T contained C14 : 0 iso and C16 : 0 as major fatty acids. Phenotypic, chemotaxonomic, and molecular properties of strain CBA3647T suggest that it represents a novel species in the genus Peptostreptococcus, which has been named Peptostreptococcus equinus sp. nov. The type strain is CBA3647T (=KACC 22891T= JCM 35846T).

Advances in Culturomics Research on the Human Gut Microbiome: Optimizing Medium Composition and Culture Techniques for Enhanced Microbial Discovery.

Despite considerable advancements achieved using next-generation sequencing technologies in exploring microbial diversity, several species of the gut microbiome remain unknown. In this transformative era, culturomics has risen to prominence as a pivotal approach in unveiling realms of microbial diversity that were previously deemed inaccessible. Utilizing innovative strategies to optimize growth and culture medium composition, scientists have successfully cultured hard-to-cultivate microbes. This progress has fostered the discovery and understanding of elusive microbial entities, highlighting their essential role in human health and disease paradigms. In this review, we emphasize the importance of culturomics research on the gut microbiome and provide new theories and insights for expanding microbial diversity via the optimization of cultivation conditions.

Selective influence of garlic as a key ingredient in kimchi on lactic acid bacteria in a fermentation model system.

Garlic is an essential ingredient added to kimchi, a fermented vegetable, in small amounts owing to its sensory and antibacterial properties. This study aimed to elucidate the complex relationship between garlic and specific lactic acid bacteria (LAB) and the resulting metabolite changes in a controlled kimchi model system using nine strains as mixed and individual starters. The group without garlic using mixed starters showed the highest LAB growth activity, which influenced lactic acid production, pH, and titratable acidity. The group without garlic also showed differences in the composition of bacteria, such as Latilactobacillus sakei, Levilactobacillus brevis, unclassified Leuconostoc, and Weissella koreensis, during the fermentation period. In addition, the altering patterns of metabolites in the group without garlic during fermentation differed from those in the group with garlic. In addition, the metabolic profile of L. brevis group was mostly different from that of the other strains in the controlled model kimchi system using individual starters, suggesting that changes in LAB composition by garlic could subsequently affect metabolites during fermentation. This study provides valuable insights into the complex interactions among food ingredients, LAB succession, and metabolite production during fermentation.

Cutibacterium equinum sp. nov., isolated from horse faeces.

Strain CBA3108T is a Gram-positive, non-spore-forming, obligately anaerobic bacterium isolated from horse faecal samples obtained in Jeju Island, Republic of Korea. The cells of CBA3108T are non-motile short rods that have been assessed as catalase-positive and oxidase-negative. Growth of the strain occurs under the following conditions: 25-45 °C (optimum, 35 °C); pH 6-9 (optimum, pH 6); and in the presence of 0-6 % (w/v) NaCl (optimum, 2%). Major fatty acids in the strain include C15 : 0 iso and C15 : 0 iso DMA, while major polar lipids include phosphatidylglycerol, diphosphatidylglycerol and phosphatidylcholine. Based on phylogenetic analysis using 16S rRNA gene sequences, strain CBA3108T forms a phyletic lineage distinct from other closely related species within the genus Cutibacterium. It was found to be most closely related to Cutibacterium avidum ATCC 25577T (98.27 % 16S rRNA gene sequence similarity) and other strains within the genus (≤98.0 %). The genomic DNA G+C content of strain CBA3108T was 63.2 mol%. The in silico DNA-DNA hybridization values of strain CBA3108T with C. avidum ATCC 25577T, C. porci WCA-380-WT-3AT and C. acnes subsp. acnes DSM 1897T were 33.6, 21.7 and 22.7 %, respectively. Its phenotypic, chemotaxonomic and molecular properties support the hypothesis that strain CBA3108T represents a novel species in the genus Cutibacterium, for which we propose the name Cutibacterium equinum sp. nov. The type strain is CBA3108T (=KACC 22889T=JCM 35966T).

Effects of sediment and temperature on the long-term aging process of rice wine: Microbial and metabolic insights.

Rice wine is low in alcohol content (6 %), rich in nutrients, and contains live microorganisms; therefore, it is generally produced without an aging process during manufacturing. In this study, we investigated the microbial and metabolic changes that occur during the long-term aging of undiluted rice wine with different amount of sediment and aging temperatures. Rice wine samples with higher amounts of sediment had higher yeast counts and alcohol content, indicating a crucial role of sediment in providing nutrients for yeast survival during aging. Furthermore, the rice wine samples that were aged at 12 °C exhibited a notable increase in the relative abundance of Lactobacillus after 100 days of aging. Metabolic profiling revealed that the production of metabolites during rice wine aging was greatly influenced by the amount of sediment and aging temperature, with most metabolites showing a strong correlation with these factors. This study provides valuable insights into the impact of sediment and temperature on the microbial and metabolic changes that occur during the long-term aging of rice wine.

Peptoniphilus equinus sp. nov., a novel Gram-stain-positive anaerobic coccus isolated from the faeces of a thoroughbred racehorse.

A novel, anaerobic, Gram-stain-positive coccoid strain, CBA3646T, was isolated from the faeces of a thoroughbred racehorse. Phylogenetic analysis based on 16S rRNA gene sequencing yielded results indicative of CBA3646T representing a member of the genus Peptoniphilus, with the species most closely related to it being Peptoniphilus asaccharolyticus DSM 20463T, with a similarity of 94.79 %. DNA-DNA relatedness and average nucleotide identity values between CBA3646T and P. asaccharolyticus DSM 20463T were 21.4 and 67.6 %, respectively. CBA3646T has a circular chromosomal genome of 1 709 189 bp (45.5 mol% DNA G+C content), containing 1652 genes in total, 1584 predicted protein-coding genes, 3 complete rRNA loci and 47 tRNA genes. The cells were non-motile diplococci, catalase-positive and oxidase-negative. Growth of CBA3646T was observed at 20-40 °C (optimal temperature, 35 °C) and in the presence of 0-4 % (w/v) NaCl (optimum concentration, 1 %). The major fatty acids (>10 %) of CBA3646T were C16 : 0, C18 : 1ω9c and C18 : 1ω9c dimethyl acetal, with its major polar lipids being diphosphatidylglycerol and phosphatidylglycerol. The elucidated phylogenetic, physiological, chemotaxonomic and molecular properties are indicative of strain CBA3646T representing a novel species of the genus Peptoniphilus, or which the name Peptoniphilus equinus sp. nov. is proposed. The type strain is CBA3646T (= KACC 22890T = JCM 35845T).

In-depth metataxonomic investigation reveals low richness, high intervariability, and diverse phylotype candidates of archaea in the human urogenital tract.

The urogenital microbiota is the potential principal factor in the pathophysiology of urinary tract infection and the protection of urinary tract health. Little is known about the urogenital archaeome although several reports have indicated that the archaeomes of various regions of the human body are associated with health. Accordingly, we aimed to determine the presence and diversity of archaeomes in the human urogenital tract. To explore the urogenital archaeome, voided urine specimens from 373 asymptomatic Korean individuals were used. No difference was observed in body mass index, age, or gender, according to presence of archaea. Analysis of archaeal 16S rRNA gene amplicons of archaea positive samples consisted of simple community structures, including diverse archaea, such as the phyla Methanobacteriota, Thermoproteota, and Halobacteriota. Asymptomatic individuals showed high participant-dependent intervariability in their urogenital archaeomes. The mean relative archaeal abundance was estimated to be 0.89%, and fluorescence in situ hybridisation micrographs provided evidence of archaeal cells in the human urogenital tract. In addition, the urogenital archaeome shared partial taxonomic compositional characteristics with those of the other body sites. In this study, Methanobacteriota, Thermoproteota, and Halobacteriota were suggested as inhabitants of the human urogenital tract, and a distinct human urogenital archaeome was characterised. These findings expand our knowledge of archaea-host associations in the human urogenital tract and may lead to novel insights into the role of archaea in urinary tract health.

Machine learning model for predicting age in healthy individuals using age-related gut microbes and urine metabolites.

Age-related gut microbes and urine metabolites were investigated in 568 healthy individuals using metataxonomics and metabolomics. The richness and evenness of the fecal microbiota significantly increased with age, and the abundance of 16 genera differed between the young and old groups. Additionally, 17 urine metabolites contributed to the differences between the young and old groups. Among the microbes that differed by age, Bacteroides and Prevotella 9 were confirmed to be correlated with some urine metabolites. The machine learning algorithm eXtreme gradient boosting (XGBoost) was shown to produce the best performing age predictors, with a mean absolute error of 5.48 years. The accuracy of the model improved to 4.93 years with the inclusion of urine metabolite data. This study shows that the gut microbiota and urine metabolic profiles can be used to predict the age of healthy individuals with relatively good accuracy.

Comprehensive elucidation of the terroir of Korean kimchi through the study of recipes, metabolites, microbiota, and sensory characteristics.

The aim of this study was to investigate the differences in characteristics of the fermented food kimchi based on the regions where it is produced. A total of 108 kimchi samples were collected from five different provinces in Korea to analyze the recipes, metabolites, microbes, and sensory characteristics. Overall, 18 ingredients (including salted anchovy and seaweed), 7 quality indicators (such as salinity and moisture content), 14 genera of microorganisms (mainly Tetragenococcus and Weissella belonging to LAB), and 38 metabolites contributed to the characteristics of kimchi by region. Kimchi from the southern and northern regions showed distinct metabolite profile (collected 108 kimchi) and flavor profile differences (kimchi manufactured using the standard regional recipes). This is the first study to investigate the terroir effect of kimchi by identifying differences in ingredients, metabolites, microbes, and sensory characteristics based on the region of production, and the correlations between these factors.

Bacteroides faecium sp. nov. isolated from human faeces.

A novel bacterial strain, CBA7301T, was isolated from human faeces and was characterised using a polyphasic taxonomic approach. A phylogenetic analysis based on 16S rRNA gene sequences revealed that CBA7301T represented a member of the genus Bacteroides, in the family Bacteroidaceae. The similarity between the 16S rRNA gene sequence of CBA7301T and that of its most closely related species, Bacteroides faecichinchillae JCM 17102T, was 96.2 %, and the average nucleotide identity between these two strains was 77.9 %. The genome size was 6 782 182 bp, and the DNA G+C content was 42.5 mol%. Cells of CBA7301T were Gram-stain-negative, strictly anaerobic and rod-shaped. The optimal growth of this organism occurred at 30-35 °C, pH 7.0 and 0.5 % (w/v) NaCl. The respiratory quinone was menaquinone 10. The predominant polar lipids were phosphatidylethanolamine, phospholipids and aminophospholipids. The major cellular fatty acid was anteiso-C15 : 0. According to the results of the polyphasic taxonomic analysis, CBA7301T represents a novel species of the genus Bacteroides, which we named Bacteroides faecium sp. nov. The type strain is CBA7301T (=KCCM 43355T=ATCC TSD-227T).

Does kimchi deserve the status of a probiotic food?

Kimchi is a traditional fermented vegetable side dish in Korea and has become a global health food. Kimchi undergoes spontaneous fermentation, mainly by lactic acid bacteria (LAB) originating from its raw ingredients. Numerous LAB, including the genera Leuconostoc, Weissella, and Lactobacillus, participate in kimchi fermentation, reaching approximately 9-10 log colony forming units per gram or milliliter of food. The several health benefits of LAB (e.g., antioxidant and anti-inflammatory properties) combined with their probiotic potential in complex diseases including obesity, cancer, atopic dermatitis, and immunomodulatory effect have generated an interest in the health effects of LAB present in kimchi. In order to estimate the potential of kimchi as a probiotic food, we comprehensively surveyed the health functionalities of kimchi and kimchi LAB, and their effects on human gut environment, highlighting the probiotics function.

Probiotic Lactobacilli ameliorate alcohol-induced hepatic damage via gut microbial alteration.

Alcoholic liver disease (ALD), which includes fatty liver, cirrhosis, steatosis, fibrosis, and hepatocellular carcinoma, is a global health problem. The probiotic effects of lactic acid bacteria (LAB) are well-known; however, their protective effect against ALD remains unclear. Therefore, in this study, our objective was to assess the protective effects of LAB on ALD. To this end, mice were fed either a normal diet or an alcohol diet for 10 days (to induce ALD) accompanied by vehicle treatment (the NC and AC groups) or kimchi-derived LAB (Lactiplantibacillus plantarum DSR J266 and Levilactobacillus brevis DSR J301, the AL group; or Lacticaseibacillus rhamnosus GG, the AG group). Our results showed that mice in the AC group showed significantly higher serum aspartate aminotransferase and alanine aminotransferase levels than those in the normal diet groups; however, their levels in the AL and AG groups were relatively lower. We also observed that the AL and AG groups showed relatively lower interleukin-6 levels than the AC group. Additionally, AC group showed the accumulation of several fat vesicles in the liver, while the AL and AG groups showed remarkably lower numbers of fat vesicles. The relative abundance of Enterococcus feacalis, which showed association with liver injury, significantly increased in the AC group compared with its levels in the normal diet groups. However, the AG group showed a decreased relative abundance in this regard, confirming that LAB exerted an improvement effect on gut microbial community. These findings suggested that via gut microbiota alteration, the ingestion of LAB can alleviate the ill effects of alcohol consumption, including inflammation, liver damage, gut dysbiosis, and abnormal intestinal nutrient metabolism.

Brachybacterium kimchii sp. nov. and Brachybacterium halotolerans subsp. kimchii subsp. nov., isolated from the Korean fermented vegetables, kimchi, and description of Brachybacterium halotolerans subsp. halotolerans subsp. nov.

Two Gram-stain-positive, oxidase-negative, catalase-positive, and coccus-shaped bacterial strains, designated CBA3104T and CBA3105T, were isolated from kimchi. Strain CBA3104T and CBA3105T grew at 10-35°C (optimum, 25°C and 30°C, respectively), at pH 6.0-8.5 (optimum, pH 6.5), and in the presence of 0-15% (w/v) NaCl (optimum, 5%). A phylogenetic analysis based on 16S rRNA gene sequences revealed that strain CBA3104T formed a distinct phylogenetic lineage within the genus Brachybacterium whereas strain CBA3105T was closely positioned with Brachybacterium halotolerans MASK1Z-5T. The 16S rRNA gene sequence similarity between strains CBA3104T and CBA3105T was 99.9%, but ANI and dDDH values between strains CBA3104T and CBA3105T were 93.61% and 51.5%, respectively. Strain CBA3104T showed lower ANI and dDDH values than species delineation against three closely related strains and type species of the genus Brachybacterium, however, strain CBA3105T showed 96.63% ANI value and 69.6% dDDH value with Brachybacterium halotolerans MASK1Z-5T. Among biochemical analysis results, strain CBA3104T could uniquely utilize bromo-succinic acid whereas only strain CBA3105T was positive for alkaline phosphatase and α-fucosidase among two novel strains, closely related strains, and type species of the genus Brachybacterium. Compared with strain CBA3105T and Brachybacterium halotolerans JCM 34339T, strain CBA3105T was differentially positive for acid production of D-arabinose, D-adonitol, and potassium 5-ketogluconate and enzyme activity of ß-glucuronidase. Both strains contained menaquinone-7 as the dominant quinone. The cell-wall peptidoglycan of two novel strains contained meso-diaminopimelic acid. The major fatty acids of strains CBA3104T and CBA3105T were anteiso-C15:0, anteiso-C17:0, and iso-C16:0. The major polar lipids of both strains were phosphatidylglycerol and diphosphatidylglycerol. Strain CBA3104T possessed a uniquely higher abundance of tRNA (97 tRNAs) than four Brachybacterium strains used for comparative taxonomic analysis (54-62 tRNAs). Both the CBA3104T and CBA3105T strain harbored various oxidoreductase, transferase, hydrolase, and lyase as strain-specific functional genes compared to closely related strains and Brachybacterium type species. The results of biochemical/physiological, chemotaxonomic, and genomic analyses demonstrated that strains CBA3104T and CBA3105T represent a novel species of the genus Brachybacterium and a novel subspecies of B. halotolerans, respectively, for which the names Brachybacterium kimchii sp. nov. and B. halotolerans subsp. kimchii subsp. nov. are proposed. The type strains of the novel species and the novel subspecies are CBA3104T (= KCCM 43417T = JCM 34759T) and CBA3105T (= KCCM 43418T =JCM 34760T), respectively.

Sala cibi gen. nov., sp. nov., an extremely halophilic archaeon isolated from solar salt.

Two novel halophilic archaeal strains, CBA1133T and CBA1134, were isolated from solar salt in South Korea. The 16S rRNA gene sequences of the isolates were identical to each other and were closely related to the genera Natronomonas (92.3-93.5%), Salinirubellus (92.2%), Halomarina (91.3-92.0%), and Haloglomus (91.4%). The isolated strains were coccoid, Gram-stain-negative, aerobic, oxidase-positive, and catalase-negative. Growth occurred under temperatures of 25-50°C (optimum, 45°C), NaCl levels of 10-30% (optimum, 15%), pH levels of 6.0-8.5 (optimum, 7.0), and MgCl2 concentrations of 0-500 mM (optimum, 100 mM). Digital DNA-DNA hybridization values between the strains and related genera ranged from 18.3% to 22.7%. The major polar lipids of the strains were phosphatidyl glycerol, phosphatidyl glycerol phosphate methyl ester, and phosphatidyl glycerol sulfate. Genomic, phenotypic, physiological, and biochemical analyses of the isolates revealed that they represent a novel genus and species in the family Halobacteriaceae. The type strain is CBA1133T (= KACC 22148T = JCM 34265T), for which the name Sala cibi gen. nov., sp. nov. is proposed.

Role of combinated lactic acid bacteria in bacterial, viral, and metabolite dynamics during fermentation of vegetable food, kimchi.

Lactic acid bacteria (LAB) in kimchi, a traditional Korean food, are major fermentative microorganisms affecting the quality, safety, and nutritional and organoleptic properties of the final product. In this study, we determined the role of three key LAB strains, Leuconostoc gelidum, Latilactobacillus sakei, Weissella koreensis originated from different raw ingredients during natural fermentation, as opposed to an axenic environment. Starter cultures were inoculated into food with wild indigenous microbial communities, and the dynamics of bacterial communities and metabolites were analyzed during fermentation. As bacteriophages within the food viral community directly affect fermentation by influencing bacterial function and composition, the diversity and composition of DNA viral communities were compared with those of corresponding bacterial communities using a metagenomic approach. Our results provide insights into the ecological role of LAB starters in food fermentation and the potential impact of bacteriophages as modulators of bacterial communities associated with the fermentation properties of kimchi.

Host phylogeny, habitat, and diet are main drivers of the cephalopod and mollusk gut microbiome.

BACKGROUND: Invertebrates are a very attractive subject for studying host-microbe interactions because of their simple gut microbial community and host diversity. Studying the composition of invertebrate gut microbiota and the determining factors is essential for understanding their symbiotic mechanism. Cephalopods are invertebrates that have similar biological properties to vertebrates such as closed circulation system, an advanced nervous system, and a well-differentiated digestive system. However, it is not currently known whether their microbiomes have more in common with vertebrates or invertebrates. This study reports on the microbial composition of six cephalopod species and compares them with other mollusk and marine fish microbiomes to investigate the factors that shape the gut microbiota. RESULTS: Each cephalopod gut consisted of a distinct consortium of microbes, with Photobacterium and Mycoplasma identified as core taxa. The gut microbial composition of cephalopod reflected their host phylogeny, the importance of which was supported by a detailed oligotype-level analysis of operational taxonomic units assigned to Photobacterium and Mycoplasma. Photobacterium typically inhabited multiple hosts, whereas Mycoplasma tended to show host-specific colonization. Furthermore, we showed that class Cephalopoda has a distinct gut microbial community from those of other mollusk groups or marine fish. We also showed that the gut microbiota of phylum Mollusca was determined by host phylogeny, habitat, and diet. CONCLUSION: We have provided the first comparative analysis of cephalopod and mollusk gut microbial communities. The gut microbial community of cephalopods is composed of distinctive microbes and is strongly associated with their phylogeny. The Photobacterium and Mycoplasma genera are core taxa within the cephalopod gut microbiota. Collectively, our findings provide evidence that cephalopod and mollusk gut microbiomes reflect host phylogeny, habitat, and diet. It is hoped that these data can contribute to future studies on invertebrate-microbe interactions.

Long-term population dynamics of viable microbes in a closed ecosystem of fermented vegetables.

This study was performed to investigate the succession of various microorganisms naturally present in raw ingredients and the changes in metabolites following long-term fermentation of kimchi. Kimchi was stored at 4 °C for 500 days, and the composition of the microbial community and the nature of metabolites were analyzed using metataxonomics and metabolomics. We confirmed that the taxa belonging to Leuconostoc and Weissella were dominant in the early stages of fermentation, while Latilactobacillus and Levilactobacillus were dominant in the middle and late fermentation stages, respectively. In the eukaryotic community, Cladosporium was dominant in the early stages, while Pichia and Hanseniaspora tended to increase in the middle and late fermentation stages. The longitudinal metabolite profile demonstrated that about half (55.7%) of the metabolites present in kimchi after 500 days of fermentation were produced within 15 days of fermentation due to rapid fermentation in the initial stage. These results revealed that even in a closed environment, the viable microbiota in fermented vegetables are not static but dynamic, and the composition of metabolites evolves accordingly during long-term fermentation.

Host habitat is the major determinant of the gut microbiome of fish.

BACKGROUND: Our understanding of the gut microbiota of animals is largely based on studies of mammals. To better understand the evolutionary basis of symbiotic relationships between animal hosts and indigenous microbes, it is necessary to investigate the gut microbiota of non-mammalian vertebrate species. In particular, fish have the highest species diversity among groups of vertebrates, with approximately 33,000 species. In this study, we comprehensively characterized gut bacterial communities in fish. RESULTS: We analyzed 227 individual fish representing 14 orders, 42 families, 79 genera, and 85 species. The fish gut microbiota was dominated by Proteobacteria (51.7%) and Firmicutes (13.5%), different from the dominant taxa reported in terrestrial vertebrates (Firmicutes and Bacteroidetes). The gut microbial community in fish was more strongly shaped by host habitat than by host taxonomy or trophic level. Using a machine learning approach trained on the microbial community composition or predicted functional profiles, we found that the host habitat exhibited the highest classification accuracy. Principal coordinate analysis revealed that the gut bacterial community of fish differs significantly from those of other vertebrate classes (reptiles, birds, and mammals). CONCLUSIONS: Collectively, these data provide a reference for future studies of the gut microbiome of aquatic animals as well as insights into the relationship between fish and their gut bacteria, including the key role of host habitat and the distinct compositions in comparison with those of mammals, reptiles, and birds. Video Abstract.

Pathogenomics of Streptococcus ilei sp. nov., a newly identified pathogen ubiquitous in human microbiome.

Viridans group streptococci are a serious health concern because most of these bacteria cause life-threatening infections, especially in immunocompromised and hospitalized individuals. We focused on two alpha-hemolytic Streptococcus strains (I-G2 and I-P16) newly isolated from an ileostomy effluent of a colorectal cancer patient. We examined their pathogenic potential by investigating their prevalence in human and assessing their pathogenicity in a mouse model. We also predicted their virulence factors and pathogenic features by using comparative genomic analysis and in vitro tests. Using polyphasic and systematic approaches, we identified the isolates as belonging to a novel Streptococcus species and designated it as Streptococcus ilei. Metagenomic survey based on taxonomic assignment of datasets from the Human Microbiome Project revealed that S. ilei is present in most human population and at various body sites but is especially abundant in the oral cavity. Intraperitoneal injection of S. ilei was lethal to otherwise healthy C57BL/6J mice. Pathogenomics and in vitro assays revealed that S. ilei possesses a unique set of virulence factors. In agreement with the in vivo and in vitro data, which indicated that S. ilei strain I-G2 is more pathogenic than strain I-P16, only the former displayed the streptococcal group A antigen. We here newly identified S. ilei sp. nov., and described its prevalence in human, virulence factors, and pathogenicity. This will help to prevent S. ilei strain misidentification in the future, and improve the understanding and management of streptococcal infections.