{ "id": "epqvvr2edb", "slug": "actinomyces-viscosus", "title": "Actinomyces viscosus", "updated": "2026-10-21", "topic_path": [ { "id": "life", "label": "Life and health", "api_url": "https://www.edgepedia.org/api/v1/topics/life" }, { "id": "life.microbes", "label": "Microorganisms and fungi", "api_url": "https://www.edgepedia.org/api/v1/topics/life.microbes" }, { "id": "life.microbes.bacteria", "label": "Bacteria", "api_url": "https://www.edgepedia.org/api/v1/topics/life.microbes.bacteria" }, { "id": "life.microbes.bacteria.bacteria.phyla.actinomycetota", "label": "Actinomycetota (Actinobacteria)", "api_url": "https://www.edgepedia.org/api/v1/topics/life.microbes.bacteria.bacteria.phyla.actinomycetota" } ], "geo": [ { "id": "geo.us.t1946.life.microbes.bacteria", "label": "United States · 1946 to 2000: Bacteria", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946.life.microbes.bacteria", "path": [ { "id": "geo.us", "label": "United States", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us" }, { "id": "geo.us.t1946", "label": "United States · 1946 to 2000", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946" }, { "id": "geo.us.t1946.life", "label": "Life and health", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946.life" }, { "id": "geo.us.t1946.life.microbes", "label": "Microorganisms and fungi", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946.life.microbes" }, { "id": "geo.us.t1946.life.microbes.bacteria", "label": "Bacteria", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946.life.microbes.bacteria" } ] } ], "excerpt": "Actinomyces viscosus, formerly Odontomyces viscosus, is a high G+C Gram-positive oral bacterium of humans and hamsters; its human strains are taxonomically uncertain and mostly belong to Actinomyces oris.", "snippet": "Actinomyces viscosus, formerly Odontomyces viscosus, is a high G+C Gram-positive oral bacterium of humans and hamsters; its human strains are taxonomically uncertain and mostly belong to Actinomyces oris.", "node": "life.microbes.bacteria.bacteria.phyla.actinomycetota", "markdown": "# Actinomyces viscosus\n\n**Actinomyces viscosus** is a species of high G+C Gram-positive bacterium in the genus *Actinomyces*, family Actinomycetaceae, found in the oral cavity of humans and hamsters and isolated from actinomycotic lesions in swine, cats, and dogs.[1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=1656&mode=Info) • [2](https://meshb-prev.nlm.nih.gov/record/ui?ui=D016924) It was originally described as \"Odontomyces viscosus\" by Howell and colleagues in 1965 and renamed *Actinomyces viscosus* by Georg, Pine, and Gerencser in 1969.[1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=1656&mode=Info) • [3](https://journals.asm.org/doi/epdf/10.1128/am.18.1.80-87.1969) The species is taxonomically entangled with *Actinomyces naeslundii* and *Actinomyces oris*, and human strains identified as *A. viscosus* are now considered most likely to be members of *A. oris*.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC4402957/)\n\n| Key fact | Detail |\n|---|---|\n| Current name | *Actinomyces viscosus* (Howell et al. 1965) Georg et al. 1969; homotypic synonym \"Odontomyces viscosus\"[1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=1656&mode=Info) |\n| Type strain | ATCC 15987, also held as CCUG 14476, CIP 103147, DSM 43327, JCM 8353, and NCTC 10951[1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=1656&mode=Info) |\n| Group | High G+C Gram-positive bacteria; lineage Bacteria, Bacillati, Actinomycetota, Actinomycetes, Actinomycetales, Actinomycetaceae[1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=1656&mode=Info) |\n| Habitat | Oral cavity of man and hamsters; isolated from actinomycotic lesions in swine, cats, and dogs[2](https://meshb-prev.nlm.nih.gov/record/ui?ui=D016924) |\n| Taxonomic status | Human strains assigned to \"A. viscosus\" are taxonomically uncertain and most likely belong to *A. oris*[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC4402957/) |\n| Ecological role | Coaggregates with *Fusobacterium*, *Prevotella*, and *Veillonella*, contributing structural integrity to dental plaque[5](https://journals.asm.org/doi/10.1128/cmr.00100-14) |\n\n## Nomenclature and history\n\nThe organism was first isolated from subgingival plaque in hamsters and described by Howell and Howell and Jordan as the \"hamster organism.\" Howell and colleagues classified it as *Odontomyces viscosus*, judging that it differed considerably from *Nocardia* and *Rothia* and, although it closely resembled *Actinomyces naeslundii*, differed by being aerobic and catalase-positive. The Subgroup on Taxonomy of the Microaerophilic Actinomycetes recommended broadening the generic description of *Actinomyces* to include catalase-positive organisms, on which basis Georg, Pine, and Gerencser proposed the new combination *Actinomyces viscosus*.[3](https://journals.asm.org/doi/epdf/10.1128/am.18.1.80-87.1969) The original name \"Odontomyces viscosus\" remains an effectively published but not validly published name under the [International Code of Nomenclature of Prokaryotes](https://www.edgepedia.org/international-code-of-nomenclature-of-prokaryotes).[1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=1656&mode=Info)\n\nCatalase-positive bacteria similar to the hamster organism were subsequently reported from the human mouth, beginning the association of the species with human oral microbiology.[3](https://journals.asm.org/doi/epdf/10.1128/am.18.1.80-87.1969)\n\n## Relationship to A. naeslundii and A. oris\n\nThe *A. naeslundii*/*A. viscosus* group has long been known to be heterogeneous. DNA-[DNA hybridization](https://www.edgepedia.org/dna-hybridization) studies showed that human \"*A. viscosus*\" strains were more closely related to *A. naeslundii* genospecies 2 than to *A. viscosus* itself, and that *A. viscosus* includes strains resident in animals only. Multilocus sequence analysis then narrowed the definition of *A. naeslundii* and led to the proposal of *A. oris* for strains formerly assigned to genospecies 2 and *A. johnsonii* for strains formerly assigned to genospecies WVA963. Human strains have nevertheless continued to be assigned to *A. viscosus*, and most of these are likely to be members of *A. oris*; reviews therefore print the name in quotation marks to flag the uncertainty.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC4402957/)\n\nEarlier phenetic work anticipated this closeness. In a 1973 numerical taxonomy study of 76 isolates and 47 reference strains characterized by 77 features, the reference strains labeled *A. naeslundii* and *A. viscosus* agreed at the 92.5% level, meaning they formed a single species by that criterion.[6](https://www.microbiologyresearch.org/content/journal/micro/10.1099/00221287-76-1-43) [Ribotyping](https://www.edgepedia.org/ribotyping) studies likewise noted that species delineation remains unclear, particularly regarding *A. naeslundii* and *A. viscosus*.[7](https://pubmed.ncbi.nlm.nih.gov/7537133/) The close relationship also complicates identification in practice: [MALDI-TOF mass spectrometry](https://www.edgepedia.org/maldi-tof-mass-spectrometry) shows good concordance with 16S rRNA gene sequencing for most *Actinomyces* species but not for the closely related *A. naeslundii*, *A. viscosus*, and *A. oris*.[8](https://pubmed.ncbi.nlm.nih.gov/30261272/)\n\n## Ecology and clinical significance\n\nIn the human mouth, *A. viscosus* and *A. naeslundii* are among the most common organisms found in dental plaque, saliva, and mucosal surfaces.[9](https://www.sciencedirect.com/topics/immunology-and-microbiology/actinomyces-viscosus) The Human Oral Microbiome Database lists the species as cultivated, with low abundance in the oral cavity and a note of supragingival plaque.[10](https://www.homd.org/taxa/tax_description?otid=688)\n\nLike other *Actinomyces* species, it interacts with other dental plaque bacteria, including representatives of *Fusobacterium*, *Prevotella*, and *[Veillonella](https://www.edgepedia.org/veillonella)*, by coaggregation (bacteria of different species sticking together into clumps), a process that provides structural integrity to the plaque.[5](https://journals.asm.org/doi/10.1128/cmr.00100-14) Strong biofilm production has been described for *A. israelii*, *A. naeslundii*, and *A. viscosus*; the biofilm may contribute to the chronic nature and recurrence of these infections and protects the bacteria against the diffusion of antibiotic molecules.[11](https://www.mdpi.com/2304-6767/7/3/85/pdf?version=1567331339)\n\nIn clinical identification schemes, *A. viscosus* is among the species that MALDI-TOF systems may identify if validated for common species, though molecular methods such as 16S rRNA gene sequencing are recommended for definitive identification of invasive isolates.[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC4767983/) In one multiyear series of 115 invasive *Actinomyces* infections, a single isolate was identified as *A. viscosus*, about 1% of the total.[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC4767983/) The MeSH scope note records that the species has been identified as a causative agent of animal diseases.[2](https://meshb-prev.nlm.nih.gov/record/ui?ui=D016924)\n\n## Genome resources\n\nWhole or partial genome sequences are available for strains of the species, including the type strain NCTC 10951 and the strain C505, both tracked in NCBI and the [Integrated Microbial Genomes](https://www.edgepedia.org/integrated-microbial-genomes-system) database.[1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=1656&mode=Info) • [13](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=562973)\n\n## References\n\n1. [NCBI Taxonomy Browser: Actinomyces viscosus (Taxonomy ID 1656)](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=1656&mode=Info)\n2. [MeSH Browser: Actinomyces viscosus (D016924)](https://meshb-prev.nlm.nih.gov/record/ui?ui=D016924)\n3. [Identification of Human Strains of Actinomyces viscosus, Applied Microbiology (1969)](https://journals.asm.org/doi/epdf/10.1128/am.18.1.80-87.1969)\n4. [Actinomyces and Related Organisms in Human Infections, Clinical Microbiology Reviews (2015)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4402957/)\n5. [Actinomyces and Related Organisms in Human Infections, Clinical Microbiology Reviews](https://journals.asm.org/doi/10.1128/cmr.00100-14)\n6. [Holmberg & Hallander (1973), Numerical Taxonomy and Laboratory Identification of Bacterionema matruchotii, Rothia dentocariosa, Actinomyces naeslundii, Actinomyces viscosus, and Some Related Bacteria, Microbiology 76(1)](https://www.microbiologyresearch.org/content/journal/micro/10.1099/00221287-76-1-43)\n7. [rRNA gene restriction patterns as possible taxonomic tools for the genus Actinomyces (PubMed)](https://pubmed.ncbi.nlm.nih.gov/7537133/)\n8. [Identification and diversity of Actinomyces species in a clinical microbiology laboratory in the MALDI-TOF MS era (PubMed)](https://pubmed.ncbi.nlm.nih.gov/30261272/)\n9. [Actinomyces viscosus, ScienceDirect Topics overview](https://www.sciencedirect.com/topics/immunology-and-microbiology/actinomyces-viscosus)\n10. [HOMD Taxon Info: Actinomyces viscosus (HMT-0688)](https://www.homd.org/taxa/tax_description?otid=688)\n11. [Microbiological and Clinical Aspects of Cervicofacial Actinomyces Infections: An Overview, Dentistry Journal (2019)](https://www.mdpi.com/2304-6767/7/3/85/pdf?version=1567331339)\n12. [Species-Level Identification of Actinomyces Isolates Causing Invasive Infections: Vitek MS versus 16S rRNA Gene Sequencing (2016)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4767983/)\n13. [NCBI Taxonomy Browser: Actinomyces viscosus C505 (Taxonomy ID 562973)](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=562973)\n\n---\n*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Bacteria › Actinomycetota (Actinobacteria)*\n\n*Initially written Oct 21, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/actinomyces-viscosus", "markdown_url": "https://www.edgepedia.org/actinomyces-viscosus.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Actinomyces viscosus\", Edgepedia (EdgeChat), https://www.edgepedia.org/actinomyces-viscosus. Edgepedia Community License 1.0.", "credit_md": "\"[Actinomyces viscosus](https://www.edgepedia.org/actinomyces-viscosus)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/actinomyces-viscosus](https://www.edgepedia.org/actinomyces-viscosus). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Actinomyces viscosus\", Edgepedia (EdgeChat), https://www.edgepedia.org/actinomyces-viscosus. Edgepedia Community License 1.0.", "speakable": "Actinomyces viscosus, formerly Odontomyces viscosus, is a high G+C Gram-positive oral bacterium of humans and hamsters; its human strains are taxonomically uncertain and mostly belong to Actinomyces oris." }