Microbiology & infectious diseases

Filamentous bacteria


Deputy Editor-in-Chief: Patricia Tsang, M.D., M.B.A.
 Natalie Larsen, M.D.
Sixto M. Leal, Jr., M.D., Ph.D.

Last author update: 16 August 2022
Last staff update: 17 January 2023

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PubMed Search: Actinomyces

 Natalie Larsen, M.D.
Sixto M. Leal, Jr., M.D., Ph.D.
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Cite this page: Larsen N, Perez V, Leal SM. Actinomyces. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/microbiologyactinomyces.html. Accessed May 19th, 2024.
Definition / general
  • Gram positive genus containing over 40 species
  • Taxonomy: genera Actinobacteria, family Actinomycetaceae
  • Common species:
    • Actinomyces israelii
    • Actinomyces bovis
    • Actinomyces gerencseriae
    • Actinomyces graevenitzii
    • Actinomyces odontolyticus
    • Actinomyces meyeri
    • Actinomyces neuii
    • Actinomyces turicensis
Essential features
  • Gram positive anaerobic bacilli with filamentous branching (Clin Microbiol Rev 2015;28:419)
  • Anaerobic; modified acid fast stain negative
  • Grow in tissue as intertwined aggregates (termed sulfur granules) to exclude oxygen
  • Normal flora in tonsils; pathogenic elsewhere
  • Actinomycosis is characterized by pyogranulomatous inflammation
  • Most common causes of actinomycosis include: A. israelii, A. gerencseriae and A. graevenitzii
  • A. meyeri, A. neuii and A. turicensis are emerging pathogens
  • Reside on mucosal surfaces: oral cavity, tonsillar crypts, genitourinary tract
  • Opportunistic infection in skin, jaw bones, chest and abdominal cavities, lungs, liver, uterus
  • Common involvement in polymicrobial infections
  • Affects immunocompetent and immunocompromised hosts
  • More common in developing nations
  • Associated with poor dentition, smoking, heavy alcohol consumption, bisphosphonate related osteonecrosis of the jaw, osteoradionecrosis, prolonged use of intrauterine contraceptive devices
  • Reference: Clin Microbiol Rev 2015;28:419
  • Orocervicofacial; > 50% of infections (Oral Surg 2017;123:586)
  • Disseminated:
    • Chest and abdominal cavities
    • Lungs
    • Liver
    • CNS
  • Uterus
  • Infection initiated upon access to tissues via trauma, surgical procedures, foreign bodies
  • Formation of dense intertwined filamentous bacterial aggregates called sulfur granules exclude oxygen, host cells and immune mediators (antibodies, etc.), thus enabling optimal growth
  • Reference: Infect Drug Resist 2014;7:183
Clinical features
  • Indolent, slowly progressing pyogranulomatous disease
  • Orocervicofacial: abscess formation with sinus tracts, purulent discharge, fistulae, tissue fibrosis; lumpy jaw formation
  • Pneumonia: chronic cough, dyspnea, fibrosis, cavitation (Clin Microbiol Rev 2015;28:419)
  • Disseminated: low grade fever, weight loss; tissue specific symptoms
  • Uterus: purulent vaginal discharge, foul odor, cramping
  • Most pathogens will grow on chocolate blood agar at 37 °C within 5 days
  • Some aerotolerant Actinomyces species will grow on aerobic culture within 2 days
  • Most exhibit nondistinct tan-white diphtheroid-like colonies; gamma hemolysis
  • A. israelli exhibits a pathognomonic molar tooth colony appearance
  • Key feature: gram positive filamentous bacteria; staining negative with modified acid fast stain
    • Acid fast stains include Kinyoun and Ziehl-Neelsen
    • Actinomyces species lack high levels of mycolic acid in the cell wall and do not retain the carbol fuschin dye when using a weaker acid for decolorization (i.e., modified Kinyoun acid fast stain), resulting in a negative stain
    • Nocardia, another gram positive filamentous bacteria, have abundant mycolic acid in the cell wall, can retain the carbol fuschin dye and stain positive with modified Kinyoun
  • MALDI TOF mass spectrometry enables species level identification
  • Reference: Infect Drug Resist 2014;7:183
Case reports
Microscopic (histologic) description
  • Characteristic large basophilic filamentous bacterial aggregates (sulfur granules) that stain positive on GMS but negative with FITE stain and the traditional AFB stain
  • Commensal organism frequently encountered histologically in tonsillar crypts, resembling starbursts
Microscopic (histologic) images

Contributed by Natalie Larsen, M.D. and Sixto M. Leal, Jr., M.D., Ph.D.
Inflammation surrounding filamentous bacteria

Inflammation surrounding filamentous bacteria

Sulfur granule formation in tonsillar tissue

Sulfur granule formation in tonsillar tissue

Molecular / cytogenetics description
  • 16S rRNA sequencing is the current gold standard for species level identification in bacteria
Differential diagnosis
  • Nocardiosis:
    • Note aerobic growth and positive modified acid fast stain
  • Botryomycosis:
    • Note bacterial cocci (staphylococci) in tissue
  • Tuberculosis:
  • Mold infection:
    • Note increased width (4 - 10 µm) of hyphae or yeast-like structures
  • Malignancy:
    • Note cellular atypia and absence of organisms
Board review style question #1
A 43 year old woman presents to the emergency department with weight loss, dyspnea and a mild cough. A sputum culture Gram stain shows gram positive filamentous bacteria. Which stain is helpful for distinguishing the 2 major genera of filamentous bacteria?

  1. GMS
  2. Gram stain
  3. Modified Kinyoun
  4. Ziehl-Neelsen
Board review style answer #1
C. Modified Kinyoun. Actinomyces species, unlike Nocardia species, do not have high levels of mycolic acid in the cell wall and will not retain the carbol fuschin dye when using a weaker acid for decolorization (i.e., modified acid fast stain), resulting in a negative stain. Nocardia will retain the carbol fuschin dye and will stain positive. Both will stain gram positive, although Nocardia yield a beaded appearance due to the mycolic acid on the cell wall. Both will be positive by GMS performed on tissue and negative by Ziehl-Neelsen.

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Reference: Actinomyces
Board review style question #2

A homeless man with poor dentition presented with jaw pain with swelling, fistula formation and purulence. On exam, a small granule was expressed from this lesion and sent to microbiology. Half of the sample was crushed and stained as shown above and the other half was submitted for culture. A Gram stain of the slide revealed filamentous structures. Which culture conditions are optimal to isolate the likely etiologic agent?

  1. BCYE agar under aerobic conditions
  2. Blood agar under microaerophilic conditions
  3. CDC anaerobic agar under aerobic conditions
  4. Chocolate agar under anaerobic conditions
Board review style answer #2
D. Chocolate agar under anaerobic conditions. Actinomyces species grow optimally under anaerobic conditions. Some facultative anaerobes and aerotolerant species in this genus may grow in the presence of oxygen but extended culture may be required.

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Reference: Actinomyces
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