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Microbiology & infectious diseases

Gram positive bacteria

Cutibacterium acnes


Meng Pu, Ph.D.
Elizabeth M. Garrett, Ph.D.

Last author update: 10 November 2023
Last staff update: 10 November 2023

Copyright: 2023, PathologyOutlines.com, Inc.

PubMed Search: Cutibacterium acnes

Meng Pu, Ph.D.
Elizabeth M. Garrett, Ph.D.
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Table of Contents
Definition / general | Essential features | Epidemiology | Sites | Pathophysiology | Clinical features | Diagnosis | Laboratory | Case reports | Treatment | Clinical images | Microscopic (histologic) description | Microscopic (histologic) images | Positive stains | Differential diagnosis | Board review style question #1 | Board review style answer #1
Cite this page: Pu M, Garrett EM. Cutibacterium acnes. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/microcutibacteriumacne.html. Accessed July 15th, 2024.
Definition / general
  • Gram positive anaerobic bacilli that are a normal component of skin flora
  • Rare cause of infection, predominantly prosthetic joint infections (PJI)
Essential features
  • Anaerobic and lipophilic Gram positive bacterium that colonizes sebaceous glands in the skin
  • Common cause of bacterial culture contamination
  • May cause infection in association with abiotic surfaces such as prosthetic joints and shunts
  • PJI with Cutibacterium acnes are typically indolent and may not exhibit common signs of infection
Epidemiology
  • Normal component of human skin flora (Am J Clin Dermatol 2020;21:18, J Shoulder Elbow Surg 2009;18:897)
    • Colonizes sebaceous glands
    • Maintains skin health by producing propionic acid; acidic skin pH protects from pathogens
    • Most prevalent on skin in high sebum areas including nose, shoulders and back
    • Men have greater prevalence of detectable skin colonization than women
  • An opportunistic pathogen, infrequently causing infection in the absence of surgical implants
  • ~10% of PJI are attributed to C. acnes (Orthopedics 2020;43:52)
Sites
  • Significant cause of PJI (Orthopedics 2020;43:52)
    • Shoulders are most common site of infection, likely due to high abundance of C. acnes colonization in skin of that area
    • Second most common microorganism causing shoulder PJI after coagulase negative Staphlococcus
    • Rare cause of hip, knee and ankle PJI
  • Also causes spinal infections in association with hardware
  • Other common sources of infection include ventriculoperitoneal shunts, implanted cardiac devices and breast implants
  • Contributes to the development of acne vulgaris (Am J Clin Dermatol 2020;21:18)
    • Presence of C. acnes alone does not cause acne
    • Some causes of acne linked to dysbiosis of normal flora, loss of microbial diversity and overabundance of some C. acnes strains
Pathophysiology
  • PJI likely develops from introduction of C. acnes during surgical arthroplasty (Orthopedics 2020;43:52)
    • Likely due to contamination of implant or surgical tools from subdermal layers of the patient's skin
    • Standard skin preparation may not eradicate C. acnes prior to surgery
    • Longer procedure time is associated with higher risk of infection
  • C. acnes readily forms biofilms on abiotic surfaces (Biofilm 2021;4:100063)
    • Increased production of virulence factors, such as lipases, in biofilm
    • Resists immune clearance and antibiotic treatment
    • In vivo biofilm formation linked to treatment failure in PJI
Clinical features
  • PJI due to C. acnes are typically indolent infections that develop slowly and present weeks to months after surgery (Orthopedics 2020;43:52)
    • Most common complaints are persistent pain and stiffness
    • May mimic aseptic loosening in presentation
    • May lack typical signs of infection such as erythema, purulence and drainage or formation of sinus tracts
Diagnosis
  • Culture and Gram stain of affected joint for PJI
    • Multiple periprosthetic tissue specimens are recommended along with hardware as available, for improved sensitivity and specificity
    • Explanted joints or hardware may also be cultured using sonication or a vortex mixer to disrupt biofilms while preserving microbial viability
  • Sensitivity of culture for PJI may be reduced due to previous antibiotic therapy, the presence of biofilms, insufficient incubation time or lack of anaerobic culture conditions
  • Recovery in culture may indicate contamination, which necessitates clinical correlation
    • May indicate contamination during collection or processing or may represent colonization of an abiotic surface without infection
    • 2 or more deep specimens or higher colony counts for semiquantitative cultures may correlate with true infection
  • Typical inflammatory markers such as C reactive protein, erythrocyte sedimentation rate and white blood cell count may be within normal limits
  • Molecular approaches such as broad range 16s sequencing or targeted C. acnes PCR may aid diagnosis but should be interpreted with caution due to risk of detecting contamination
    • No FDA cleared assays for molecular detection of C. acnes
  • References: Clin Microbiol Rev 2014;27:302, Orthopedics 2020;43:52, J Shoulder Elbow Surg 2020;29:1920
Laboratory
  • Culture (Carroll: Manual of Clinical Microbiology, 12th Edition, 2019)
    • Optimal recovery via anaerobic culture conditions, though may grow poorly under aerobic conditions
    • Can be recovered on rich media (e.g., sheep's blood agar), anaerobic media supplemented with hemin (e.g., Brucella agar) and anaerobic media selective for Gram positives (e.g., phenylethyl alcohol blood agar [PEA])
    • Recovery from periprosthetic tissue specimens may be improved by using thioglycolate broth or blood culture media
    • Disruption of biofilms from explanted prosthetic joints or hardware, using vortex mixer or sonication, improves culture sensitivity (J Clin Microbiol 2009;47:1878, J Clin Microbiol 2013;51:591)
      • Should be performed in a sterile, sealed, hard walled container (not bag) with sterile saline or Ringer's solution
      • Culture from sonicate fluid can be more sensitive than periprosthetic tissue
    • Recommended incubation time of 14 days for surgical specimens (Clin Microbiol Rev 2014;27:302)
      • Primary isolation of C. acnes from specimens may require a longer time period than growth from subculture due to slow growth from metabolically dormant biofilms
      • Time to recovery may also depend on media type
  • Identification (Carroll: Manual of Clinical Microbiology, 12th Edition, 2019)
    • Gram stain: pleomorphic, thin, nonbranching, non-spore forming Gram positive rods
      • May be coryneform
      • Bacilli clump to form arrangements that resemble spiders
    • Colony morphology: white, shiny, slightly domed colonies
    • Catalase test positive
    • Can be identified using MALDI-TOF mass spectrometry
Case reports
Treatment
Clinical images

Contributed by Meredith Schade, M.D. and Elizabeth Garrett, Ph.D.
Shoulder PJI

Shoulder PJI

Colony morphology

Colony morphology

Microscopic (histologic) description
  • Thin, pleomorphic, non-spore forming Gram positive rod
    • Bacilli clump in arrangements described as squashed spiders
Microscopic (histologic) images

Contributed by Elizabeth Garrett, Ph.D.
<i>C. acnes</i> Gram stain

C. acnes Gram stain

<i>C. acnes</i> Gram stain   <i>C. acnes</i> Gram stain

C. acnes in tissue

Positive stains
Differential diagnosis
  • Common agents of PJI (Clin Microbiol Rev 2014;27:302):
  • Aseptic joint failure or aseptic loosening (Clin Infect Dis 2013;56:e1):
    • Septic joint loosening often associated with elevated inflammatory markers, including C reactive protein and erythrocyte sedimentation rate
      • These markers may be negative in C. acnes PSI
    • Synovial fluid analysis (including cell count and differential leukocyte count) indicating a predominance of neutrophils
    • Identification of one organism from multiple deep cultures supports infection
    • Purulence or presence of a sinus tract likely indicates infection
Board review style question #1
Brq image


A 57 year old patient presents with 9 weeks of shoulder pain and stiffness following total shoulder arthroplasty. Periprosthetic tissue specimens are collected during surgery and are sent for bacterial culture. After 6 days, the anaerobic cultures are growing a pleiomorphic, non-spore forming Gram positive rod from the thioglycolate broth but the aerobic cultures are not. Which of the following is most likely to be the organism?

  1. Bacillus subtilis
  2. Clostridium perfringens
  3. Cutibacterium acnes
  4. Staphylococcus epidermidis
    Board review style answer #1
    C. Cutibacterium acnes. The question describes an anaerobic, non-spore forming Gram positive rod, which is true of C. acnes. The Gram stain shown has clumped arrangements of Gram positive rods that are characteristic of C. acnes. C. acnes is a leading cause of prosthetic joint infections, commonly in the shoulder. Answer A is incorrect because Bacillus subtilis is spore forming and will grow in aerobic conditions and it is also a rare cause of invasive infections. Answer B is incorrect because Clostridium perfringens spore forming does not display the morphology seen on the Gram stain and is a rare cause of joint infection. Answer D is incorrect because S. epidermidis, though a common cause of joint infections, is a Gram positive cocci and will grow in aerobic cultures.

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