Arcanobacterium haemolyticum-Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes


Arcanobacterium haemolyticum is a gram-positive, rod-shaped bacterium that can cause infections in humans. Here’s a brief introduction:

  1. Taxonomy: Formerly it was classified under the genus Corynebacterium and was named Corynebacterium haemolyticum. It was later moved to its own genus, Arcanobacterium, due to distinct characteristics.
  2. Clinical Significance: A. haemolyticum is primarily recognized as a cause of pharyngitis (sore throat) in adolescents and young adults. The symptoms of this pharyngitis can sometimes mimic those caused by Streptococcus pyogenes (Group A Streptococcus), which is a more common cause of pharyngitis.In addition to pharyngitis, A. haemolyticum can cause other infections such as skin and soft tissue infections, wound infections, and more rarely, invasive infections like septicemia.
  3. Identification in the Laboratory: A. haemolyticum is characteristically beta-hemolytic on sheep blood agar, meaning it breaks down the red blood cells in the agar and produces a clear zone around the bacterial colonies. This is one of the primary features that differentiate it from other Arcanobacterium species.
  4. Treatment: A. haemolyticum is typically susceptible to various antibiotics, including penicillin, making it treatable in most cases. However, as with any bacterial infection, it’s essential to have an accurate diagnosis to guide effective treatment.
  5. Epidemiology: Though it is not as common as some other bacterial pathogens, it is believed to colonize the throats of a small percentage of healthy individuals. This means that these individuals carry the bacterium without showing any symptoms.
  6. Transmission: The exact mode of transmission is not fully understood, but it’s believed to spread via respiratory droplets or through close personal contact.


Arcanobacterium haemolyticum is characterized by the following morphological features:

  1. Gram Staining: It is a Gram-positive bacterium, which means it retains the violet crystal stain when subjected to a Gram stain procedure.
  2. Shape: Arcanobacterium haemolyticum is rod-shaped, often described as a coryneform or diphtheroid bacterium because of its resemblance to bacteria in the genus Corynebacterium. Its appearance can sometimes be slightly club-shaped.
  3. Arrangement: The bacteria often appear as single rods or in short chains.
  4. Hemolysis: On blood agar, A. haemolyticum is typically beta-hemolytic, which means it produces an enzyme that lyses red blood cells, resulting in a clear (transparent) zone around bacterial colonies.
  5. Size: The size of individual cells can vary, but they typically measure between 0.6 to 1.0 micrometers in width and 1.0 to 2.0 micrometers in length.
  6. Surface Characteristics: The colonies of A. haemolyticum are typically small, smooth, and shiny when grown on agar.
  7. Motility: Arcanobacterium haemolyticum is non-motile, meaning it does not move on its own.
  8. Spore Formation: This bacterium is non-spore-forming, which means it doesn’t produce spores as a survival mechanism.


Arcanobacterium haemolyticum is primarily associated with human infections. While it’s not as widely recognized or prevalent as certain other pathogens, it has specific clinical significance. Here’s a breakdown of its pathogenicity:

  1. Pharyngitis: A. haemolyticum is most commonly associated with pharyngitis (sore throat) in adolescents and young adults. The clinical presentation can be similar to pharyngitis caused by Streptococcus pyogenes, making it essential to differentiate between the two for appropriate treatment.
  2. Skin and Soft Tissue Infections: This bacterium can also lead to skin and soft tissue infections, especially in people with compromised immune systems. These infections can include cellulitis, abscesses, and wound infections.
  3. Other Infections: More rarely, A. haemolyticum can cause other invasive infections like septicemia, osteomyelitis, pneumonia, and endocarditis. These are less common but can be severe.
  4. Virulence Factors:
    • Hemolysin: This bacterium produces a hemolysin that can lyse red blood cells. This hemolytic activity is believed to play a role in its pathogenicity, especially in pharyngeal infections.
    • Adhesion: Like many pathogenic bacteria, A. haemolyticum has surface proteins that facilitate its adherence to host cells, contributing to its ability to cause disease.
  5. Epidemiology: While it’s known to cause infections, A. haemolyticum can also be found as part of the normal flora in some individuals. This means that they carry the bacterium without showing any symptoms. This colonization can serve as a reservoir for potential infections.
  6. Transmission: The exact transmission route isn’t completely understood, but it’s thought to spread through respiratory droplets or through close personal contact. Given its association with pharyngitis, it’s plausible that close contact or sharing of utensils could facilitate its spread.

Lab Diagnosis

The laboratory diagnosis of Arcanobacterium haemolyticum infection involves both traditional microbiological methods and more modern techniques. Here’s a general overview:

  1. Sample Collection:
    • For suspected pharyngitis, a throat swab is taken.
    • For skin and soft tissue infections, a swab of the infected area, pus from an abscess, or tissue biopsy might be collected.
    • For other infections, relevant clinical samples like blood (for bacteremia) or other body fluids are collected.
  2. Culture:
    • The primary medium for cultivation is Blood Agar. A. haemolyticum characteristically exhibits beta-hemolysis on blood agar, which means it produces a clear zone around the colonies due to the complete lysis of red blood cells.
    • The bacterium grows best under aerobic or microaerophilic conditions at 37°C.
  3. Microscopy:
    • A Gram stain of the culture will show Gram-positive rods. These rods can sometimes resemble diphtheroids or Corynebacterium species.
  4. Biochemical Tests:
    • A. haemolyticum is catalase-negative, which differentiates it from many other Gram-positive rods.
    • It’s also CAMP test-positive, showing an enhanced zone of hemolysis when streaked perpendicular to a beta-hemolytic Staphylococcus aureus on a blood agar plate.
  5. Reverse CAMP Test: This is a characteristic test for A. haemolyticum. Instead of using Staphylococcus aureus, Rhodococcus equi is used. A. haemolyticum will inhibit the hemolysis caused by R. equi, producing a characteristic “shadow” of reduced hemolysis.
  6. Antimicrobial Susceptibility Testing:
    • Once the bacterium is isolated, it’s essential to determine its susceptibility to various antibiotics, guiding clinicians in choosing the appropriate treatment.
  7. Molecular Methods:
    • PCR (Polymerase Chain Reaction) can be used to identify specific genetic markers of A. haemolyticum, providing rapid and accurate identification.
    • MALDI-TOF MS (Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry) is another modern technique used in some labs for rapid bacterial identification based on protein profiles.
  8. Serological Tests:
    • Not commonly used for the diagnosis of A. haemolyticum infections, but serological methods can be developed to detect antibodies against the bacterium in the patient’s serum.


The treatment of infections caused by Arcanobacterium haemolyticum typically involves antibiotics. Here’s a general overview of the treatment options:

  1. Penicillin: A. haemolyticum is usually sensitive to penicillin, making it a first-line choice for many infections caused by this bacterium. This can be in the form of oral penicillin or, for more severe infections, intravenous penicillin.
  2. Macrolides: For patients allergic to penicillin, macrolide antibiotics like erythromycin or azithromycin are often effective alternatives. However, there have been reports of macrolide resistance in some strains, so susceptibility testing is crucial.
  3. Other Antibiotics: Clindamycin and tetracycline have also been used to treat A. haemolyticum infections. The choice might depend on the specific infection site, patient allergies, or antibiotic resistance patterns.
  4. Antimicrobial Susceptibility Testing: It’s essential to test the isolated strain for antibiotic susceptibility to ensure the chosen antibiotic is effective. This is especially important given the potential for resistance and the need to ensure optimal patient outcomes.
  5. Supportive Care: In addition to antibiotics, patients may require supportive care, depending on the nature and severity of the infection. For example, pain relievers or fever reducers might be given for pharyngitis. For more severe infections like septicemia, hospitalization and broader supportive measures may be necessary.
  6. Drainage: In the case of abscesses or other localized infections with a significant build-up of pus, surgical drainage might be needed.
  7. Follow-up: Patients should be monitored to ensure that the infection is resolving, and no complications arise. If there’s no improvement or if the condition worsens, the treatment plan might need adjustment.


Prevention of Arcanobacterium haemolyticum infections largely hinges on general measures to avoid the spread of infectious agents. While there isn’t a vaccine or specific preventive treatment for A. haemolyticum infections, the following strategies can help reduce the risk:

  1. Good Hygiene:
    • Handwashing: Regularly wash hands with soap and water for at least 20 seconds, especially after coughing or sneezing, before eating, and after using the bathroom.
    • Hand Sanitizers: Use alcohol-based hand sanitizers if soap and water aren’t available.
  2. Respiratory Hygiene:
    • Cover the mouth and nose with a tissue or the elbow when coughing or sneezing. This practice is essential, given that A. haemolyticum can cause pharyngitis and might be spread through respiratory droplets.
    • Dispose of tissues properly after use and wash hands immediately.
  3. Avoid Close Contact:
    • Avoid close contact with individuals who are sick or showing symptoms of an infection.
    • If you’re sick or have symptoms of pharyngitis or another infection, it’s best to stay home and avoid spreading the illness.
  4. Don’t Share Personal Items:
    • Avoid sharing personal items like toothbrushes, drinking cups, utensils, or other objects that might come into contact with the mouth, especially if someone is showing symptoms.
  5. Wound Care:
    • For skin infections, keep wounds clean and covered until they heal. Avoid touching or picking at the wound.
    • If the wound appears to be infected (e.g., increased redness, swelling, pus, or if it feels warm to the touch), seek medical attention.
  6. Awareness:
    • Clinicians should be aware of the potential for A. haemolyticum infections, especially in adolescents and young adults presenting with pharyngitis, to ensure appropriate diagnosis and treatment.
    • Awareness can also help in guiding empirical antibiotic therapy in patients with suspected bacterial pharyngitis, especially in settings where A. haemolyticum is prevalent.
  7. Surveillance and Research:
    • Continued surveillance and research on A. haemolyticum can provide insights into its epidemiology and risk factors, guiding better preventive measures in the future.


Here are the keynotes on Arcanobacterium haemolyticum:

  1. Classification:
    • Gram-positive rod-shaped bacterium.
    • Previously classified under the genus Corynebacterium due to its coryneform appearance.
  2. Morphology:
    • Gram-positive rods, often appearing in short chains.
    • Exhibits beta-hemolysis on blood agar.
  3. Epidemiology:
    • Primarily associated with human infections.
    • Can be found as part of the normal flora in some individuals.
  4. Clinical Manifestations:
    • Most commonly causes pharyngitis in adolescents and young adults.
    • Can also cause skin and soft tissue infections.
    • Rarely, can lead to more invasive infections like septicemia, osteomyelitis, and endocarditis.
  5. Diagnosis:
    • Cultured on blood agar; beta-hemolysis is a hallmark.
    • Gram stain reveals Gram-positive rods.
    • Catalase-negative; positive CAMP test.
    • Molecular methods, such as PCR, can provide rapid identification.
  6. Treatment:
    • Typically sensitive to penicillin.
    • Macrolides (e.g., erythromycin) are alternatives, but resistance has been reported.
    • Antimicrobial susceptibility testing is vital to guide treatment choices.
  7. Prevention:
    • Emphasis on good hygiene, especially handwashing.
    • Respiratory hygiene to prevent the spread through respiratory droplets.
    • Avoid sharing personal items.
  8. Special Characteristics:
    • Notably, A. haemolyticum has a unique “reverse CAMP test” reaction with Rhodococcus equi.
    • Despite its potential pathogenicity, it’s often overshadowed by the more common Streptococcus pyogenes in pharyngitis diagnoses.

Further Readings

  1. Textbooks:
    • “Manual of Clinical Microbiology” (latest edition) by ASM Press. This comprehensive textbook provides detailed information on various microorganisms, including A. haemolyticum, their laboratory diagnosis, and clinical significance.
    • “Principles and Practice of Infectious Diseases” by Mandell, Douglas, and Bennett. This textbook offers in-depth insights into infectious diseases, their causative agents, and treatment strategies.
  2. Research Articles & Reviews:
    • Search PubMed ( using the term “Arcanobacterium haemolyticum.” You’ll find numerous research articles, case reports, and review papers on the topic.
  3. Clinical Guidelines:
    • Clinical and Laboratory Standards Institute (CLSI): CLSI periodically publishes guidelines for the diagnosis and antimicrobial susceptibility testing of pathogens, which might include updates on A. haemolyticum.
  4. Centers for Disease Control and Prevention (CDC):
    • While the CDC might not have an entire page dedicated to A. haemolyticum, their website ( contains vast resources on infectious diseases, and searching their database can yield valuable information.
  5. World Health Organization (WHO):
    • Similar to the CDC, the WHO ( offers a wealth of information on infectious diseases. While A. haemolyticum might not be a primary focus, their resources on bacterial infections and antibiotic resistance can provide a broader context.
  6. Online Microbiology Databases:
    • The Pathogen-Host Interaction Database (PHI-base) or other similar databases can offer specific insights into the interactions of A. haemolyticum with its human hosts.
  7. Universities & Institutions:
    • Many academic institutions or organizations dedicated to microbiology and infectious diseases have online resources or course materials available to the public. Checking their websites might yield valuable information or references.
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