Infectious Disease and Microbiology - Arcanobacterium Species

Basics

Arcanobacterium species are uncommon Gram-positive bacteria capable of causing human infections. The most clinically important species is Arcanobacterium haemolyticum, which is particularly associated with acute pharyngitis accompanied by a characteristic skin rash.

Important species include A. haemolyticum, A. pyogenes, and A. bernardiae.

Microbiologic Characteristics

Arcanobacterium species are aerobic, Gram-positive bacilli. They may be overlooked in routine cultures because infections are relatively uncommon and their clinical presentation can resemble more familiar bacterial illnesses.

Epidemiology

Arcanobacterium infections have been reported worldwide but remain relatively rare.

The incubation period is not clearly established.

Infections

Pharyngitis

Pharyngitis is the classic clinical manifestation of A. haemolyticum infection.

Patients may develop:

• Sore throat

• Pharyngeal erythema

• Fever

• Cervical lymphadenopathy

• General constitutional symptoms

An important distinguishing feature is the development of an erythematous rash in approximately half of affected patients.

The eruption may have a:

• Morbilliform appearance, resembling measles

or

• Scarlatiniform appearance, resembling the rash of scarlet fever

Because of this presentation, A. haemolyticum pharyngitis can easily be confused with group A streptococcal pharyngitis and scarlet fever.

Endocarditis

Although uncommon, A. haemolyticum can cause invasive infection, including infective endocarditis.

Possible manifestations include prolonged fever, constitutional symptoms, a new or changing cardiac murmur, and other complications associated with bloodstream infection.

Diagnosis

Diagnosis is established primarily by bacterial culture.

When Arcanobacterium infection is suspected, appropriate clinical specimens should be obtained depending on the presentation.

For pharyngitis:

→ Throat culture

For suspected invasive infection:

→ Blood cultures and other appropriate specimens

Because the organism may not initially be suspected, communication with the microbiology laboratory can be useful.

Differential Diagnosis

The differential diagnosis of A. haemolyticum pharyngitis includes:

• Group A streptococcal pharyngitis

• Scarlet fever

• Infectious mononucleosis

• Viral pharyngitis

• Diphtheria

• Other bacterial causes of pharyngitis

The combination of pharyngitis plus a morbilliform or scarlatiniform rash is an important diagnostic clue.

Treatment

Macrolides

Macrolide antibiotics are important treatment options.

Examples include:

• Erythromycin

• Azithromycin

These agents are particularly useful for A. haemolyticum pharyngitis when the organism is susceptible.

Additional Treatment

Other potentially active agents include:

• Clindamycin

• Tetracyclines

Treatment of serious invasive disease, such as endocarditis, should be guided by antimicrobial susceptibility testing and the site and severity of infection.

Prognosis

Most uncomplicated cases of pharyngitis respond well to appropriate antimicrobial therapy.

Invasive infections are uncommon but potentially serious and require more intensive evaluation and treatment.

High-Yield Clinical Pattern

Pharyngitis + erythematous morbilliform or scarlatiniform rash

→ Think of Arcanobacterium haemolyticum, particularly when the presentation resembles streptococcal scarlet fever.

High-Yield Microbiology

Arcanobacterium:

→ Gram-positive bacillus

→ Aerobic

→ Worldwide distribution

→ Uncommon human pathogen

→ A. haemolyticum is the major species associated with pharyngitis

Exam Essentials

Genus:

→ Arcanobacterium

Important species:

→ A. haemolyticum

→ A. pyogenes

→ A. bernardiae

Microbiology:

→ Aerobic Gram-positive bacillus

Classic infection:

→ Pharyngitis

Characteristic associated finding:

→ Morbilliform or scarlatiniform rash

Frequency of rash:

→ Approximately 50% of cases

Important invasive complication:

→ Endocarditis

Diagnosis:

→ Culture

Main treatment options:

→ Erythromycin or azithromycin

Alternatives:

→ Clindamycin or tetracycline

Key clinical pearl: A. haemolyticum should be considered in a patient with acute pharyngitis and a scarlet fever-like rash, especially when routine testing for more common causes does not explain the illness.