|Arcanobacterium haemolyticum colonies on a blood agar plate. Beta-hemolysis is demonstrated with transmitted light.|
Arcanobacterium haemolyticum, formerly known as Corynebacterium hæmolyticum, is a bacterium classified as a Gram-positive bacillus and is catalase-negative, aerobic, beta-haemolytic, nonmotile. It is part of the normal flora of the pharynx, but may cause head and neck infections, pharyngitis and sinusitis.
It was first described by MacClean et al. in 1946 from US servicemen and peoples of the South Pacific suffering from sore throat. Due to is resemblance to another type of bacteria, Corynebacterium, A. haemolyticum was initially classified as C. pyogenes subspecies hominus. Controversies regarding classification were resolved in 1982 when a new genus, Arcanobacterium (enigmatic bacterium) was created based on its peptidoglycan, fatty acid, and DNA characteristics.
Since its initial description, the spectrum of diseases caused by A. haemolyticum has been expanded to include sepsis and osteomyelitis. Organisms are Gram-positive, facultative anaerobic, catalase negative rods (but transition to the coccal shape occurs as the organism grows) with arrangements described as matchbox or Chinese letters arrangements. Growth is enhanced in blood and by carbon dioxide.
Hemolysis is detected on human blood agar plates, and routine plating of specimens suspected of containing A. haemolyticum on human blood agar is suggested to distinguish it from Streptococcus pyogenes as A. haemolyticum can easily be confused with this organism. Microscopic morphology differentiates can be use to differentiate them, since Arcanobacterium are bacilar and Streptococcus are coccus.
A. haemolyticum infection is most common in 15- to 25-year-old persons and manifests as exudative pharyngitis and/or tonsillitis accompanied by cervical lymphadenopathy. Symptoms look like those of β-hemolytic streptococci or viral infection. A rash of the chest and of the abdomen, neck, or extremities is seen in 20% to 25% of cases enhancing the risk of diagnostic error as streptococcal infection or penicillin allergy, when β-lactam therapy is initiated without exact diagnosis.
A. haemolyticum often occurs in polymicrobic infections together with typical respiratory pathogens such as streptococci. The isolation of classical pathogens from specimens that also contain A. haemolyticum might be in part responsible for the tendency to miss the organism.
A. haemolyticum is the cause of pharyngitis (sore throat) in up to 2.5% of cases. In one study, it was the causative agent of pharyngitis in 1.4% of military conscripts. It is rarely found in the skin or throat of healthy people, meaning it is not a member of the usual bacterial flora.
Little is known about the means by which A. haemolyticum causes infection or the associated skin manifestations. The organism is known to produce uncharacterized hemolytic agent(s), a neuraminidase and a phospholipase D (PLD) acting preferentially on sphingomyelin. PLD is known to result in tissue damage, but the role in disease of the cytotoxic effect caused by this extracellular toxin is not established.
The use of parenteral antimicrobial drugs must be limited to serious infections.
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