Unusual throat isolates...(Reverse CAMP Test)

All β-haemolytic colonies isolated from throat swabs which do not Lancefield group should be Gram stained to rule out the possibility of Arcanobacterium haemolyticum.

Lab Identification:

  • Arcanobacterium haemolyticum is a catalase-negative, gram-positive to gram-variable rod that does not form spores and is nonmotile.
  • It is β-haemolytic, but expression can vary by culture media and conditions, with haemolysis best observed on human blood agar. Growth is enhanced in the presence of CO2. It is known for forming dark pits under the colonies. Poor growth on tellurite helps to differentiate it from C. diphtheriae.
  • Colony morphology has been described as either rough or smooth type. Rough type colonies are rough appearing, non-haemolytic, β-glucuronidase positive, and do not ferment sucrose and trehalose. Smooth type are smooth appearing colonies, β-haemolytic, β-glucuronidase negative, and ferment sucrose or trehalose. Both types ferment glucose and maltose. Rough type colonies are most frequently associated with respiratory isolates; smooth biotypes are most frequently associated with wound isolates.
  • A. haemolyticum does not ferment xylose, which differentiates it from A. pyogenes. The α-mannosidase test has been proposed as a useful tool for identification. A positive test identifies A. haemolyticum and differentiates it from from Arcanobacterium (Actinomyces) pyogenes, and other coryneform-like bacteria including Rhodococcus equi and Erysipelothrix rhusiopathiae. Because of the presence of phospholipase D activity similar to C. ulcerans and C. pseudotuberculosis, the reverse CAMP test will be positive, with inhibition of the haemolytic zone of a β-lysin-producing strain of Staphylococcus aureus1. Other secreted toxins include neuraminidase and a haemolysin.

Clinical Features:

  • Recurrent sore throat
    • Pharyngitis and tonsilitis in the 15-25 age group with some associated lymphadenopathy
  • SSTIs
  • Sepsis
  • Osteomyelitis
  • Skin rash that might be confused with a penicillin allergy

Management:

  • Isolates are sensitive to penicillin in vitro, but therapeutic failures are reported with single agent therapy2. Either penicillin with gentamicin (which has been shown to be bacateriocidal), or macrolides or cephalosporins.

AMH.