MICROBIOLOGY

• Arcanobacterium haemolyticum: Agent mostly causes pharyngitis or soft tissue infections.
  • The major human pathogen in this group.
  • Gram-positive facultative anaerobe bacillus, not thought to be part of routine human oral or skin flora.
    • Organisms are thin, curved and may have rudimentary branching.
    • β-hemolytic, need to evaluate when catalase-negative and no Lancefield group antigens found (which would indicate non-streptococcal origin).
      • Growth improved on blood-enriched media, 37ºC with 5-10% CO₂.
      • Hemolysis best observed on human or horse blood in a CO₂-enriched atmosphere.
        • Frequently overlooked on plates with commensal oral flora.
    • Sometimes Gram variable on staining.
    • Formerly called Corynebacterium hæmolyticum.
      • Based on 16S rRNA sequencing, organism distinguished from Corynebacterium spp.
• Other Arcanobacterium spp. may occasionally cause human infection.
  • A. pyogenes: mostly an animal pathogen as a major cause of mastitis in livestock, but an occasional cause of human infection including soft tissues and other sites. Most often described in rural environs.
  • A. bernardiae: non-branching on Gram stain. Rare occurrences described causing bacteremia, musculoskeletal and eye infections.
• Trueperella bernardiae: previously transferred to the Arcanobcaterium genus, but now reclassified. A rare cause of human infection (wound, bloodstream, joint, renal).
  • Considered part of normal skin and oral flora.
  • Also frequently not identified as listed as a Corynebacterium spp.