MICROBIOLOGY

• This pleomorphic, non-motile Gram-positive coccobacillus [Fig 1] has undergone several name changes.
  • Formerly known as Corynebacterium equi, then Rhodococcus equi, which is still predominantly used, though there is debate about moving to Rhodoccus hoagii (due to the realization that R. equi had an earlier heterotypic synonym in Corynebacterium hoagii).
• Ubiquitous in the environment.
• Variably acid-fast staining.
• Organisms vary from coccoid to bacillary form depending on growth conditions.
  • Solid media: coccoid, salmon-pink color colonies [Fig 2] may not appear until 4-7 days after incubation.
  • Liquid media: bacillary and branching [Fig 3]. Average time to culture positivity with MGIT broth, 10 days.
• Modes of acquisition: inhalation, local inoculation, or ingestion.
  • A common cause of bronchopneumonia in foals, also swine and cattle.
• It can be overlooked as contaminant diphtheroids and discarded.
  • The lab should be aware if clinically suspected.
  • It can also be confused with Micrococcus, Bacillus, Nocardia, or Mycobacteria species.
  • MALDI-TOF MS and 16S rRNA sequencing are increasingly important to distinguish these organisms.
    • Species identification and AST are important because recent data suggest different susceptibility profiles across Rhodococcus species, and non-equi isolates are increasingly recognized, although their clinical significance can be variable[1].