Currently, 50 species of Rhodococcus exist. Only a few species have been implicated in human infections.
Rhodococcus was once included in the genus Mycobacterium. Recently, several have been redefined as Gordonia and Tsukamurella.
R. equi is most commonly associated with human and animal infections. (It is a major cause of bronchopneumonia in young foals.) Rhodococcus infections may cause severe infections among the immunocompromised, especially among those infected with the human immunodeficiency virus (HIV).
| Characteristics | Comments |
| Appearance of Gram stain | Gram-positive, cocci or short rods (diphtheroid-like), minimal branching |
| Appearance of modified acid-fast stain (MAS) | Usually, weakly acid-fast (red color) |
| Most often associated with clinical disease | Rhodococcus equi (R. erythropolis, R. rhodochrous, R. aurantiacus, R. erythropolis, R. globerulus, R. gordoniae, R. corynebacteroides, may also be implicated.) |
| Colonial growth | Non-hemolytic. No aerial hyphae; rough, smooth or mucoid (slimy), round; often pigmented (buff, pale yellow, coral, orange, red, some may be colorless) |
| Phenotypic characterization47 | Gelatin hydrolysis negative Lysozyme resistance variable Urea hydrolysis is usually positive Reduces nitrates Carbohydrate reduction negative |
| Temperature of optimal growth | Optimal temperature is 30°C |
48. CDC/Linder. Image #15190. This photomicrograph of a Gram-stained specimen, revealed numerous, Gram-positive,
Rhodococcus equi, formerly
Corynebacterium equi, coryneform bacteria. PHIL public domain. Created 1977. Accessed October 27, 2022.
https://phil.cdc.gov/Details.aspx?pid=1519049. CDC/Clark. Image #1553. This image depicts a blood agar plate (BAP), which had been inoculated with Rhodococcus equi. PHIL public domain. Created 1977. Accessed November 5, 2022. https://phil.cdc.gov/Details.aspx?pid=1553
