What are the characteristic colony morphologies of Rhodococcus and Nocardia, and how can they be differentiated in a culture, particularly in an immunocompromised patient?

Differentiating Rhodococcus and Nocardia Colony Morphology

Rhodococcus colonies typically appear smooth, mucoid, or rough with a salmon-pink to orange pigmentation, while Nocardia colonies are characteristically chalky-white, dry, and wrinkled with aerial hyphae that may fragment into bacillary forms—both are aerobic actinomycetes but can be distinguished by colony appearance, acid-fast staining patterns, and biochemical testing.

Colony Morphology Characteristics

Rhodococcus

• Colonial appearance varies significantly with smooth, rough, or mucoidal morphotypes depending on the strain 1, 2
• Colonies often display salmon-pink to orange pigmentation on non-selective media 1
• Smooth strains produce glistening, moist colonies while rough strains appear dry with irregular edges 2
• Mucoidal strains exhibit viscous, sticky colony texture 2
• Grows well on non-selective media as an obligate aerobic organism 1

Nocardia

• Colonies are typically chalky-white, dry, and wrinkled with a powdery or velvety appearance 3
• Exhibits filamentous branching structures that fragment into bacillary or coccoid forms 3
• May develop aerial hyphae giving colonies a fuzzy or cottony appearance 3
• Growth is slower compared to Rhodococcus, often requiring extended incubation 4

Key Differentiation Methods

Microscopic Examination

• Both are Gram-positive, partially acid-fast organisms 1, 3
• Nocardia shows more prominent filamentous branching that fragments into bacillary forms 3
• Rhodococcus appears as non-motile, catalase-positive cocci or short rods without extensive branching 1

Biochemical Testing

Eleven characteristics reliably differentiate these genera 5:

• Arylsulfatase activity (14-day test): Nocardia species show variable results while Rhodococcus is typically positive 5
• Catalase activity (semiquantitative): Both positive but intensity differs 5
• Beta-esterase activity: Distinct patterns between genera 5
• Pyrazinamidase activity: Useful for species-level differentiation 5
• Citrate utilization: Different patterns as sole carbon source 5
• 2,3-butylene glycol utilization: Helps distinguish species 5
• 5-fluorouracil resistance (20 μg/ml): Variable between species 5

Molecular Methods

• 16S rRNA gene sequencing is the most powerful method for genus identification but cannot differentiate all Rhodococcus species 1
• Whole genome sequencing definitively resolves taxonomic complexities when biochemical methods are insufficient 1
• Molecular probes are not routinely available for these organisms unlike for mycobacteria 4

Clinical Context Considerations

Immunocompromised Patients

• Rhodococcus equi primarily causes pneumonia in immunocompromised patients, similar to its role in foal pneumonia 1
• Nocardia species cause chronic progressive infections with abscess formation, fistulous tracts, and draining sinuses in immunocompromised hosts 3
• Both are "great masqueraders" with diagnosis often delayed due to their fungal-like appearance 3

Laboratory Communication

• Direct communication between clinician and laboratory is critical for determining the need for extensive identification efforts 4
• Context of specimen source (sputum, sterile site, single vs. multiple isolates) guides the intensity of identification work 4
• Consider sending isolates to reference laboratories for definitive identification when local capabilities are limited 4

Common Pitfalls

• Do not rely solely on colony morphology as Rhodococcus exhibits multiple morphotypes that can mimic other organisms 2
• Avoid premature dismissal of single isolates from non-sterile sites without clinical correlation, especially in immunocompromised patients 4
• Recognize that conventional biochemical testing alone is time-consuming and may not identify newly described species—molecular methods are increasingly necessary 4, 1
• Both organisms may be mistaken for fungi due to their filamentous growth patterns, leading to inappropriate antifungal therapy 3