| Organism | Gram Stain | Partial Acid-Fast Stain | Colony Morphology (at 35-37°C) at 40-48hr |
| Aerobic Actinomyces | Gram-positive filamentous/branched fragment into rod-shaped or coccoid forms | Negative | White, flat, round, smooth, or slightly granular some species may be alpha-hemolytic on blood agar |
| Aerotolerant Bifidobacterium | Gram-positive short or long rods | Negative | White, glistening, umbonate, and mucoid |
| Varibaculum | Gram-positive short, straight, or curved diphtheroid rods | Negative | Most are anaerobic. Small, white, glistening colonies |
| Nocardia | Branched, Gram-positive filaments that may stain variable and can fragment into shorter bacilli | Positive (may be negative first but most are positive with age) | Dry, crumply, wrinkled with chalky white, orange, or tan color |
| Tsukamurella | Gram-positive short to long bacilli, can be diphtheroid no branching on routine media | Weak positive (20-100% per oil lens field) | Dry, sometimes spready with age, white to orange |
| Actinomadura | Gram-positive short bacilli, intertwining branching with short chains of spores | Negative | Wrinkled with leathery surface usually no aerial mycelium may be mucoid and “molar-tooth" with red, pink, yellow, white, orange, or tan color |
| Rhodococcus | Gram-positive, some are diphtheroid (coccobacillary), some are more hyphal in appearance, and some are branched | Weak positive (10-20% per oil lens field) | Non-hemolytic round often mucoid, salmon-pink/red color in 4-7 days |
| Gordonia | Gram-positive, thin, beaded coccobacillus or diphtheroid | Weak Positive | Slimy, smooth, glossy to irregular and rough. Colonies may be white, yellow, tan, orange, red, or pink |
| Dermatophilus | Branched filaments fine and tapered filaments | Negative | Round, sticky white colonies that turn orange and ß-hemolytic |
| Nocardiopsis | Branching with internal spores | Negative | Coarsely wrinkled or folded orange, blue, brown, yellow, gray, or white in color |
| Micromonospora | Gram-positive filamentous, branching with singular spores | Negative | Initially light orange, becoming orange, red, brown, or purple with the production of spores as culture ages |
