Williamsia is similar to Rhodococcus and Gordonia, and is rarely associated with human disease. Nine species have been known to cause human infections.
Table 7. Characteristics of Williamsia species.
| Characteristics | Comments |
| Appearance on Gram stain | Short gram-positive rods or coccobacilli; not branching |
Appearance on modified acid-fast stain
| Not acid-fast |
Most often associated with clinical disease34
| W. muralus is associated with pulmonary and endophthalmitis; W. serinedens has been associated with perinatal sepsis |
| Colonial growth | Smooth, yellow to orange (or red) colonies |
| Phenotypic characterization | Not helpful; use molecular testing such as 16S rRNA gene sequencing |
| Temperature of optimal growth | 10–37°C |
34. Carroll, K.C. & Pfaller, M.A. (Eds.). (2023). Manual of Clinical Microbiology (13th ed.). ASM Press.
