Thiomargarita namibiensis

Thiomargarita namibiensis is a gram-negative, facultative anaerobic, coccoid bacterium found in South Africa's ocean sediments of the continental shelf of Namibia. The genus name Thiomargarita means "sulfur pearl." This refers to the cells' appearance as they contain microscopic elemental sulfur granules just below the cell wall that refract light creating a pearly iridescent luster. The cells are each covered in a mucus sheath aligned in a chain, resembling loose strings of pearls. The species name namibiensis means "of Namibia".

It is the second largest bacterium ever discovered, at 0.1–0.3 mm (100–300 μm) in diameter on average, but can attain up to 0.75 mm (750 μm), making it large enough to be visible to the naked eye. Thiomargarita namibiensis is nonpathogenic.

Thiomargarita namibiensis is categorized as a mesophile because it prefers moderate temperatures, which typically range between 20-45 degrees Celsius. The organism shows neutrophilic characteristics by favoring environments with neutral pH levels like 6.5-7.5.