Thiomargarita namibiensis is the largest microbe currently known. It can reach up to 0.75 mm which is visible to the naked eye! It was discovered in the ocean sediments of Walvis Bay, Namibia in 1997. Being this large is normally a disadvantage for a bacterium because its nutrient requirements are larger than it is capable of acquiring. Thiomargarita namibiensis gets around this by maintaining unusually large vacuoles (bubbles filled with nutrients). If you look at high-detail images of the bacterium (e.g. here), you see strings of sulfur granules, which resemble pearls—which is where it gets the name Thiomargarita: from L. thivm "sulfur" (from Gr. θεῖον theion "sulfur") + L. margarita "pearl" (from Gr. μαργαρίτης margarítēs "pearl").
 See http://en.wikipedia.org/wiki/Microbe.
 Schulz, H. N. (2002) "Thiomargarita namibiensis: Giant Microbe Holding Its Breath." ASM News, 68 (3): 122–127.
 Schulz, H. N, Brinkhoff, T., Ferdelman, T. G., Mariné, M. H., Teske, A., and Jorgensen, B. B. (1999), "Dense populations of a giant sulfur bacterium in Namibian shelf sediments" (just abstract), Science, 284 (5413): 493–495.
 See http://en.wikipedia.org/wiki/Thiomargarita namibiensis.
 See http://www.bacterio.cict.fr/t/thiomargarita.html.
Stained micrograph of Thiomargarita namibiensis is by (as best I can ascertain) Bronwen Currie (for NASA and thus in the public domain), available at http://disc.sci.gsfc.nasa.gov/oceancolor/additional/science-focus/ocean-color/science focus.shtml/sulfur plume.shtml.