Binary fission and counting microbes.

(a) Division of a bacterial cell as viewed by scanning electron microscopy. (b) When samples are spread onto the surfaces of Petri dishes that contain nutrients, single cells of bacteria or archaea may divide repeatedly to form visible colonies, which can be counted. The number of colonies is an estimate of the number of culturable cells in the original sample. (c) If a fluorescence microscope is available, cells can be counted directly by applying a fluorescent stain that binds to cell DNA. Each cell glows brightly when illuminated with ultraviolet light.