Caulobacter crescentus bacteria, coloured transmission electron micrograph (TEM). This Gram-negative oligotrophic bacterium is found in fresh water, soil, and sea water. It has two forms: a mobile ('swarmer') cell with a hair-like flagellum for swimming, and an immobile ('stalked') cell that attaches to surfaces. Only stalked cells are able to reproduce, which they do by replication. Swarmer cells are able to search for nutrients in nutrient-poor environments. They can also differentiate into stalked cells as they mature. Caulobacter is a model organism for studying the regulation of the cell cycle, asymmetric cell division, and cellular differentiation.