Fig. 1.

Photorhabdus cells that initiate mutualism in the nematode intestine are small-cell variant M-form cells. (A) Photorhabdus cells that initiate mutualism (green) on the ninth intestinal ring cells, left and right, INT9L and INTR, posterior intestinal cells and transients (red) are present throughout the lumen of the nematode. (B)M-form cells develop small colonies, and transient P-formcells develop large colonies. (C) Small colonies consist of primarily small cells, and large colonies consist of primarily larger cells.