False-colour transmission electron micrograph (TEM) of the rod-shaped, gram-negative bacteria Serratia marcescens, showing the characteristic convex curve of the end walls displayed by the 2 bacteria at centre. Species of Serratia occur as commensal fauna in the intestinal mucous membranes of man & mammals. S. marcescens is occasionally pathogenic in humans, presenting in long term, chronically sick patients in whom they may cause septicaemia. Previous medical interest in Serratiasp. existed only because they were likely to be identified in areas where other potential pathogens were found, and needed to be isolated. Magnification: x12'000 at 35mm, x24'000 at 6x7cm.