Shigella flexneri. Coloured transmission electron micrograph (TEM) of a Shigella flexneri bacterium undergoing cell division. S. flexneri is the commonest cause of bacillary dysentery (shigell- osis) in the world's developing countries. S. flexneri is a gram-negative, non-motile, bacilli (rod-shaped) bacterium. It destructively invades the intestinal mucosa, especially in young child- ren. Transmission is by oral ingestion of the faeces of a sufferer, either through contamination of the hands or, less commonly, in food and water. As few as 200 bacilli can establish disease, which is characterised by diarrhoea and abdominal pain. Magnification: x30,000 at 6x4.5cm size.