Yersinia pestis. Coloured Scanning Electron Micro- graph (SEM) of clusters of the bacteria Yersinia pestis, cause of bubonic plague (the Black Death of the Middle Ages). The bacteria are rod-shaped, Gram-negative, non-motile bacilli. Yersinia pestis is primarily carried by the fleas of rats. Transfer to humans occurs when such fleas feed on human blood. Infection is rapid, causing swollen lymph nodes, and leading to septicaemia and pulmonary infection. Extensive control measures against rats and their fleas have eliminated plague from Europe, but it still occurs in other regions of the world. Magnification: x6500 at 6x6cm size.