Serratia marcescens is a common microbe which lives in soil, water, on plants, and in animals. Because of its bright red color and because it may grow on bread and communion wafers stored in a damp place, it has contributed to some interesting "miracles". In the images above the bacteria are shown captured on a bacterial filter as photographed under a scanning electron microscope (left) whereas the micrograph at right shows them in false colours as they grew on a soda cracker.

These bacteria were considered for many years to be non-pathogenic. Because of their red pigment, they were used to trace bacterial transmission. In a study called "Operation Sea-Spray", the US Army filled balloons with S. marcescens and burst them over San Francisco in 1951 and 1952. A severe incidence of pneumonia and urinary tract infections followed shortly afterwards - and the bacterium has not been considered harmless anymore. Several S. marcescens sites bring additional information on this red bacterium.

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