Serratia

Serratia marcescens is a Gram negative bacterium, a human pathogen of the family Enterobacteriaceae. It is involved in nosocomial infections, particularly urinary tract infections and wound infections.

Most strains are resistant to several antibiotics because of the presence of R-factors on plasmids.

In the 1950s it was erroneously believed to be non-pathogenic and its reddish coloration was used in school experiments to track infections.

Because of its red pigmentation, and its ability to grow on bread, it has been evoked as a naturalistic explanation of Medieval accounts of the "miraculous" appearance of blood on the eucharist.