Serratia marcescens can be identified by the presence of red pigment that resembles blood on objects that have not been exposed to living creatures or which are incapable of bleeding. In the lab, it can also be identified by test results that establish it as gram negative, by its production of lipase and gelatinase, and by its ability to perform casein hydrolysis.
Serratia marcescens is a gram negative bacteria, meaning that it has an outer membrane and lacks teichoic acids. It is primarily distinguished from other gram negative bacteria by its ability to create metalloproteinase, a class of metallic enzymes, using a metabolic process called casein hydrolysis. It also performs several different fermentation processes that, in combination, heavily indicate its precense. It uses one such process to produce lactic acid.
Outside of the lab, S. marcescens is readily identified by the bacterial colony's bloody red appearance. It often grows on starchy or sugary foods, such as grain. It was discovered and named in 1819 after one such colony was found growing on cornmeal mush. It has also been implicated in many cases of religious icons that appear to bleed. The pigment that gives the bacterial colony its bright red coloration is prodigiosin, so its presence in lab cultures should be taken as a strong sign of S. marcescens infection.