Miraculous Microbes: They Make Holy Statues "Bleed"—and Can Be Deadly, Too

Features | More Science

A sinister bacterium implicated in Catholic miracles and "blood"-tainted polenta also kills coral, insects, and are even are up to no good in your contact lens case.

By Jennifer Frazer ?| November 11, 2011

A pigmented strain and various mutant versions of Serratia marcescens. Image: Robert Shanks, University of Pittsburgh School of Medicine

The Killer Bacteria Hall of Fame no doubt houses the usual suspects: Yersinia pestis, perpetrator of the Plague; Treponema pallidum, the spiral-shaped culprit in syphilis; and Vibrio cholerae, the swimmer that causes cholera. But you have probably never heard of one of the inductees.

Serratia marcescens is a forgotten but ubiquitous bacterium that can produce a red pigment called prodigiosin and likes to hang out as a pink film in the shower grout and toilet bowls of less-than-scrupulously clean homes. The pigment is so persistent that giant amoebas called slime molds that dine on S. marcescens turn red just as flamingoes that eat shrimp turn pink. Yet the picture emerging of this unsung organism is increasingly sinister.

These bacterium first attracted scientific attention in early modern times when it was found oozing out of damp Italian statues, communion wafers and, of all things, polenta doing its best impersonation of "blood." And blood it was taken to be?usually miraculously?until a pharmacist named Bartolomeo Bizio started trying to get to the bottom of what peasants declared to be an outbreak of diabolically cursed polenta in 1819.

Bizio believed a microorganism was responsible. In the test chamber, he found the bacterium happily chowing down on polenta while cranking out red pigment. Believing it to be a fungus, he named it Serratia in honor of Italian physicist Serafino Serrati, and marcescens because of the pigment's tendency to fade or decay rapidly.

Fast forward to the middle of the 20th century. In the early 1950s the U.S. government decided it would be a good idea to use S. marcescens in a bioweapon dispersal experiment dubbed Operation Sea-Spray. They burst balloons filled with Serratia over San Francisco Bay. Chosen because the red pigment makes it easily traceable, the supposedly innocuous bacterium so generously sprinkled over the bay was subsequently linked to several respiratory infections and at least one death.

Since then the bacterium has been widely found to be an opportunistic human pathogen, capitalizing on its prowess in forming tight-knit surface communities called biofilms wherever it can. It infects urethras through catheters, lungs via respirators, and premature babies by way of hospital caregivers. S. marcescens turns out to be one of the top 10 causes of all hospital-acquired respiratory, neonatal and surgical infections, said Robert Shanks, associate professor of ophthalmology at the University of Pittsburgh School of Medicine, who studies S. marcescens.

It has also been found irritating or infecting the corneas of contact lens wearers who fail to clean their cases with enough diligence (or at all). "What I think is sort of strange about S. marcescens is so many people have them in their contact lens cases," said Regis Kowalski, an ocular microbiologist at the University of Pittsburgh Medical Center. Although it often lives there harmlessly, S. marcescens is the third-most common cause of ocular keratitis, a corneal infection usually caused by poorly cleaned cases.

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