Serratia marcescens used to be the workhorse of the biology laboratory. It was thought to be benign and saprophytic - scavenging only on dead or decaying tissue. Researchers used to put their hands in huge colonies of it and shake hands, then see how much bacteria had been transferred from one hand to another. In the latter half of the 20th century, people noticed that it was commonly involved in serious hospital infections. In 2004, it contaminated a company's entire stock of flu vaccines. The contamination was discovered early and the vaccines were never shipped, but there was a shortage of flu vaccines.
It seems the s. marcescens has even more to apologize for. The bacterium is especially at home in musty, starchy environments, like stockpiles of bread. Ever since the 13th century, there have been stories of communion wafers being broken, and priests finding red tinting inside. They believed that it was the blood of Christ on the wafer, symbolizing that it had already become the body of Christ. In 1819, a pharmacist called Bartolomeo Bizio, working with regular bread, proved that the red tinting that people sometimes found on their bread was actually a red-tinted bacteria setting up a thriving colony in the loaf. Full of national pride, he chose to name the bacteria after Seralino Serrati, a physicist and the inventor of the steam boat. Marcescens comes from a Latin word that means "to decay," and indicates that the bacteria lived in decaying matter.
So if your bread looks like it's bleeding, just go ahead and throw it out. It's not holy.
Image: Public Health Image Library