Bacteria need iron to cause infections. The body has defense mechanisms to make it harder for germs to suck iron out of someone’s blood or other tissues. But deadly germs can get around that so-called iron blockade, and understanding how might lead to better treatments.

University of Chicago microbiologists report Thursday in the journal Science that the staph germ — a leading cause of pneumonia and other infections — fuels itself with iron in a previously unknown way.

Early in staph infections, the germs blow open red blood cells. The Chicago researchers found staph then snatches their oxygen- and iron-carrying component, called heme, and discovered the genes that govern the process.

When they weakened those genes, staph no longer sickened worms or mice, said lead researcher Eric P. Skaar. Next step is hunting drugs to block staph’s iron-stealing ability.

Where does that ancient remedy of bloodletting come in?

The discovery suggests that bloodletting, done early enough, may have slowed staph infections by starving germs of iron, National Institutes of Health iron specialist Tracy Rouault wrote in a review of Skaar’s research.

Nobody’s suggesting bleeding staph patients today. Now derided as a nonsensical if not barbaric custom, bloodletting was abandoned in the mid-20th century after antibiotics were invented.

But the mystery persists: “How could a procedure popular for 2,500 years have really been completely worthless?” Rouault asked.