Acting like miniature trees that soak up sunlight and release oxygen, photosynthetic bacteria injected into the heart may lighten the damage from heart attacks, a new study in rats suggests.
When researchers injected the bacteria into rats’ hearts, the microbes restored oxygen to heart tissue after blood supply was cut off as in a heart attack, researchers at Stanford University report June 14 in Science Advances.
“It’s really out of the box,” says Himadri Pakrasi, a systems biologist at Washington University in St. Louis who was not involved in the research. “It reads like science fiction to me, but it’s fantastic if it works.”
The organism, called Synechococcus elongatus, has been used recently to produce biofuels, but this may be the first time the cyanobacteria have ever been used in a medical setting, he says.
Other researchers also reacted enthusiastically to the study. “It’s outrageous, but outrageous in a good way,” says Susan Golden, who studies cyanobacteria at the University of California, San Diego. Cardiovascular scientist Matthias Nahrendorf of Massachusetts General Hospital in Boston says, “I enjoy the idea. It’s really fresh.”
Bringing oxygen to starved tissues is what Stanford cardiovascular surgeon Joseph Woo had in mind when he and colleagues dreamed up the plan to put light-harvesting bacteria into the heart. In a heart attack, clogged arteries or blood clots cut off blood flow to the organ. Without oxygen supplied by the blood, heart cells die.
Woo wanted a way for the heart to make its own oxygen or access another supply until doctors could open blocked vessels and restore blood flow. Plants make oxygen from carbon dioxide and sunlight, so Woo wondered, “Why not bring the tree to your heart?”
He and colleagues started by grinding up kale and spinach to harvest chloroplasts, the organelles within plant cells that carry out photosynthesis. But the chloroplasts didn’t survive outside the cells. That’s when the researchers learned about S. elongatus, a photosynthetic organism that Golden and other researchers have long used to study circadian rhythms.
After finding that cyanobacteria could provide oxygen to heart cells in a lab dish, the next step was to see how the cyanobacteria would fare in an animal. The researchers stopped blood flow to part of rats’ hearts and after 15 minutes injected either cyanobacteria or a saline solution. The bacteria increased oxygen in heart tissue to about three times the levels measured right after the heart attack, while saline-treated rats had almost no increase in oxygen.
And that was in the dark: When researchers exposed the heart to light, rats that got the bacteria had 25 times higher oxygen levels than they did after the heart attack. Four weeks after the treatment, these rats had less heart damage than untreated rodents, indicating long-term benefits. In fact, the hearts of photosynthesis-treated rats were beating strongly: Blood flow out of the heart was 30 percent higher in rats treated with cyanobacteria and light than those treated with the bacteria in the dark. That extra blood flow could make the difference between life and death for some patients, Woo says. The results indicate that the bacteria need light to supply heart cells with enough oxygen to stave off damage. That presents a difficulty if the cyanobacteria are ever to be used in people: Getting light into the heart is a major hurdle.
“It will be next to impossible to open the chest to light,” says Nahrendorf. “A day on the beach won’t do the trick.” Woo says the researchers are working with engineers at Stanford to make devices that can shine light through bones and skin to reach the heart and other deep tissues.
Injecting bacteria into the heart is also a risky proposition. “What you’re doing is infecting a tissue, and that’s rarely a good thing,” says Nahrendorf. But the cyanobacteria were cleared from the rats’ bodies within 24 hours and didn’t provoke the immune system to attack the heart, the researchers found. Some other cyanobacteria produce toxins, says Golden. “But this organism is benign,” she says.
Cyanobacteria might also supply oxygen to tissues in other diseases, such as brain injuries, strokes or nonhealing wounds in people with diabetes, says Arnar Geirsson, a cardiovascular scientist at Yale University. Photosynthetic bacteria might also help preserve organs for transplant.
“I’m quite impressed,” Geirsson says. “It’s a really unique way to deliver oxygen.”