The Bacteria That Remembered

"This one's dead again," Maya said, tilting the jar so Soren could see. The starter inside had gone gray and flat, a thin sad puddle instead of the bubbling froth it was supposed to be. "Third time. Same flour, same water, same everything."

Soren leaned over the second jar, the one that was working. It smelled like green apples and rain. "This one's fine. We fed them the same. From the same bag."

"So it's not the food."

"No."

They were sitting on upturned crates behind the community garden shed, where Mr. Adebayo let them keep their experiment as long as they didn't leave the lids off and attract raccoons. Mr. Adebayo grew prize tomatoes and thought their sourdough project was a waste of good flour, and he said so, cheerfully, at least once a week.

"Bacteria and yeast," Maya said. "That's all a starter is. Little living things eating flour."

"So something's killing the little living things in this jar and not that one."

"Something got in." Maya turned the dead jar in the light. "Something too small to see."

Soren pulled out his notebook and drew two jars, one with bubbles and one without. Then he stopped, pen hovering. "You know what eats bacteria?"

"Bigger bacteria?"

"Viruses. There are viruses that only attack bacteria. Bacteriophages. Mr. Adebayo told me. He said phages are in everything, in dirt, in water, everywhere. Millions in a spoonful."

Maya went quiet. She looked at the dead jar, then the living one, then back. "So a virus got into the flour bacteria and killed them."

"Maybe."

"Then why not both jars? Same flour. If the virus was in the flour, both jars get it."

Soren tapped his pen against the crate. "Maybe both jars got it. Maybe this one" - he pointed at the living jar - "fought it off."

"Bacteria don't fight. They're too simple. They just eat and split."

"That's what I thought too." He flipped back three pages in the notebook. "But Mr. Adebayo said something weird when I asked. He said the reason his tomatoes don't all die from the same blight every year is that living things keep a kind of memory of what attacked them before. He meant it about plants. But I keep thinking about it wrong."

"Wrong how?"

"Like it should be true for the bacteria too. But bacteria can't remember. They don't have brains. They don't have anything."

Maya stood up. She did that when she was chasing something. "Say the living jar came from the older starter. The one we've been feeding for a month."

Soren checked his notes. "It did. The dead one we started fresh last week."

"So the old one has been alive longer. Been through more." She was walking now, small circles in the dirt. "If a phage attacked the old starter three weeks ago and most of the bacteria died, but a few survived - the few that could fight it off -"

"Then the survivors are the ones we kept feeding."

"And the new jar never met the phage before. No survivors. No fighters. It just died." She stopped. "But that's still not memory. That's just the tough ones living and the weak ones dying."

Soren shook his head slowly. "Unless the toughness is the memory."

They looked at each other.

"Say that again," Maya said.

"When the phage attacked the old starter, the bacteria that survived didn't just get lucky. What if they grabbed a piece of the phage - a scrap of it - and kept it? Like keeping a photograph of a burglar. And then when the same phage comes back, they check every intruder against the photographs they've saved, and if it matches, they cut it apart before it can do anything."

Maya's mouth opened. "They'd have to store the scraps somewhere."

"In their own DNA. Where else? It's the only place a bacterium keeps anything."

"So a bacterium would have" - she pressed her hands together, thinking - "a whole library. In its own body. Of every virus that ever tried to kill it. Little pieces of the enemy filed away."

"And it passes the library to its babies when it splits. So the babies are born already knowing."

"Already knowing an enemy they never met." Maya sat back down hard on the crate. "That's memory. That's actual memory, in a thing with no brain."

Mr. Adebayo came around the shed with a watering can. "You two still fussing over that dead pudding?"

"Mr. Adebayo," Soren said, "can bacteria remember a virus? Like, keep a piece of it inside them to recognize it later?"

Mr. Adebayo laughed He set the watering can down. "You know, I read something about that. There's a real thing. They cut a bit of the virus and store it in a row, in their own genes. Spacers, I think they're called. Whole line of them. Scientists found a way to use it for something big, cutting DNA on purpose, but I don't remember the -"

"CRISPR," Maya said. She didn't know where the word came from. She'd heard it somewhere, a video, a science show. "That's CRISPR. My cousin talked about it. The gene-editing thing. The thing that can fix diseases."

"That's the one." Mr. Adebayo picked his can back up. "Funny. All that fancy medicine. And it started as bacteria keeping scars." He wandered off toward his tomatoes.

Maya and Soren sat very still.

"The gene-editing thing," Maya said quietly. "The thing scientists use to cut DNA wherever they want. To fix broken genes in kids who are sick."

"It was never invented," Soren said. "Nobody invented it. It was already here. In the dead jar. In the dirt. In every spoonful of everything, for a billion years, doing exactly this."

"We didn't build the scissors." Maya looked at the living jar, bubbling faintly, apple-sweet, full of survivors carrying a library of enemies they'd already beaten. "We found them. Somebody just noticed what the bacteria were already doing and thought, we could use that."

Soren looked at the dead jar in his hands. The bacteria in it had never met the phage, so they had no photographs, no library, nothing filed away. They'd met the intruder blind.

Maya picked up a fresh spoonful from the living jar and stirred it into the dead one, folding the survivors and their whole invisible library into the ruined puddle. She set the lid back on and looked at Soren.

"Now this jar remembers too," she said.