The toothbrush should be silent when I'm asleep, yet at 11:11pm it emits a short, cheeky beep. It's lying on the charger, the bristles light up with a pulse and the app displays a strange message: Plate movement detected. Not in my mouth, because I flossed thoroughly, rinsed thoroughly and brushed for an equal two minutes. My name is Lena, I have braces and have been obsessed with biofilm ever since the orthodontist showed me the staining tablets. Usually I guide the toothbrush, but today it guided me, flashing like a miniature compass. A sketch of our school appears on the map in the app, an arrow pulsing by the closed prophylaxis office. Rumour has it that an acid sensor experiment was once conducted there, which never made it into production. I put the phone down, but the pulse doesn't stop, as if an invisible tooth GPS is tracking something just beyond the wall. My mum is shouting from the kitchen for me to sleep because we're preparing the stall for the School Health Day in the morning. I roll my eyes, put my toothbrush in the case and promise myself I'll check it with Oscar tomorrow. Oskar likes drones and things that squeak; he once called bacteria my flock of electronic sheep. In the office we are greeted by Mrs Mira, a hygienist who can defuse panic with one joke about flossing. She brings a plate scanner, staining pads and a new poster: Two minutes, twice a day, wash your tongue too. We test the equipment on each other, laugh with violet smiles, and the app continues to blink from my pocket. As Oskar turns to get his cup of liquid, I get another ping and a message: Activity at room 13. In the evening I return to the school as a decoration volunteer, get a key and an empty corridor to myself. The office smells of mint and plastic, and I search the metal cupboard for lights, ribbons and our poster. I come across a locked locker; the toothbrush in the case starts vibrating, as if it can sense bacteria at a distance. I slip an old card between the padlock and the lid, hear a click, and inside lies something that shouldn't be here. A small box labelled Colony S, next to it a magnifying glass, a bundle of prints and a mirror with oddly scratched edges. I open the box; inside is a vial with a drop of dye, more intense than all the others, as if it had swallowed a whole berry. I sprinkle the mirror, the toothy streaks spilling out in a pattern that resembles a school plan covered in gummy canals. The toothbrush lights up blue and displays an option: Antibiofilm mode - run now or get directions? Behind the door, the handle taps; someone's footsteps approach through the corridor, and a drop of dye begins to slowly creep towards me.