Why is it a bicycle that summons gastrea in the dark bullet?

This is because the gasterozoa have a deep obsession with bicycles. In their previous memories, the bicycle was their only good friend.

The gastrea in the dark bullet, a creature born after being infected with a terrifying virus that is highly infectious through the blood and can rewrite the DNA of living things, suddenly broke out all over the world in 2021. In a short period of time, the human world was destroyed to the point of near destruction. Infected organisms will mutate in a short period of time and mix the infected person's DNA with the DNA of other animals, turning into terrifying and brutal monsters.

Extended information:

"Pitch Black Bullet" is a light novel originally written by Japanese light novelist Kanzaki Shiden and illustrated by Ukai Saki. In 2021, humans lost the war with the viral parasites "Gastrula" and were expelled to a narrow territory, living an elusive existence with fear and despair.

In such a closed and dark world, Satomi Rentaro, a boy living in the Tokyo area, is a member of the "Doman Police", an expert on gastroenterology, engaged in crisis work. His partner is the somewhat precocious little girl Aihara Enju.