I think the reason some people might believe this claim is because we’re taught in school that the moon’s gravity causes the tides. I think the reasoning goes, “well if the moon’s gravity can affect the tides, surely it can affect smaller things too”
If the bacteria has a mass of 1 millionth the mass of a human, the gravitational effect the moon has on it, would similarly be 1 millionth the effect it has on a human.
Stupid physics. 🙄