Buddhist here. The goal of Buddhism is to escape the cycle of rebirth. Monks and ascetics are generally trying to go out (to nibbana, “extinguishment”) rather than up (to rebirth in one of the heavenly realms), which tends to be a goal of laypeople. Though it depends to some extent on which branch of Buddhism. Even after reaching a heavenly realm and having a godlike existence, eventually (after a very long time) they will die and fall back into a lower realm with more suffering, which is why escaping the cycle is the ultimate goal.
In Buddhism we don’t consider humans a higher form of life than animals or those in the hell realm. All living beings are seen equally, as we all suffer and we all want to be free from suffering. Regarding E. coli, I think most Buddhists would say you can only be reborn into a body with a sufficient nervous system for the brain to support the mind processes required for experience, so bacteria are probably too simple.
But saying “you” are reborn isn’t quite right as there is no self which goes through this process, just the causal process itself links the past and present life.
Causality in Buddhism is based on specific conditionality, which is usually summarised as:
So when there is heat, a fuel and oxygen, fire arises. Take one of those away, and the fire passes away. Everything is like this in Buddhism. There can be dependency loops, so some processes are self-sustaining once they have begun until the loop is broken.
We sometimes talk about “mindstreams” which you could think of as bundles of conditions which fulfil the requirements for being alive.
If there is no self, then nothing distinguishes your mindstream from mine, other than the specific conditions which make it up, which explain why you experience different things than I do.
Say if I introduce a male friend to a female friend, then I die shortly after. Later on, they get together and have a baby. When I die, all that has happened is one or some of the conditions required for my life have ceased. But whatever other conditions were present still remain, including that I introduced those friends. Other conditions combine with that one, leading to them eventually having the baby. Is that baby me? No, there was never a “me” at any point in the process. But there is a causal connection.