Homo Bacterium

Across the vastness of space, there are plenty of strange and wondrous creatures. Most vexing of all are the denizens of a "planet" (if one can even call it that) called Earth. Positively puny, this rock and its surrounding galaxy are minuscule compared to the rest of the universe. So minuscule, in fact, that its natives are more akin to bacteria than people. Humans (scientific name Homo Bacterium) are the universe's smallest sentient beings. Thousands of years ago, the people of Ahlet discovered humanity, mistaking them for strangely organized nano organisms at first. While not completely untrue, they did take samples of the species back to their home planet for study.
The Onoi who inhabit Ahlet soon discovered many uses for humans, as well as their sentience. While far more primitive than the space-faring civilization, their intelligence meant they could carry out tasks that were simply too small and precise for the rest of the universe. Cleaning at a microscopic level. Providing pleasure. Boosting gut health. All of these tasks and more, humans seemed designed for. In fact, one scientist suggested that life on Earth was originally seeded by the sperm of an ancient alien, its organelles evolving in a unique environment into proper life. From then on, humans became a part of every day life across the universe, spreading into every home, body, and toy. Although human sentience was eventually recognized, their usefulness was far too important to be overlooked. Their place in the universe was set, and no one wanted to change it.
Earth was proposed off limits, as any interference in the bacteria's affairs could be disastrous. As for the descendants of humans who were already abducted, however, they were bred and distributed across the universe. Things continued this way for generations until two college students by the names of Xanqrill and Roce descended upon the Earth and wreaked havoc in their drunken state. From then on, the secret was out, and the universal community had no choice but to reveal the truth to humanity in an attempt to undo the damage these students wrought. From then on, the universe had to distinguish between free humans and bred ones.
Free humans are exactly as one might assume: free to live their lives as they wish. Many have integrated into the intergalactic community over the last few human generations (though most aliens have lived through the entirety of this period). Bred humans are those who have been farmed like livestock, unable to exist on their own in the universe. They are used in products like before. Some have campaigned for human rights to extend to them, but the universe has grown so used to them that few want to discard this way of life. After all, more humans have been used as shoe cleaning powder than have ever existed on Earth.

Species

Humans

Humans are microscopic compared to the rest of the universe, their planet no more than the size of a handheld ball. As such, they are more akin to bacteria than proper life, with cells that can hardly be observed with the strongest microscopes. They are of endless fascination to microbiologists. Humans can survive within an alien's body with ease, able to subsist off of the body's natural processes. For example, a sperm cell, roughly 3 times larger than a human, can feed a family for a week, offering everything they need to survive.
Interestingly, humans live a fraction of the lifespan of even the most short-lived species in the universe. Idling out at a mere 100 years, they pale in comparison to the millennia-long lifespans of the average denizen of their universe. This lent to the theory that they are short lived microorganisms, reproducing at an incredible rate. To foster their reproduction, time dilation chambers are used to quickly make their populations rise before harvest.

Uses

Bred humans are used in everything these days, from cleaning products to pleasure. Many have improved their gut health by introducing human colonies to such an environment. Humans introduced to one's reproductive system increase fertility. And of course, they can be used to clean anything from noses to shoes. Many clothing companies weave human cities into their fabric by the trillions to provide self-cleaning garments.
Humans are typically disposable, since there are so many trillions of them. Just as many have carved out lives in the bodies, homes, and clothing of aliens, however. At their size, they are relatively hard to notice after all.

Onoi

The Onoi are pig-like aliens who inhabit the planet Ahlet. The first to discover humanity, they are lauded as the chief experts on the microorganisms.

SizeTech

SizeTech is a broad term for devices that are capable of altering mass to extreme degrees. Shrinking and growth are possible, with few restrictions. Your average person can pick up a shrink ray for less than a thousand dollars, and there are industrial devices capable of shrinking entire planets to even smaller sizes than Earth. Chiefly, sizetech is used to enlarge humans to better interact with the universe at large. While they are microscopic by nature, a device approved for human use can be utilized to make a human full size for a limited time.

Size Sickness

A condition that affects humans exclusively, do their naturally small size. If grown too large for too long, they will experience nausea, vertigo, and in some cases, even death. As such, most devices approved for human use have built in timers to ensure that the human they're used on will not experience violent size sickness.
The time that a human can spend at a certain size is based on how large they are.