Bacteria experience a completely different world to us. We see things on a comparatively huge scale, sampling the air with our noses, feeling the textures of surfaces that it would take billions of bacteria to cover with a simple brush of the hand and taking in light of enough frequencies to form a detailed picture of the world. But what do bacteria see? How do they experience the world they inhabit?
(For this post I'm using 'sense' as 'responds to external stimuli' rather than involving brain things)
Every living thing senses it's environment. The ones that couldn't will have be snuck up on and eaten by those that could, so didn't spread their genes leading to a very short branch on the evolutionary tree! We have our many senses, (the whole five senses thing is outdated and wrong, just like the taste map thing. Close your eyes and wave your hands about. You still know where they are, without seeing or touching them! That one's called proprioception!) Trees sense light and gravity, animals can sense electric currents and use sonar. Bacteria and the other microbes all sense things too; not with sensory organs but with components of their single celled bodies.
Cyanobacteria have been shown to respond to light; with the spherical shape of their cells they focus the light like a really basic eye, and can 'see' where it's coming from. They've evolved this so they can move into the light, as they photosynthesise like plants (in fact it's likely that chloroplasts, the things plant cells use to photosynthesise, were once free living relatives of cyanobacteria) so they need light to grow. Some protists detect light sources in a different way; they have organelles that cast a shadow onto themselves, giving a direction for the light just like a sundial!
Other bacteria take iron from their surroundings, deliberately crystallise them and line them up in chains. Just like the iron needle in a compass, they align to the Earth's magnetic field! This lets them find their way around. You would think that microbes wouldn't need to know where north is, butfindy use it to help look for areas of lakes and rivers with the right amount of oxygen. They aren't finding North, they're finding Down!
The big one, the major bacterial sense, is detecting chemicals in the environment. When they find something they like the look of, they move towards it. Minerals, sugars and other nutrients all draw hungry bacteria to them. It's like walking past a bakery and not being able to resisting the smell! We have a bit more conscious control to keep walking, though!
This isn't an exhaustive list, by any means! Bacteria have a huge range of external and internal stimuli that they interact with constantly, just like we do.