Spots of opsin chemical (what we have in our retinas) on or just under the cell surface, combined with some transpot mechanism (not nerves) to get the info that a set of molecules has tripped due to being exposed to light to the motive mechanism of the bacterium.
Yup. Sure stretches your preconceptions about "single celled" doesn't it?
Not at all uncommon. There are far more bacteria, and KINDS of bacteria, than there are animals big enough for humans to see. Just, literally, a whole world of life there. What they need to have in their vraious niches is what they have evolved to have. Evolution didn't stop when chordates branced off.
(Added later:) See this excellent discussion of the variety of eyes and other visual mechalisms in the animal kingdom. http://scienceblogs.com/pharyngula/2...ent_divers.php