The killing curse's job is to kill, plain and simple.
Plant life, though mostly not sentient so far as we know (Dryads anyone?) even by magical standards, would be killed, if not by the curse, then at the very least trees would be set on fire and burn. End of story.
Now Viruses and bacteria are something else entirely. AK was designed as an "opponent killing machine" or rather a "duel winning machine", quick and clean, no spilled guts and foul bodily fluids, no maimed or charred flesh, no crushed bones, or howls of agony. I think there's a chance it might work IF AK hit a vial full of microbes. Intent may be required to kill a specific person, but if the intended target dodges the curse and, say a Hippogriff gets hit instead he'll drop like a stone. AK is for all intents and purposes a magical death ray Hate and malice may be be the required "spark" to light the fuse of that peculiar rocket, but once it's been cast there is no way to cancel or delay the lethal effect because it did not hit the right target. This is not a smart missile whose warhead can be neutralized.
If you want to kill viruses and bacteria it may just be a matter of using the right spell, remember AK was created at a time when admittedly no one knew about microbes and viruses; no microscopes. Though wizards could have discovered it, as a rule they don't seem to apply the scientific method even for magical research and theory. I think wizards would rather use potions, herbs, healing ritual or magically enhanced natural healing. It is interesting though, that the same way there are magical bugs and parasites (e.g., Doxies), there are magical viruses and bacteria causing potentially deadly diseases such a Dragon pox (Abraxas Malfoy / if this disease affects a Dragon it must be very potent) which wizards seem to be vaguely worried about as opposed to common cold (do I smell potential for magical biological warfare?) and other Muggle diseases easily taken care of with a pepper up potion or the like.
My point? There might be a "sanitizing" charm say "Sanito Corpus" or "Expello/Expecto viruses" "Anihilo Microbiens"; pure speculation though.