Hierarchical organization of life:
Cells ---> tissues ---> organs ---> organ systems ---> organism.
Technically bacteria and viruses are not organisms, as they are only single celled. That they are sometimes written as: single celled organisms, doesn't help matters. They should really be called: single celled creatures, instead.
"Creature" suggests animal to me. How about "organellisms"?
Cheers, John
-- Palaeontography: http://palaeo.jconway.co.uk