Sentence:  Many cells , <WSD>bacteria</WSD> , and other microorganisms are transparent to visible light and must be stained for microscopic investigation . 
Meaning of the word marked with <WSD> tags:  (microbiology) single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission; important as pathogens and for biochemical properties; taxonomy is difficult; often considered to be plants