What does it mean when PDB lists multiple organisms for the same structure? I'm browsing through ribosomes which have been modeled with Cryoem on PDB and am pretty confused by the fact that for some, PDB, or rather the authors of the deposition probably, put multiple organisms from different domains of life... How does one interpret that?

So far as i understand bos taurus is a cow and e.coli is a bacterium. So eukarya and bacteria.. Thank you.
Since this is a 40S ribosomal protein, and therefore eukaryotic-specific (as the description says) the listing of E. coli is not intuitive as you note. If you go down to "Macromolecules" and click on "Nucleic Acids/Hybrids" you can see the listed E. coli sequence is just 28bp, and all Us (poly(U) tail?). So, you can safely ignore the bacterial annotation I think.