E. Coli is classified as a prokaryote, having only one cell and no nuclei. This unicellular bacteria is microscopic. Since it has only one cell, it doesn't have organs or membranes. E. Coli and other prokaryotic bacteria can live in places that would kill an multicellular organism, such as in places with high radiation or places that are extremely hot, like natural hotsprings. Salmonella is another unicellular organism you may be familiar with.
Unlike unicellular organisms, multicellular organisms are classified as eukaryontes and have many cell types; they are large enough to see with the naked eye.