Glycogen, a randomly branched glucose polymer, provides energy storage in organisms. It forms small beta particles which in animals bind to form composite alpha particles, which give better glucose release. Simulations imply beta particle size is controlled only by activities and sizes of glycogen biosynthetic enzymes and sizes of polymer chains. Thus, storing more glucose requires forming more beta particles, which are expected to sometimes form alpha particles. No alpha particles have been reported in bacteria, but the extraction techniques might have caused degradation. Using milder glycogen extraction techniques on Escherichia coli, transmission electron microscopy and size-exclusion chromatography showed alpha particles, consistent with this hypothesis for alpha-particle formation. Molecular density and size distributions show similarities with animal glycogen, despite very different metabolic processes. These general polymer constraints are such that any organismwhich needs to store and then release glucose will have similar alpha and beta particle structures: a type of convergent evolution.