“It has been shown … the antibacterial action of penicillin in vitro and in vivo may be markedly … by the presence of appropriately low concentrations of cobalt that do not themselves inhibit proliferation of the test organisms (Pratt and …, 1947b; Pratt, Dufrenoy, and Strait, 1948; Strait, Dufrenoy, and Pratt, 1948).”