This work deals with materials on the utilization of carbon dioxide and hydrogen from gas-air mixture by Yersinia pseudotuberculosis and Listeria monocytogenes in the process of their cultivation in Hirsch's mineral medium. As follows from the data presented in these materials, the above-mentioned bacterial species are heterotrophic when the infectious process is reproduced in the body of warm-blooded animals and chemolithoautotrophic under the conditions of their life in environmental objects. Such a wide range of their metabolic plasticity ensures the dual character of their saprophytic and parasitic nature.