Study of bacterial suspensions of Escherichia coli M 17 and Brevibacterium flavum E 531 has revealed the existence of temperature intervals in which the extra-cellular medium is in a special state characterized by the fact that the molecules of the extracellular proteins produce a generalized conformational change increasing the hydrophobicity of the surface molecules with the water of the extracellular medium passing to a stabilized state. It was established that in the same temperature intervals the total content of the ultraviolet absorbing metabolites in the extracellular medium falls as a result of activation of the transport systems of micro-organisms. It is shown that the temperature intervals in which these features of state are observed differ for the bacteria studied and correlate with the temperature optima of their development.