Responses of Microorganisms to Sterols and Steroids

Abstract

The wide distribution and synthesis of sterols among microorganisms signify the importance of these compounds in metabolism and synthesis of protoplasm. Sterols accumulate or their synthesis is stimulated in certain microorganisms under conditions of stress. Such synthesis of sterol has adaptive significance and at least in the case of yeast endogenous ergosterol is intimately linked with the maintenance of aerobic respiratory function. The maintenance of respiratory function would insure an energy supply for survival under unfavorable conditions. Growth responses of microorganisms to both sterols and steroids range from "vitamin-like" effects to antimicrobial effects. Many microorganisms require a sterol for normal growth and either incorporate it as such or metabolize it to a metabolically "useful" form when supplied with the proper sterol. In nonrequiring microorganisms, catalytic quantities (10-6 [image] or less) of supplied steroids or sterols often stimulate growth or respiration (or both); whereas higher concentrations (10-4 [image] or more) generally inhibit and may result in mutation or death. The "effect" may vary with the concentration. There are certain steroids which appear to be antimicrobial over a wide concentration range (high concentrations are the most effective). Among the highly active antimicrobial steroids are diethyl-stilbestrol, deoxycorticosterone, 21-21-dimethoxy progesterone and certain aza steroids. Antimicrobial properties of these steroids may be primarily related to purely physicochemical effects associated with a particular molecular configuration or to the fact that certain microorganisms cannot metabolize these molecules to nontoxic forms. Aerobacter aerogenes apparently liberates a substance that modifies the solubility of exogenously supplied cholesterol since a significant portion of the sterol is found unchanged in the saponifiable fraction of the growth medium after growth. Antimicrobial steroids in some cases are probably those whose solubility characteristics cannot be altered by cells. The role of sterols and steroids in the regulation of microbial cell metabolism is not yet well understood, although endogenous sterol is maintenance of respiratory function. An early effect of these compounds may be involved with permeability functions at the cell surface. Both sterol-requiring and nonsterol -requiring Mycoplasma adsorb sterols primarily into the membrane fraction where the sterols have a "structural role". Sterols or steroids may "fix" the membrane into a more permeable state or, work in conjunction with specific "per meases." Such per meases could be involved with inorganic cations such as Na+ or K+ in some cells and more complicated substances such as C substrates or amino-acids in other cells.