Inhibition ofAzotobacter salinestrïsgrowth by zinc under iron-limited conditions

Abstract

The growth yield of Azotobacter salinestris, a Na+-dependent, microaerophilic nitrogen-fixing bacterium, was inhibited more than 60% by 5 μM Zn2+. This organism was much more sensitive to Zn2+than the obligate aerobe Azotobacter vinelandii. Inhibition of A. salinestris was most evident in iron-limited cells and exogenously added Fe2+was more effective than Fe3+in preventing inhibition by Zn2+. While Zn2+decreased the Fe content of the cells, decreased the activity of the soluble cytoplasmic ferric reductase, and altered the intracellular Fe2+/Fe3+ratio, which in turn increased siderophore production, none of these effects appeared severe enough to account for growth inhibition. However, Zn2+also was observed to be a powerful inhibitor of Fe-limited whole cell respiration. As the cells became more Fe sufficient, this inhibition of respiration was decreased. Growth of A. salinestris also was inhibited by Cd2+ > Zn2+ > Cu2+ > Cr2+ > Ni2+ > Co2+, and inhibition by these ions also was reversed by exogenous Fe2+or Fe3+. Examination of isolated cell membranes showed that the sensitivity of A. salinestris NADH oxidase activity to Zn2+and other respiratory poisons changed as the cells became Fe sufficient, but a similar change did not occur in A. vinelandii. It is proposed that Fe-limited A. salinestris cells present a sensitive target for Zn2+inhibition, possibly a sulfhydryl group in a terminal oxidase, but this target is lost or is of decreased importance in Fe-sufficient cells.Key words: ferric reductase, iron uptake, respiratory poison, zinc, microaerophile.