Inhibition of Bacterial Growth by Potent Synergism between Organic Acids and Transition Metals

Zinc is a metal co-factor used by cells for reactions that require a redox-stable ion and as a structural factor, stabilizing proteins folding and oligomerization. It is estimated that ~10% of the human proteome consists of potential Zn-binding proteins while in E. coli, as much as 3% of the total proteins expressed in the logarithmic growth phase are Zn-binding proteins. Despite its central roles, Zn²⁺ is needed at low intracellular concentrations. At high intracellular concentrations zinc is highly toxic. Thus, its levels in the cell are tightly regulated, mainly by transmembrane efflux pumps like P_IB-ATPases. These ATPases maintain a delicate balance between import and toxic overload of both essential and non-essential metals.

Organic acids are known to inhibit bacterial growth. They permeate into cells, undergo de-protonation, disrupt the proton motive force (∆pH) and membrane potential (∆ψ) and also directly inhibit metabolic pathways. Accordingly, organic acids are often used as food preservatives. Moreover, there is a substantial commercial interest on identifying conditions enabling more efficient bioreactor production of organic acids, as possible alternatives to common petrochemicals.

Here we report that Zn²⁺/Cd²⁺-sensitive E. coli are also more sensitive to organic acids. The data suggests that organic acids increase the influx of Zn²⁺ ions into bacterial cells, thus leading to increased metal sensitivity. P_IB-ATPases reduces the intracellular metal concentrations to tolerable levels. A synergistic toxic effect between Acetate and Zinc was observed in several bacteria species.

These results provide a stepping-stone for a more efficient use of organic acids as antibacterial agents and for their cost-effective bioreactor production.