Oxidation-Reduction Potentials in Relation to the Growth of an Aerobic Form of Bacteria

Abstract

A vacuum-tube potentiometer circuit is described, for avoiding polarization in making electro-metric measurements of oxidation-reduction potentials of biological systems. The studies indicate that the oxidation-reduction potentials of the medium exert a decisive influence on the ability of certain aerobic bacteria to initiate growth. 2 basic media were studied: a yeast-water mannitol, and a mannitol-nitrate medium. The former was the more reducing, and supported growth with high dilutions of the organisms; the latter permitted growth in similar dilutions only after the potential was properly reduced. It was possible to poise the yeast-water mannitol medium with H2O2 at potentials sufficiently high to inhibit growth completely, and yet recover the organisms after 80 hrs.'' exposure to these conditions, by reducing the potentials of the systems. Commercial agar is not electromotively inert; it not only appears to afford physical advantages to the organisms in initiating growth, due to its colloidal nature, but also to act as a mild reducing agent under the conditions of these exps. Large inocula possess greater ability to overcome unfavorable oxidation-reduction conditions than small. This seems to hold true in media either too oxidized or too reduced for optimum growth. Oxidation-reduction potentials which limit bacterial growth seem to be somewhat dependent on the other factors characteristic of the medium. More suitable ion-balance or more available N source, e.g., appear to facilitate wider adjustments of unfavorable potentials by the bacteria themselves. Potential-time courses developed by rhizobia were followed by electrode measurements. The factors responsible for the negative drift of potentials in the bacterial cultures are discussed. The behavior of the rhizobia toward atmospheric 0 is determined largely by the oxidation-reduction character of the medium, as indicated by growth levels established by the organisms in media possessing different oxidation-reduction properties. Pellicle formation in liquid media seems to be definitely correlated with oxidation-reduction potentials of the system in the case of rhizobia. Bacteria collect at the surface in a pellicle as the potential of the medium falls, due to their metabolic activities.