Selection of Oxalate-Degrading Rumen Bacteria in Continuous Cultures1

Abstract

Continuous cultures of mixed rumen bacteria, established with inocula of ruminal contents from sheep fed alfalfa hay, were fed gradually increasing quantities of oxalate, either as that present in halogeton or as sodium oxalate. Mixed bacterial populations established during a 3- to 4-day adaptation period had the capacity to degrade oxalate at rates (2 to 9 µmoles/ml/hr) that were 10- to 100-fold greater than rates in the inoculum. Oxalate degradation was positively correlated with methane production, suggesting that hydrogen from oxalate was used for methanogenesis. Adaptation to oxalate appeared to be due to selection of increased proportions of oxalate-degrading bacteria, but no change that correlated with the selection was detected microscopically. In vitro adaptation appears to be similar to that observed in the rumens of animals fed oxalate. It is likely that the ability to use oxalate as an energy source is the basis for the selection observed. A bacterium that degrades oxalate to CO2 and formate and is dependent upon oxalate for growth has been isolated from one of these oxalate-adapted continuous cultures. These results help to explain an interaction (between gut microbes, toxic substance and host) that probably is of survival value to the host. A better understanding of this interaction may eventually contribute toward development of improved management practices on forages that have a high content of oxalate.