Some effects of arsenic on the rumen microflora; an in vitro study

The rate of fermentation of the rumen microflora was inhibited almost 30% by 5 micrograms/ml of arsenic added in the form of arsenite, although 304 micrograms/ml was required to cause 50% inhibition. Arsenate was less inhibitory. The rate of fermentation of a separated bacterial fraction was inhibited 37% by 1 microgram of arsenite per millilitre, whereas 100 micrograms/ml had little effect on the fermentation of a separated protozoal fraction. Similar results were obtained for arsenate. Both fractions had the capacity to take up arsenate, but the protozoa took it up more readily to a higher intracellular concentration. Both arsenate and arsenite inhibited the growth of a number of rumen bacteria in pure culture at concentrations as low as 5 micrograms of arsenic per millilitre. The greater resistance of Megasphaera elsdenii to arsenate as compared with that of Bacteroides succinogenes was not related to the inability to take up the element. In conclusion, the concentrations of arsenic causing a significant inhibitory effect on the fermentative activity and growth of some rumen bacteria are less than that reported to be toxic to ruminant animals.