An ascorbate-reduced medium for nitrogen metabolism studies with Selenomonas ruminantium

Abstract

A basal medium for the growth of rumen selenomonads in nitrogen metabolism studies is presented. Mercaptoethanol, thioglycolate and ascorbate were inititally considered as nonnitrogenous reductants to replace cysteine in anaerobic media commonly used for rumen bacterial growth studies. Unlike media containing mercaptoethanol and thioglycolate, media reduced with 1.4–4.2 mM ascorbate were found to support growth rates of Selenomonas ruminantium HD 4 which were consistently similar to growth rates obtained when this strain was grown in conventional medium containing 1.4 mM cysteine. Strains D, PC18 and GA192 also grew well in media reduced with 4.2 mM ascorbate. Ascorbate appeared to be nonmetabolizable since no growth occurred in ascorbate-reduced medium in the absence of NH 4Cl or glucose. The ability to achieve nitrogen-limited growth with this reductant was confirmed with a defined medium containing graded levels of NH 4Cl. Growth in the ascorbate medium allowed for formation of the typical fermentation products of this strain as well as redox balances (0.98±0.10) which were similar to those obtained with a medium containing cysteine.