ARABINOGALACTAN UTILIZATION IN CONTINUOUS CULTURES OF BIFIDOBACTERIUM-LONGUM - EFFECT OF COCULTURE WITH BACTEROIDES-THETAIOTAOMICRON

Studies showed that the plant cell wall polysaccharide arabinogalactan supported growth of Bifidobacterium longum in batch culture. Galactos e was also utilized, but not arabinose, the other major constituent su gar of the polymer. Enzymes required for hydrolysis of arabinogalactan ('arabinogalactanase', alpha-arabinopyranosidase, beta-galactosidase) were inducible and cell-associated in B. longum, and their expression was repressed by glucose. Considerable amounts of alpha-arabinopyrano sidase and beta-galactosidase were synthesized during growth on arabin ogalactan, but only low levels of arabinogalactanase were detected. B. longum only grew on arabinogalactan in continuous culture under putat ive carbon-excess conditions. In C-Limited chemostats, the bifidobacte rium could not establish unless Bacteroides thetaiotaomicron was prese nt in co-culture. The relationship between the two organisms was not s imply commensal; at low specific growth rates, bacteroides cell popula tion densities were approximately 30% lower than those recorded in axe nic culture, indicating the existence of competitive interactions with the bifidobacterium. In contrast, at high specific growth rates, a mu tualistic association was observed, in that Bnct, thetaiotaomicron was maintained in the chemostats at high dilution rates if bifidobacteria were also present. Measurements of residual carbohydrate in spent cul ture fluid from C-Limited chemostats indicated that a large part of th e arabinogalactan molecule could not be broken down by either B. longu m or Bact. thetaiotaomicron alone, or in co-culture. Formate and aceta te were the major fermentation products of B. longum cultured in the p resence of high concentrations of arabinogalactan, confirming that the se bacteria were growing under energy-limited conditions.