Cellobiose versus Glucose Utilization bytheRuminal Bacterium Ruminococcus albust

Cellulose degradation andmetabolism intherumen can beadversely affected bythepresence ofsoluble sugars,butrelatively little information isavailable on substrate preferences ofcellulolytic bacteria. When the ruminal bacterium Ruminococcus albus was incubated witha combination ofcelobiose andglucose, the organism preferentialy utilized thedisaccharide. Thispreference appeared toberelated torepression of glucose uptake systems incellobiose-grown cells. Glucose transport kinetics exhibited low-andhigh-affinity uptake, andhigh-affinity transport was apparently driven byATP hydrolysis. Bacterial yield incontinuous culture was asmuchas38% greater whentheorganism was grown on celiobiose versus glucose, andthe increased yield could bepartially attributed toconstitutive cellobiose phosphorylase activity. Themaintenance coefficient ofglucose-grown cells was significantly greater thanthatofcellsprovided withcellobiose (0.225 g of glucose pergofprotein perhversus0.042 g ofcellobiose per g ofprotein per h),andthis result suggested that more energywas devoted toglucose uptake. Substrate affinities were examined incarbon-excess continuous culture, andaffinities forglucose andcellobiose were relatively low(0.97 and3.16mM,respectively). Although R.albusmaintained a protonmotive force ofapproximately 60mV frompH6.7to5.5,growthceased below pH6.0, andthis inhibition ofgrowth may havebeencaused bya depletion ofATP atlowpH. Although plants arean abundant andrenewable sourceof organic energy,less than50%ofplant matterisdegraded by mammaliandigestive enzymes. Mostofthisindigestible material consists ofplant cell walls whichcanbeutilized by ruminant animals thatharbor cellulolytic microorganisms in their digestive tracts. Several bacterial species, Fibrobacter succinogenes, Ruminococcus flavefaciens, andRuminococcus albus, are mostcommonly associated withcellulose degradation intherumen (2). Ruminococci are foundat concentrations of107to109CFU/gofrumen contents and constitute up to10%ofthetotal bacterial isolates ineither high-roughage or high-concentrate diets (7). Although most ruminococci ferment cellobiose, some species can also metabolize soluble sugars(e.g., glucose) arising fromstarch degradation (12). R albusmetabolizes glucose viatheEmbden-Meyerhoff-Parnas pathway(13), buttransport mecha