When attempting to increase yields of desirable end-products during fermentation, there is the possibility that increased concentrations of one product redirects metabolism towards the synthesis of less desired products. Changes in growth, final end-product concentrations, and activities of enzymes involved in pyruvate catabolism and fermentative end-product formation were studied in Clostridium thermocellum in response to the addition of individual end-products (H(2), acetate, ethanol, formate, and lactate) to the growth medium. These were added to the growth medium at concentrations ten times greater than those found at the end of growth in cultures grown under carbon-limited conditions using cellobiose (1.1 gl(-1)) as model soluble substrate. Although growth rate and final cell biomass decreased significantly with the addition of all end-products, addition of individual end-products had less pronounced effects on growth. Metabolic shifts, represented by changes in final end-product concentrations, were observed; H(2) and acetate yields increased in the presence of exogenous ethanol and lactate, while ethanol yields increased in the presence of exogenous hydrogen (H(2)), acetate, and lactate. Late exponential phase enzyme activity data of enzymes involved in pyruvate catabolism and end-product formation revealed no changes in enzyme levels greater than 2-fold in response to the presence of any given end-product, with the exception of pyruvate:formate lyase (PFL), ferredoxin-dependent hydrogenase (Fd-H(2)ase), and pyruvate:ferredoxin oxidoreductase (PFO): PFL and Fd-H(2)ase activities increased 2-fold in the presence of ethanol, while PFO activity decreased by 57% in the presence of sodium formate. Changes in enzyme levels did not necessarily correlate with changes in final end-product yields, suggesting that changes in final end-product yields may be governed by thermodynamic considerations rather than levels of enzyme expressed under the conditions tested. We demonstrate that bacterial metabolism may be manipulated in order to selectively improve desired product yields.