Summary: A sp. and a sp. possessing crossing substrate saturation curves for l-lactate were isolated from fresh water by chemostat enrichment. Their and μ values for l-lactate were: sp., 23 μm and 0.35 h, respectively; sp., 91 μm and 0.64 h, respectively. Under l-lactate limitation, sp. outgrew sp. at dilution rates () above 0.29 h, but the converse occurred at lower values. The advantage of sp. increased with decreasing until, at = 0.05 h (i.e. l-lactate concentration of approximately 1 μm), sp. was eliminated from the culture essentially as a non-growing population. In sp. the for l-lactate transport (5.8 μm) was threefold lower than in sp. (20 μm); sp. also possessed a higher for the transport of this substrate. The surface to volume ratio was higher in sp. and increased more markedly than in sp. in response to decreasing Thus, a more efficient scavenging capacity contributes to the advantage of sp. at low concentrations of the carbon source. Although most of the enzymes of l-lactate catabolism were more active in sp., NADH oxidase activity was about twice as high in sp.; and, unlike sp., the cytochrome content of this bacterium increased markedly with decreasing A more active and/or more efficient respiratory chain may therefore also play a role in the advantage of sp. The other factors which appear to be involved include a lower energy of maintenance of sp. [0.016 g l-lactate (g cell dry wt) h compared with 0.066 in sp.] and a lower minimal growth rate.


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