Efficiency and Stability of Exopolysaccharide Production from Different Carbon Sources by Erwinia herbicola

Shell Research Ltd, Sittingbourne Research Centre, Sittingbourne, Kent ME9 8AG, UK ABSTRACT Summary: Under conditions of glucose limitation the maximum yield of Erwinia herbicola from oxygen ( Y max o 2 ) and glucose ( Y max gic ) was 31 g mol –1 and 72 g mol –1 respectively. This corresponded to a relative ATP/O quotient between 1.1 and 1.5. The cytochrome profile indicated the absence of c -type cytochromes which is consistent with this low ATP/O quotient. During nitrogen-limited growth on glucose, the rate of polysaccharide production (0.34-0.37 g g –1 h –1 ) was independent of growth rate, whereas the rate of production of gluconate and 2-ketogluconate increased with growth rate. The ATP/O quotient of 1.25 derived from glucose-limited growth was used to calculate the theoretical yields of exopolysaccharide from fructose and oxygen. The values obtained were close to the observed yields once an allowance had been made for cell production. Thus the growth efficiency of E. herbicola appears to be unaltered during carbon- or nitrogen-limited growth on fructose. At an ATP/O quotient of 1.25 the synthesis of the acid moiety of the exopolysaccharide provides a significant proportion (51-65%) of the ATP needed for polymerization of the sugar backbone. Nevertheless, the rate of ATP turnover is large in relation to that required for cell production under these conditions. Polymer production may serve as a means of turning over ATP but it is not essential for growth as mutants that do not produce exopolysaccharide but turnover ATP by some other means are readily selected. The rate of ATP turnover rather than exopolysaccharide production appears to be physiologically important. Present address: Koninklijke Shell Laboratorium, Badhuisweg 3, 1003 AA Amsterdam, The Netherlands.