Synthesis and degradation of intracellular polyglucose in Streptococcus salivarius

Abstract

The incorporation of large quantities of glucose into an intracellular polymer similar in composition to glycogen has been demonstrated with growing and resting cells of Streptococcus salivarius isolated from human saliva. Characteristics of the synthetic process have been determined with resting cells grown in minimal glucose broth and then incubated with glucose-¹⁴C in a nitrogen-free buffer medium. At an optimum pH of 7.0, cells rapidly synthesized large quantities of intracellular polyglucose incorporating glucose at an initial rate of 5.6 μmoles/h per mg dry weight of cells. At exogenous glucose concentrations of 11.0 μmoles per milligram dry cells or above, resting cells would not incorporate more than 2.75 μmoles of glucose into intracellular polysaccharide. The incorporation of this amount of glucose represents a 50% increase in cell size on a dry weight basis and reflects the physical limit of these cells without cell division. Almost 90% of this intracellular material was utilizable glucose in polymer form with the remaining material being incorporated into structural components. The enzymes for polyglucose synthesis were found to be constitutive, although the rate and degree of synthesis was reduced after prolonged exposure to carbon sources other than glucose, possibly reflecting an alteration in the glucose transport system. Growth at high levels of glucose also suppressed the rate of polyglucose synthesis as well as the rate of glucose utilization. Rapid degradation of the intracellular polyglucose was always observed in the absence of exogenous carbohydrate with the last molecules of glucose synthesized into the polymer being the first to be degraded upon exhaustion of the exogenous glucose. With resting cells of S. salivarius, the exogenous glucose not incorporated into polyglucose was stoichiometrically converted to lactic acid, but only 80–85% of the endogenous polyglucose was converted to this end-product in the absence of exogenous glucose, with the remaining 15–20% being degraded to small amounts of α-ketoglutarate, citrate, pyruvate, and an unknown acid. In this organism, intracellular polyglucose synthesis and degradation appear to be controlled processes, as experiments designed to measure the specific activity of lactic acid, produced by cells grown on high and low levels of glucose and metabolizing radioactive glucose, have shown that synthesis and degradation of intracellular polyglucose do not proceed simultaneously.