Sucrose transport by Streptococcus mutans. Evidence for multiple transport systems

Abstract

The transport of sucrose by selected mutant and wild-type cells of Streptococcus mutans was studied using washed cocci harvested at appropriate phases of growth, incubated in the presence of fluoride and appropriately labelled substrates. The rapid sucrose uptake observed cannot be ascribed to possible extracellular formation of hexoses from sucrose and their subsequent transport, formation of intracellular glycogen-like polysaccharide, or binding of sucrose or extracellular glucans to the cocci. Rather, there are at least three discrete transport systems for sucrose, two of which are phosphoenol pyruvate-dependent phosphotransferases with relatively low apparent K m values and the other a non-phosphotransferase (non-PTS) third transport system (termed TTS) with a relatively high apparent K m . For strain 6715-13 mutant 33, the K m values are 6.25·10 −5 M, 2.4·10 −4 M, and 3.0·10 −3 M, respectively; for strain NCTC-10449, the K m values are 7.1·10 −5 M, 2.5·10 −4 M and 3.3·10 −3 M, respectively. The two lower K m systems could not be demonstrated in mid-log phase glucose-adapted cocci, a condition known to repress sucrose-specific phosphotransferase activity, but under these conditions the highest K m system persists. Also, a mutant devoid of sucrose-specific phosphotransferase activity fails to evidence the two high affinity (low apparent K m ) systems, but still has the lowest affinity (highest K m ) system. There was essentially no uptake at 4°C indicating these processes are energy dependent. The third transport system, whose nature is unknown, appears to function under conditions of sucrose abundance and rapid growth which are known to repress phosphoenol pyruvate-dependent sucrose-specific phosphotransferase activity in S. mutans. These multiple transport systems seem well-adapted to S. mutans which is faced with fluctuating supplies of sucrose in its natural habitat on the surfaces of teeth.