In naturally competent bacteria, DNA transformation through horizontal
gene transfer is an evolutionary mechanism to receive extracellular DNA.
Bacteria need to maintain a state of competence to accept foreign DNA
and this is an energy-driven phenomenon that is tightly controlled. In
Streptococcus, competence development is a complex process that is not
fully understood. In this study, we used Streptococcus mutans, an oral
bacterium, to determine how cell density affects competence development.
We found that in S. mutans the transformation efficiency is maximum when
the transforming DNA was added at low cell density and incubated for 2.5
h before selecting for transformants. We also found that S. mutans cells
remain competent until the mid-logarithmic phase, after which the
competence decreases drastically. Surprisingly, we observed that
individual components of Clp proteolytic complexes differentially
regulate competence. If the transformation is carried out at the early
growth phase, both ClpP protease and ClpX ATPase are needed for
competence. In contrast, we found that both ClpC and ClpE negatively
affect competence. We also found that if the transformation is carried
out at the mid-logarithmic growth phase ClpX is still required for
competence but ClpP negatively affects competence. While the exact
reason for this differential effect of ClpP and ClpX on transformation
is currently unknown, we found that both ClpC and ClpE have a negative
effect on transformation, which was not reported before.