The Enterococcus faecalis Information Gate - Cellular Computing - page 114

Realization of the E. faecalis Information Gate
E. faecalis secrete various pheromones into the culture medium. Even an hour-
long culture of E. faecalis is sufficient to induce E. faecalis conjugative plasmid
transfer. E. faecalis donors also secrete pheromone inhibitors. However, the
amount of secretion is not sufficient for our purposes, so we used synthetic
pheromone peptides in this work.
We incubated OG1RF(pAM714) and OG1SSp in THB at 37 ◦ C for 18 h. After
washing twice with fresh THB, OG1RF(pAM714) was incubated for 60 min
in THB, 20

g/ml iAD1 to confirm the inhibitory
execution of the information gate. OG1RF(pAM714) was then combined with
100-fold excess of OG1SSp and incubated at 37 ◦ C for 90 min. OG1SSp was
separated from OG1RF(pAM714) by antibiotic selection with 1 mg/ml strept-
mycin. After washing twice with fresh THB, we incubated OG1SSp for 18 h to
increase the total cell number and to esnure that the conjugative plasmids were
distributed among the cells.

µ g/ml cAD1. We added 200 µ

Confirmation of the Gate Execution
During an 18-h incubation of the final step, we took a sample of the cell mixtures
and spread it on an THB agar plate containing 50

µ

g/ml erythromycin to confirm
the result of the gate execution. The total colony number on the plate was
counted (Figure 6.4). None of the OG1SSp(pAM714) colony was observed
on the following day, but they grew slowly and reached maximum growth
after a few days. In the case of OG1SSp(pCF10), the growth delay was not
observed. The presence of a high concentration of antibiotic substances creates
pressure on the bacteria that even defeats any antibiotic resistance gene. We
note that perhaps the condition should be less stringent in the selection step of
OG1SSp(pAM714).

A Logical Inverter: Execution of Two Information
Gates Simultaneously

The information gate is applicable to building a logic inverter (Figure 6.5).
The gate contains donors with plasmid P 0 and donors with plasmid P 1 . Donors
with plasmid P 1 − n (n 1 or 0) in the logic inverter are activated when the

inverter receives inhibitor I n . Activated donors conjugate with recipients and
the plasmid P 1 − n transferred to the recipients. The recipients produce inhibitors
I 1 − n in accordance with the genetic information of the newly obtained plasmid.
These then act as the input signal to the next operation.
An execution of signal inversion P 0 to P 1 was tested as follows. We cultured
E. faecalis strains OG1SSp ( P 0 ::Tc r ), OG1RF ( P 0 ::Tc r ), OG1RF ( P 1 ::Em r ),
and OG1SSp at 37 ◦ C for 18 h. First, OG1SSp ( P 0 ::Tc r ) was used as the input
cells which send inhibitors I 0 to the gate. OG1RFs were used as the plasmid

=