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in the formation of virulence determinants and secondary metabolites involved
in biofilm formation.
The primary considerations for molecular interconnection schemes are the
number of different molecules available and the level of cross-talk between the
different molecular species. Research over the last 10 years has demonstrated
that AHL signaling is widespread among Gram-negative bacteria, having been
reported in more that 29 species of the
subdivisions of the pro-
teobacteria [103]. Twenty-three individual LuxI homologs have been reported,
and the regulatory components have been shown to be conserved at the genetic
level. All of the AHLs characterized to date vary primarily in the length and
hydrophobicity of their acyl side chains [41]. Experimental evidence suggests
a high degree of specificity between the AHL and its cognate transcriptional
activator. The V. fisheri LuxR showed no activity in the presence of the P. aerug-
inosa AHL, and, similarly, the P. aeruginosa LuxR homolog, LasR, showed no
activity in the presence of the V. fisheri AHL [46].
However, more recently, evidence for cross-species communication has be-
gun to appear in the literature. In one study in which several species of bacteria
were tested for production of extracellular AHL-like activities using V. harveyi
sensor mutants, V. cholerae and V. parahaemolyticus were shown to exhibit such
activities [5]. Also, V. harveyi was shown to have two independent systems, one
highly species specific and another species nonspecific [5]. Using the V. harveyi
sensor mutant, it was shown that several strains of E. coli and S. typhimurium
produced AHL-like molecules [101]. It appears that the species-specific system
monitors the environment for other V. harveyi , whereas the nonspecific system
monitors for other species of bacteria [4].
The variety of independent AHL-like signaling processes might permit mas-
sive parallel interconnection between whole-cell information-processing de-
vices. A species-specific AHL could be produced that would diffuse throughout
a population of cells and yet only react with a single target cell or some subset
of the total population. Similarly, a species nonspecific AHL could be used to
coordinate processes in an entire population of cells. This ability to both broad-
cast and target communication provides a significant tool for the realization of
highly complex whole-cell devices.
α
,
β
, and
γ
CHIP-TO-CELL COMMUNICATION
The communication of information from chip to cell may be the most chal-
lenging of the three major information pathways presented here. However,
this information pathway is essential for the complete integration of cells as
components in engineered systems, and it possibly represents the most far-
reaching step beyond that accomplished in whole-cell biosensor systems. Here
we consider communication that may proceed in two ways: (1) information
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