296 14 Sequestered: Design and Construction of Synthetic Organelles
kleptoplasty – the theft of an organelle – and spends much of its life living not
as an animal but as a plant after acquiring its algal prey, Vaucheria litorea
[124]. This intra‐corpus symbiosis is maintained for the life of the slug by a yet
unexplained mechanism [124]. It has been proposed this mechanism may
involve the exchange of genetic material, but there are conflicting reports from
differing experimental modalities as to whether algal DNA is actually incorpo
rated into Elysia’s genome [125, 126]. This inspires a remarkable research
question: can kleptoplasty and endosymbiosis be engineered? It remains to be
seen, but work from Silver and colleagues is an intriguing first step. Agapakis
et al. found that cyanobacteria are surprisingly innocuous and do little to dis
turb viability when injected into zebrafish embryos [127]. Even more surpris
ing, cyanobacteria expressing invasin and listeriolysin can grow and divide,
intracellularly, in macrophages while generating little to no immunogenic
response.
Alternatively, one could also imagine engineering extracellular symbioses. For
example, one obvious use would be in biofuel production. There is considerable
interest in constructing an organism for consolidated bioprocessing of plant‐
based biomass into fuel. This would entail engineering an organism for both fuel
production and cellulose degradation, the major component of plant‐based bio
mass [128]. An alternative to this approach would be to develop a stable cocul
ture of two or more organisms that accomplish the same thing. This would
potentially be more modular as the chemical production pathway remains
independent of sugar consumption. Interestingly, it has been found that stable
communities composed of just a handful of bacterial species can indeed degrade
cellulose [129]. Moreover, recent work using E. coli also suggests that stable
mutualism can be predicted using metabolic flux modeling, which could help
systematize future engineering efforts [130]. As a demonstration of this bottom‐
up strategy, Mee et al. designed and constructed a 14‐member consortium of
E. coli mutants that were able to survive up to 50 days, although it was ultimately
dominated by only four strains [131]. In addition to single‐species cocultures, it
is possible to engineer stable mutualism between multiple species of microbes.
For example, a synthetic fungal–bacterial consortium consisting of lignocellu
lose‐degrading Trichoderma reesei and an isobutanol‐producing E. coli strain
can produce the branched alcohol from corn stover [132]. Most recently, Hays
and colleagues paired a heterotroph such as E. coli, Bacillus subtilis, or S. cerevi-
siae with the cyanobacteria Synechococcus elongatus PCC7942 mutant that has
increased sucrose exporting ability. The resulting synthetic consortia can survive
for a long period of time (weeks to months). In addition, by changing the hetero
troph, production of large quantity of enzyme amylase (in the case of B. subtilis)
and polyhydroxybutanoate (PHB) (E. coli) could be achieved [133]. Therefore,
this symbiotic platform shows promise as a new modular strategy for capturing
light energy in the form of bioproducts. It remains to be seen, however, how
tractable these communities will be to engineering to what extent they can be
scaled up in an industrial setting. Nevertheless, this represents an interesting and
untapped avenue for synthetic biology, perhaps informing both our understand
ing of endosymbiosis and the evolution of the cell as well as the interspecies
interactions central to ecology.