As I’ve mentioned before, the Silver lab (with help from the Megason lab) have been working on a quirky-but-cool project to induce zebrafish to accept cyanobacteria as an intracellular symbiont.  And stage one has gone surprisingly well: as Christina Agapakis points out in her blog post on the project, when they injected millions of cyanobacteria into a single-cell zebrafish embryo “the biggest surprise was that nothing happened. The embryos developed normally into a happy, swimming fish when we injected them with cyanobacteria”.  And the cyanobacteria survive too.  In contrast, if fish embryos are injected with E. coli, the embryo dies within 2 hours.

Encouraged by this, Christina wondered if it would be possible to engineer the photosynthetic bacteria to allow them to infect tissue culture cells.  Something similar has been done before, with the long-term goal of using invasive E. coli to kill cancer cells, or to deliver shRNAs for gene silencing.  Here, Agapakis et al. engineered Synechococcus elongatus to express two foreign proteins: invasin, from the bacterium responsible for the Black Death, Yersinia pestis, Yersinia pseudotuberculosis (see comments below) and listeriolysin, from Listeria monocytogenes, another important human pathogen.  Invasin, as the name implies, is responsible for getting Y. pestis Y. pseudotuberculosis into mammalian cells, while listeriolysin allows Listeria to escape from the endosomal compartment, where most bacteria get digested.  These engineered bacteria can indeed infect tissue culture cells, including a murine macrophage cell line, and even divide inside them.

As Christina points out, there are a number of non-engineered animals such as the photosynthetic salamander that have developed some kind of association, and maybe even true symbiosis, with photosynthetic bacteria.  This project has already shown us something about how such associations can arise, and might provide insight into what makes them possible.

The results are reported in a PLoS One paper this week.

Agapakis, C., Niederholtmeyer, H., Noche, R., Lieberman, T., Megason, S., Way, J., & Silver, P. (2011). Towards a Synthetic Chloroplast PLoS ONE, 6 (4) DOI: 10.1371/journal.pone.0018877

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