SYMBIOSIS Subsequent Colonization

RESEARCH HIGHLIGHTS

Nature Reviews Microbiology | AOP, published online 27 August 2013; doi:10.1038/nrmicro3114

changes prime the symbiont for Furthermore, immunomicroscopy SYMBIOSIS subsequent colonization. localized the encoding transcript The exclusive partnership and the protein to regions of the light between V. fischeri and the Hawaiian organ where V. fischeri accumulates, Breaking the ice bobtail squid (Euprymna scolopes) is and chitin by-products were enriched lifelong, but it must be reconstituted at the pores of the light organ, where in the light organ of each juvenile V. fischeri senses the chitobiose with your host squid after hatching in the ocean. gradient. This complex process involves initial After aggregation at the pores, attachment of just 3–5 V. fischeri cells but before migration into the crypt, to the mucociliary epithelium on the V. fischeri cells seem to pause as exterior of the light organ, followed they prepare or are ‘primed’ for new by aggregation of this population at environmental conditions, such as the light organ pores and subsequent a chemoattractant gradient. Kremer migration into the crypts of the et al. showed that pre-exposure of light organ, where they proliferate. V. fischeri to chitobiose enhanced Enrichment of V. fischeri from the migration along a chitobiose gradient. highly diverse marine bacterial com- This suggests that early exposure of munity has previously been shown aggregated cells to chitobiose (gener- to depend on epithelial mucus, ated by the action of chitotriosidase) including the production of a gradi- at the light organ pore primes them ent of chitobiose (a chitin break- for migration into the crypts. down product), which promotes Collectively, these findings colonization. define the molecular mechanisms To determine whether changes and spatiotemporal dynamics in host gene expression accompany underpinning the initial events that initiation of the symbiosis, Kremer lead to the establishment of the et al. compared the transcriptome of V. fischeri–squid symbiosis. The symbiotic light organs (which were mucociliary epithelium of the light exposed to thousands of environ organ has evolved a highly sensitive mental bacteria and an average and selective transcriptional response of five V. fischeri cells) to that of to the first few interacting symbionts, aposymbiotic light organs (which which results in a change in the local were exposed to environmental bac- chemical environment that promotes CORBIS teria only). The results revealed that subsequent colonization. This study protease and hydrolase transcripts illuminates the key molecular strate- The early interactions between a were increased in the presence of gies required to initiate a beneficial bacterial symbiont and its host are the few associating V. fischeri cells, symbiosis, the principles of which are a highly sensitive crucial for establishing a long-lived particularly those associated with likely to be widespread. and selective association, but the molecular chitin metabolism and most notably Christina Tobin Kåhrström mechanisms underlying this initial the gene encoding chitotriosidase. transcriptional dialogue are poorly understood. Chitotriosidase from other species response to ORIGINAL RESEARCH PAPER Kremer, N. et al. Using the well-established Vibrio is a secreted enzyme that cleaves chitin Initial symbiont contact orchestrates host-organ- the first few fischeri–squid symbiosis as a model polymers and releases chitobiose, wide transcriptional changes that prime tissue colonization. Cell Host Microbe 14, 183–194 (2013) interacting system, Kremer et al. now show that among other by-products. Purified FURTHER READING Nyholm, S. V. & Graf, J. symbionts the few V. fischeri cells which initiate E. scolopes chitotriosidase also showed Knowing your friends: invertebrate innate the association induce transcriptional catalytic activity, which was optimal at immunity fosters beneficial bacterial symbioses. Nature Rev. Microbiol. 10, 815–827 (2012) changes in the host, and these the acidic pH of the epithelial mucus.

NATURE REVIEWS | MICROBIOLOGY VOLUME 11 | OCTOBER 2013