Impact of transit time on the reproductive capacity of Euprymna scolopes as a laboratory animal

Quorum sensing inhibits interference competition among bacterial symbionts within a host

The symbioses that animals form with bacteria play important roles in health and disease, but the molecular details underlying how bacterial symbionts initially assemble within a host remain unclear.1,2,3 The bioluminescent bacterium Vibrio fischeri establishes a light-emitting symbiosis with the Hawaiian bobtail squid Euprymna scolopes by colonizing specific epithelium-lined crypt spaces within a symbiotic organ called the light organ.4 Competition for these colonization sites occurs between different strains of V. fischeri, with the lancet-like type VI secretion system (T6SS) facilitating strong competitive interference that results in strain incompatibility within a crypt space.5,6 Although recent studies have identified regulators of this T6SS, how the T6SS is controlled as symbionts assemble in vivo remains unknown.7,8 Here, we show that T6SS activity is suppressed by N-octanoyl-L-homoserine lactone (C8 HSL), which is a signaling molecule that facilitates quorum sensing in V. fischeri and is important for efficient symbiont assembly.9,10 We find that this signaling depends on the quorum-sensing regulator LitR, which lowers expression of the needle subunit Hcp, a key component of the T6SS, by repressing transcription of the T6SS regulator VasH. We show that LitR-dependent quorum sensing inhibits strain incompatibility within the squid light organ. Collectively, these results provide new insights into the mechanisms by which regulatory networks that promote symbiosis also control competition among symbionts, which in turn may affect the overall symbiont diversity that assembles within a host.

A case study assessing the impact of mating frequency on the reproductive performance of the Hawaiian bobtail squid Euprymna scolopes

BACKGROUND

The symbiosis between the Hawaiian bobtail squid Euprymna scolopes and bacterium Vibrio fischeri serves as a model for investigating the molecular mechanisms that promote the initial formation of animal-bacterial symbioses. Research with this system frequently depends on freshly hatched E. scolopes, but the husbandry factors that promote hatchling production in a mariculture facility remain underreported. Here we report on the reproductive performance of E. scolopes in response to decreased mating frequency.

RESULTS

One animal cohort was maintained in a mariculture facility for 107 days, with females assigned to either a control group (mating once every 14 days) or an experimental group (mating once every 21 days). No differences between the groups were observed in survival, the number of egg clutches laid, or hatchling counts. Each group featured multiple females that were hyper-reproductive, i.e., they generated more than 8 egg clutches while in captivity. Examination of the distributions for daily hatchling counts of individual egg clutches revealed significant variation in the hatching patterns among clutches that was independent of mating frequency. Finally, an assessment of hatchling production showed that 93.5% of total hatchlings produced by the cohort were derived from egg clutches laid within the first 70 days.

CONCLUSIONS

These results suggest a lower mating frequency does not impede hatchling production. Furthermore, the variation in hatchling production among egg clutches provides new insight into the reproductive performance of E. scolopes as a lab animal for microbiology research.

Quantification of the capacity of vibrio fischeri to establish symbiosis with Euprymna scolopes

Most animals establish long-term symbiotic associations with bacteria that are critical for normal host physiology. The symbiosis that forms between the Hawaiian squid Euprymna scolopes and the bioluminescent bacterium Vibrio fischeri serves as an important model system for investigating the molecular mechanisms that promote animal-bacterial symbioses. E. scolopes hatch from their eggs uncolonized, which has led to the development of squid-colonization assays that are based on introducing culture-grown V. fischeri cells to freshly hatched juvenile squid. Recent studies have revealed that strains often exhibit large differences in how they establish symbiosis. Therefore, we sought to develop a simplified and reproducible protocol that permits researchers to determine appropriate inoculum levels and provides a platform to standardize the assay across different laboratories. In our protocol, we adapt a method commonly used for evaluating the infectivity of pathogens to quantify the symbiotic capacity of V. fischeri strains. The resulting metric, the symbiotic dose-50 (SD50), estimates the inoculum level that is necessary for a specific V. fischeri strain to establish a light-emitting symbiosis. Relative to other protocols, our method requires 2-5-fold fewer animals. Furthermore, the power analysis presented here suggests that the protocol can detect up to a 3-fold change in the SD50 between different strains.

Quorum sensing facilitates interpopulation signaling by Vibrio fischeri within the light organ of Euprymna scolopes

Quorum sensing is an intercellular signaling mechanism that enables bacterial cells to coordinate population-level behaviors. How quorum sensing functions in natural habitats remains poorly understood. Vibrio fischeri is a bacterial symbiont of the Hawaiian bobtail squid Euprymna scolopes and depends on LuxI/LuxR quorum sensing to produce the symbiotic trait of bioluminescence. A previous study demonstrated that animals emit light when co-colonized by a Δlux mutant, which lacks several genes within the lux operon that are necessary for bioluminescence production, and a LuxI- mutant, which cannot synthesize the quorum signaling molecule N-3-oxohexanoyl-homoserine lactone. Here, we build upon that observation and show that populations of LuxI- feature elevated promoter activity for the lux operon. We find that population structures comprising of Δlux and LuxI- are attenuated within the squid, but a wild-type strain enables the LuxI- strain type to be maintained in vivo. These experimental results support a model of interpopulation signaling, which provides basic insight into how quorum sensing functions within the natural habitats found within a host.

Two enhancer binding proteins activate σ54-dependent transcription of a quorum regulatory RNA in a bacterial symbiont

To colonize a host, bacteria depend on an ensemble of signaling systems to convert information about the various environments encountered within the host into specific cellular activities. How these signaling systems coordinate transitions between cellular states in vivo remains poorly understood. To address this knowledge gap, we investigated how the bacterial symbiont Vibrio fischeri initially colonizes the light organ of the Hawaiian bobtail squid Euprymna scolopes. Previous work has shown that the small RNA Qrr1, which is a regulatory component of the quorum-sensing system in V. fischeri, promotes host colonization. Here, we report that transcriptional activation of Qrr1 is inhibited by the sensor kinase BinK, which suppresses cellular aggregation by V. fischeri prior to light organ entry. We show that Qrr1 expression depends on the alternative sigma factor σ54 and the transcription factors LuxO and SypG, which function similar to an OR logic gate, thereby ensuring Qrr1 is expressed during colonization. Finally, we provide evidence that this regulatory mechanism is widespread throughout the Vibrionaceae family. Together, our work reveals how coordination between the signaling pathways underlying aggregation and quorum-sensing promotes host colonization, which provides insight into how integration among signaling systems facilitates complex processes in bacteria.