Isolation and characterization of a visibly luminous variant ofVibrio fischeri strain ES114 from the sepiolid squidEuprymna scolopes

Spatial heterogeneity of autoinducer regulation systems

Autoinducer signals enable coordinated behaviour of bacterial populations, a phenomenon originally described as quorum sensing. Autoinducer systems are often controlled by environmental substances as nutrients or secondary metabolites (signals) from neighbouring organisms. In cell aggregates and biofilms gradients of signals and environmental substances emerge. Mathematical modelling is used to analyse the functioning of the system. We find that the autoinducer regulation network generates spatially heterogeneous behaviour, up to a kind of multicellularity-like division of work, especially under nutrient-controlled conditions. A hybrid push/pull concept is proposed to explain the ecological function. The analysis allows to explain hitherto seemingly contradicting experimental findings.

Effects of luxCDABEG induction in Vibrio fischeri: enhancement of symbiotic colonization and conditional attenuation of growth in culture

Production of bioluminescence theoretically represents a cost, energetic or otherwise, that could slow Vibrio fischeri growth; however, bioluminescence is also thought to enable full symbiotic colonization of the Euprymna scolopes light organ by V. fischeri. Previous tests of these models have proven inconclusive, partly because they compared nonisogenic strains, or undefined and/or pleiotropic mutants. To test the influence of the bioluminescence-producing lux operon on growth and symbiotic competence, we generated dark luxCDABEG mutants in strains MJ1 and ES114 without disrupting the luxR-luxI regulatory circuit. The MJ1 luxCDABEG mutant out-competed its visibly luminescent parent approximately 26% per generation in a carbon-limited chemostat. Similarly, induction of luminescence in the otherwise dim ES114 strain slowed growth relative to DeltaluxCDABEG mutants. Some culture conditions yielded no detectable effect of luminescence on growth, indicating that luminescence is not always growth limiting; however, luminescence was never found to confer an advantage in culture. In contrast to this conditional disadvantage of lux expression, ES114 achieved approximately fourfold higher populations than its luxCDABEG mutant in the light organ of E. scolopes. These results demonstrate that induction of luxCDABEG can slow V. fischeri growth under certain culture conditions and is a positive symbiotic colonization factor.

The diversity of bacterial communities associated with Atlantic cod Gadus morhua

The spatial and temporal changes in the bacterial communities associated with the Atlantic cod Gadus morhua were investigated using terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S recombinant DNA (rDNA). Epidermal mucous was sampled from 366 cod caught in three harvest locations (Baltic, Icelandic, and North Seas) over three seasons (spring 2002, autumn 2002, and spring 2003), and an automated method for the high-throughput processing of environmental samples was developed using a Qiagen BioRobot. The analysis revealed that a diverse consortium of bacteria were found on fish; gamma-proteobacteria and Cytophaga-Flavobacter-Bacteroides (CFB) species were dominant. T-RFLP peak profiles suggested that operational taxonomic units (OTUs) related to Photobacterium sp., Psychrobacter sp., and Bacteroides sp. were common to all sites in all three seasons, but there were intersite variations in community composition. Cod caught from different seas had distinct reproducible bacterial assemblages. Whereas communities from fish caught in the Baltic and Icelandic Seas were relatively stable over the three seasons, those from fish from the North Sea changed significantly over time.

Isolation and identification of Photobacterium phosphoreum from an unexpected niche: migrating salmon

Six luminous bacteria were isolated from migrating salmon in the Yukon River, Alaska. All isolates were identified as Photobacterium phosphoreum. Previous studies suggest that P. phosphoreum is an exclusively marine bacterium, while our Alaskan isolates are from salmon which migrated up to 1,228 km from the marine environment.

Alterations in Vibrio fischeri motility correlate with a delay in symbiosis initiation and are associated with additional symbiotic colonization defects

Motility is required for Vibrio fischeri cells to interact with and specifically colonize the light-emitting organ of their host, the squid Euprymna scolopes. To investigate the influence of motility on the expression of the symbiotic phenotype, we isolated mutants of the squid symbiont V. fischeri ES114 that had altered migration abilities. Spontaneous hyperswimmer (HS) mutants, which migrated more rapidly in soft agar and were hyperflagellated relative to the wild type, were isolated and grouped into three phenotypic classes. All of the HS strains tested, regardless of class, were delayed in symbiosis initiation. This result suggested that the hypermotile phenotype alone contributes to an inability to colonize squid normally. Class III HS strains showed the greatest colonization defect: they colonized squid to a level that was only 0.1 to 10% that achieved by ES114. In addition, class III strains were defective in two capabilities, hemagglutination and luminescence, that have been previously described as colonization factors in V. fischeri. Class II and III mutants also share a mucoid colony morphology; however, class II mutants can colonize E. scolopes to a level that was 40% of that achieved by ES114. Thus, the mucoid phenotype alone does not contribute to the greater defect exhibited by class III strains. When squid were exposed to ES114 and any one of the HS mutant strains as a coinoculation, the parent strain dominated the resulting symbiotic light-organ population. To further investigate the colonization defects of the HS strains, we used confocal laser-scanning microscopy to visualize V. fischeri cells in their initial interaction with E. scolopes tissue. Compared to ES114, HS strains from all three classes were delayed in two behaviors involved in colonization: (i) aggregation on host-derived mucus structures and (ii) migration to the crypts. These results suggest that, while motility is required to initiate colonization, the presence of multiple flagella may actually interfere with normal aggregation and attachment behavior. Furthermore, the pleiotropic nature of class III HS strains provides evidence that motility is coregulated with other symbiotic determinants in V. fischeri.

LuxR- and acyl-homoserine-lactone-controlled non-lux genes define a quorum-sensing regulon in Vibrio fischeri

The luminescence (lux) operon (luxICDABEG) of the symbiotic bacterium Vibrio fischeri is regulated by the transcriptional activator LuxR and two acyl-homoserine lactone (acyl-HSL) autoinducers (the luxI-dependent 3-oxo-hexanoyl-HSL [3-oxo-C6-HSL] and the ainS-dependent octanoyl-HSL [C8-HSL]) in a population density-responsive manner called quorum sensing. To identify quorum-sensing-regulated (QSR) proteins different from those encoded by lux genes, we examined the protein patterns of V. fischeri quorum-sensing mutants defective in luxI, ainS, and luxR by two-dimensional polyacrylamide gel electrophoresis. Five non-Lux QSR proteins, QsrP, RibB, AcfA, QsrV, and QSR 7, were identified; their production occurred preferentially at high population density, required both LuxR and 3-oxo-C6-HSL, and was inhibited by C8-HSL at low population density. The genes encoding two of the QSR proteins were characterized: qsrP directs cells to synthesize an apparently novel periplasmic protein, and ribB is a homolog of the Escherichia coli gene for 3,4-dihydroxy-2-butanone 4-phosphate synthase, a key enzyme for riboflavin synthesis. The qsrP and ribB promoter regions each contained a sequence similar to the lux operon lux box, a 20-bp region of dyad symmetry necessary for LuxR/3-oxo-C6-HSL-dependent activation of lux operon transcription. V. fischeri qsrP and ribB mutants exhibited no distinct phenotype in culture. However, a qsrP mutant, in competition with its parent strain, was less successful in colonizing Euprymna scolopes, the symbiotic host of V. fischeri. The newly identified QSR genes, together with the lux operon, define a LuxR/acyl-HSL-responsive quorum-sensing regulon in V. fischeri.

Lessons from a cooperative, bacterial-animal association: the Vibrio fischeri-Euprymna scolopes light organ symbiosis

Although the study of microbe-host interactions has been traditionally dominated by an interest in pathogenic associations, there is an increasing awareness of the importance of cooperative symbiotic interactions in the biology of many bacteria and their animal and plant hosts. This review examines a model system for the study of such symbioses, the light organ association between the bobtail squid Euprymna scolopes and the marine luminous bacterium Vibrio fischeri. Specifically, the initiation, establishment, and persistence of the benign bacterial infection of the juvenile host light organ are described, as are efforts to understand the mechanisms underlying this specific colonization program. Using molecular genetic techniques, mutant strains of V. fischeri have been constructed that are defective at specific stages of the development of the association. Some of the lessons that these mutants have begun to teach us about the complex and long-term nature of this cooperative venture are summarized.