Gene sequences of the pil operon reveal relationships between symbiotic strains of Vibrio fischeri

Microbial experimental evolution as a novel research approach in the Vibrionaceae and squid-Vibrio symbiosis

The Vibrionaceae are a genetically and metabolically diverse family living in aquatic habitats with a great propensity toward developing interactions with eukaryotic microbial and multicellular hosts (as either commensals, pathogens, and mutualists). The Vibrionaceae frequently possess a life history cycle where bacteria are attached to a host in one phase and then another where they are free from their host as either part of the bacterioplankton or adhered to solid substrates such as marine sediment, riverbeds, lakebeds, or floating particulate debris. These two stages in their life history exert quite distinct and separate selection pressures. When bound to solid substrates or to host cells, the Vibrionaceae can also exist as complex biofilms. The association between bioluminescent Vibrio spp. and sepiolid squids (Cephalopoda: Sepiolidae) is an experimentally tractable model to study bacteria and animal host interactions, since the symbionts and squid hosts can be maintained in the laboratory independently of one another. The bacteria can be grown in pure culture and the squid hosts raised gnotobiotically with sterile light organs. The partnership between free-living Vibrio symbionts and axenic squid hatchlings emerging from eggs must be renewed every generation of the cephalopod host. Thus, symbiotic bacteria and animal host can each be studied alone and together in union. Despite virtues provided by the Vibrionaceae and sepiolid squid-Vibrio symbiosis, these assets to evolutionary biology have yet to be fully utilized for microbial experimental evolution. Experimental evolution studies already completed are reviewed, along with exploratory topics for future study.

Gene-swapping mediates host specificity among symbiotic bacteria in a beneficial symbiosis

Environmentally acquired beneficial associations are comprised of a wide variety of symbiotic species that vary both genetically and phenotypically, and therefore have differential colonization abilities, even when symbionts are of the same species. Strain variation is common among conspecific hosts, where subtle differences can lead to competitive exclusion between closely related strains. One example where symbiont specificity is observed is in the sepiolid squid-Vibrio mutualism, where competitive dominance exists among V. fischeri isolates due to subtle genetic differences between strains. Although key symbiotic loci are responsible for the establishment of this association, the genetic mechanisms that dictate strain specificity are not fully understood. We examined several symbiotic loci (lux-bioluminescence, pil = pili, and msh-mannose sensitive hemagglutinin) from mutualistic V. fischeri strains isolated from two geographically distinct squid host species (Euprymna tasmanica-Australia and E. scolopes-Hawaii) to determine whether slight genetic differences regulated host specificity. Through colonization studies performed in naïve squid hatchlings from both hosts, we found that all loci examined are important for specificity and host recognition. Complementation of null mutations in non-native V. fischeri with loci from the native V. fischeri caused a gain in fitness, resulting in competitive dominance in the non-native host. The competitive ability of these symbiotic loci depended upon the locus tested and the specific squid species in which colonization was measured. Our results demonstrate that multiple bacterial genetic elements can determine V. fischeri strain specificity between two closely related squid hosts, indicating how important genetic variation is for regulating conspecific beneficial interactions that are acquired from the environment.

Multiple Vibrio fischeri genes are involved in biofilm formation and host colonization

Biofilms are increasingly recognized as being the predominant form for survival for most bacteria in the environment. The successful colonization of Vibrio fischeri in its squid host Euprymna tasmanica involves complex microbe-host interactions mediated by specific genes that are essential for biofilm formation and colonization. Here, structural and regulatory genes were selected to study their role in biofilm formation and host colonization. We have mutated several genes (pilT, pilU, flgF, motY, ibpA and mifB) by an insertional inactivation strategy. The results demonstrate that structural genes responsible for synthesis of type IV pili and flagella are crucial for biofilm formation and host infection. Moreover, regulatory genes affect colony aggregation by various mechanisms, including alteration of synthesis of transcriptional factors and regulation of extracellular polysaccharide production. These results reflect the significance of how genetic alterations influence communal behavior, which is important in understanding symbiotic relationships.

Characterization of two host-specific genes, mannose-sensitive hemagglutinin (mshA) and uridyl phosphate dehydrogenase (UDPDH) that are involved in the Vibrio fischeri-Euprymna tasmanica mutualism

While much has been known about the mutualistic associations between the sepiolid squid Euprymna tasmanica and the luminescent bacterium, Vibrio fischeri, less is known about the connectivity between the microscopic and molecular basis of initial attachment and persistence in the light organ. Here, we examine the possible effects of two symbiotic genes on specificity and biofilm formation of V. fischeri in squid light organs. Uridine diphosphate glucose-6-dehydrogenase (UDPDH) and mannose-sensitive hemagglutinin (mshA) mutants were generated in V. fischeri to determine whether each gene has an effect on host colonization, specificity, and biofilm formation. Both squid light organ colonization assays and transmission electron microscopy confirmed differences in host colonization between wild-type and mutant strains, and also demonstrated the importance of both UDPDH and mshA gene expression for successful light organ colonization. This furthers our understanding of the genetic factors playing important roles in this environmentally transmitted symbiosis.

Vibrio casei sp. nov., isolated from the surfaces of two French red smear soft cheeses

Three Gram-negative, rod-shaped, catalase- and oxidase-positive, facultatively anaerobic and motile bacteria, strains WS 4538, WS 4539T and WS 4540, were isolated from the surfaces of two fully ripened French red smear soft cheeses. Based on 16S rRNA gene sequence similarity, all three strains were shown to belong to the genus Vibrio. They are most closely related to Vibrio rumoiensis S-1T (96.3% similarity) and Vibrio litoralis MANO22DT (95.9%). DNA-DNA hybridization confirmed that all three isolates belong to the same species and clearly separated strain WS 4539T from V. rumoiensis DSM 19141T (38-42% relatedness) and V. litoralis DSM 17657(T) (28-37%). In contrast to their nearest relatives, the strains exhibited beta-galactosidase and aesculin hydrolase activities. A 14 bp insertion in the 16S rRNA gene sequence forms an elongated structure at helix 10 in the rRNA molecule and provides a tool for PCR-based identification of the novel species. Partial sequences of the housekeeping genes atpA, recA, rpoA and pyrH supported the conclusion that the three isolates constitute a separate species within the genus Vibrio. The name Vibrio casei sp. nov. is proposed for the novel taxon. Strain WS 4539T (=DSM 22364T =LMG 25240T; DNA G+C content 41.8 mol%) is the type strain and WS 4540 (=DSM 22378 =LMG 25241) is a reference strain.

Deciphering Evolutionary Mechanisms Between Mutualistic and Pathogenic Symbioses

The continuum between mutualistic and pathogenic symbioses has been an underlying theme for understanding the evolution of infection and disease in a number of eukaryotic-microbe associations. The ability to monitor and then predict the spread of infectious diseases may depend upon our knowledge and capabilities of anticipating the behavior of virulent pathogens by studying related, benign symbioses. For instance, the ability of a symbiotic species to infect, colonize, and proliferate efficiently in a susceptible host will depend on a number of factors that influence both partners during the infection. Levels of virulence are not only affected by the genetic and phenotypic composite of the symbiont, but also the life history, mode(s) of transmission, and environmental factors that influence colonization, such as antibiotic treatment. Population dynamics of both host and symbiont, including densities, migration, as well as competition between symbionts will also affect infection rates of the pathogen as well as change the evolutionary dynamics between host and symbiont. It is therefore important to be able to compare the evolution of virulence between a wide range of mutualistic and pathogenic systems in order to determine when and where new infections might occur, and what conditions will render the pathogen ineffective. This perspective focuses on several symbiotic models that compare mutualistic associations to pathogenic forms and the questions posed regarding their evolution and radiation. A common theme among these systems is the prevailing concept of how heritable mutations can eventually lead to novel phenotypes and eventually new species.

THE EVOLUTIONARY ECOLOGY OF A SEPIOLID SQUID-VIBRIO ASSOCIATION: FROM CELL TO ENVIRONMENT

Mutualistic relationships between bacteria and their eukaryotic hosts have existed for millions of years, and such associations can be used to understand the evolution of these beneficial partnerships. The symbiosis between sepiolid squids (Cephalopoda: Sepiolidae), and their Vibrio bacteria (gamma Proteobacteria: Vibrionaceae), has been a model system for over 20 years, giving insight as to the specificity of the association, and whether the interactions themselves give rise to such finely tuned dialog. Since the association is environmentally transmitted, selection for specificity can evolve from a number of factors; abiotic (temperature, salinity), as well as biotic (host species, receptors, cell/cell interactions). Here, we examine the transition between these forces effecting the symbiosis, and pose possible explanations as to why this association offers many attributes for understanding the role of symbiotic competence.