Biology subjects

Speare, L.

Publications and source records attributed to Speare, L..

7 recordsLinked to original sources

bioRxiv2026

A dominant coral parasite, Candidatus Aquirickettsia rohweri, resists antibiotic exposure and thermal challenge below the bleaching threshold in disease-susceptible Acropora cervicornis

The critically endangered Caribbean staghorn coral Acropora cervicornis hosts microbiomes frequently dominated by the putatively parasitic intracellular bacterium Candidatus Aquirickettsia rohweri, which is associated with reduced coral growth and heightened disease susceptibility. Whether this dominance can be disrupted through antibiotic treatment and a sequential disturbance of thermal stress, remains unknown. In this study, we exposed disease-susceptible A. cervicornis fragments to broad-spectrum antibiotics, sub-bleaching thermal stress, or the combination of an antibiotic pre-treatment followed by thermal stress, and tracked changes in microbiome composition and diversity across all experimental phases using 16S rRNA amplicon sequencing and quantitative PCR (qPCR). We find that while the minor microbial fraction exhibits sustained compositional shifts in response to treatment, Ca. Aquirickettsia rohweri is resilient to antibiotic and thermal perturbation and may in fact increase in abundance following antibiotic exposure, suggesting that its dominance is actively maintained and not readily displaced by current disease mitigation strategies.These results indicate that antibiotic intervention is unlikely to be a viable strategy for disrupting Ca. A. rohweri dominance in disease-susceptible A. cervicornis, underscoring the urgency of understanding its transmission routes to inform microbiome rescue efforts.

microbiology↗

bioRxiv2025

H-NS is a conserved repressor of the type VI secretion system in Vibrio fischeri

The type VI secretion system (T6SS) is a broadly distributed interbacterial weapon found in both beneficial and pathogenic bacteria and can enhance a microbes ability to colonize a host. Vibrio fischeri is a beneficial symbiont of fish and squid and a model organism for T6SS function, which is activated in high-viscosity conditions. Previously, we isolated an hns mutant in a transposon screen to identify regulators of the T6SS in the fish symbiont V. fischeri MJ11. The hns gene encodes the DNA-binding protein, H-NS, a conserved global regulator of gene expression that aids in adaptation to changing environments. Quantitative transcriptomes of the hns mutant and parent strains grown in liquid or hydrogel media revealed hns is required for the global transcriptional changes that occur during transition from lower to higher viscosity conditions. Furthermore, T6SS gene transcripts are more abundant in the hns mutant in both conditions, suggesting H-NS represses T6SS in the parent. Single-cell fluorescence microscopy confirmed hns mutant cells make more T6SS weapons in both liquid and hydrogel medium, where the hns mutant is more proficient at killing a competitor strain, compared to the wild-type parent. Finally, disrupting the hns gene in additional light organ isolates resulted in a similar derepression of T6SS, indicating H-NS is a conserved repressor of this interbacterial weapon. This work furthers our understanding of the role of H-NS as a global regulator during environmental shifts in a host-associated bacterial symbiont and expands the list of species where H-NS represses T6SS to include V. fischeri.

microbiology↗

bioRxiv2024

The conserved global regulator H-NS has a strain-specific impact on biofilm formation in Vibrio fischeri symbionts

Strain-level variation among host-associated bacteria often determines host range and the extent to which colonization is beneficial, benign, or pathogenic. Vibrio fischeri is a beneficial symbiont of the light organs of fish and squid with known strain-specific differences that impact host specificity, colonization efficiency, and interbacterial competition. Here, we describe how the conserved global regulator, H-NS, has a strain-specific impact on a critical colonization behavior: biofilm formation. We isolated a mutant of the fish symbiont V. fischeri MJ11 with a transposon insertion in the hns gene. This mutant formed sticky, moderately wrinkled colonies on LBS plates, a condition not known to induce biofilm in this species. A reconstructed hns mutant displayed the same wrinkled colony, which became smooth when hns was complemented in trans, indicating the hns disruption is causal for biofilm formation in MJ11. Transcriptomes revealed differential expression for the syp biofilm locus in the hns mutant, relative to the parent, suggesting biofilm may in part involve SYP polysaccharide. However, enhanced biofilm in the MJ11 hns mutant was not sufficient to allow colonization of a non-native squid host. Finally, moving the hns mutation into other V. fischeri strains, including the squid symbionts ES114 and ES401, and seawater isolate PP3, revealed strain-specific biofilm phenotypes: ES114 and ES401 hns mutants displayed minimal biofilm phenotypes while PP3 hns mutant colonies were more wrinkled than the MJ11 hns mutant. These findings together define H-NS as a novel regulator of V. fischeri symbiotic biofilm and demonstrate key strain specificity in that role. ImportanceThis work, which shows how H-NS has strain-specific impacts on biofilm in Vibrio fischeri, underscores the importance of studying multiple strains, even when examining highly conserved genes and functions. Our observation that knocking out a conserved regulator can result in a wide range of biofilm phenotypes, depending on the isolate, serves as a powerful reminder that strain-level variation is common and worthy of exploration. Indeed, uncovering the mechanisms of strain-specific phenotypic differences is essential to understand drivers of niche differentiation and bacterial evolution. Thus, it is important to carefully match the number and type of strains used in a study with the research question to accurately interpret and extrapolate the results beyond a single genotype. The additional work required for multi-strain studies is often worth the investment of time and resources, as it provides a broader view of the complexity of within-species diversity in microbial systems.

microbiology↗

bioRxiv2024

Flagella are required to coordinately activate competition and host colonization factors in response to a mechanical signal

Bacteria employ antagonistic strategies to eliminate competitors of an ecological niche. Contact-dependent mechanisms, such as the type VI secretion system (T6SS), are prevalent in host-associated bacteria, yet we know relatively little about how T6SS+ strains make contact with competitors in highly viscous environments, such as host mucus. To better understand how cells respond to and contact one another in such environments, we performed a genome-wide transposon mutant screen of the T6SS-wielding beneficial bacterial symbiont, Vibrio fischeri, and identified two sets of genes that are conditionally required for killing. LPS/capsule and flagellar-associated genes do not affect T6SS directly and are therefore not required for interbacterial killing when cell contact is forced yet are necessary for killing in high-viscosity liquid (hydrogel) where cell-cell contact must be biologically mediated. Quantitative transcriptomics revealed that V. fischeri significantly increases expression of both T6SS genes and cell surface modification factors upon transition from low-to high-viscosity media. Consistent with coincubation and fluorescence microscopy data, flagella are not required for T6SS expression in hydrogel. However, flagella play a key role in responding to the physical environment by promoting expression of the surface modification genes identified in our screen, as well as additional functional pathways important for host colonization including uptake of host-relevant iron and carbon sources, and nitric oxide detoxification enzymes. Our findings suggest that flagella may act as a mechanosensor for V. fischeri to coordinately activate competitive strategies and host colonization factors, underscoring the significance of the physical environment in directing complex bacterial behaviors. SignificanceThe physical environment can have dramatic effects on bacterial behavior, but little is known about how mechanical signals impact antagonistic interactions. Symbiotic bacteria use molecular weapons to eliminate competitors for limited space within highly viscous host tissue and mucus. To better understand how the physical environment affects competition and adhesion within eukaryotic hosts, we used quantitative transcriptomics to reveal the flagella-dependent transcriptional response to bacterial transition from lower to a higher viscosity environment. This work revealed the T6SS interbacterial weapon is coordinately activated with host colonization factors, emphasizing the importance of integrating activation of interbacterial weapons into host colonization pathways to enhance a symbionts ability to successfully colonize the host while efficiently eliminating potential competitors from the host niche.

microbiology↗

bioRxiv2023

Nutrient enrichment alters gene expression in 'Ca.' Aquarickettsia rohweri, promoting parasite expansion and horizontal transmission

Ocean warming, disease, and pollution contributed to global declines in coral abundances and diversity. In the Caribbean, corals previously dominated reefs, providing an architectural framework for diverse ecological habitats, but have significantly declined due to infectious microbial disease. Key species like coral Acropora cervicornis, are now considered critically endangered, prompting researchers to focus on scientific endeavors to identify factors that influence coral disease resistance and resilience. We previously showed that disease susceptibility, growth rates, and bleaching risk were all associated with the abundance of a single bacterial parasite, Ca. Aquarickettsia rohweri which proliferates in vivo under nutrient enrichment. Yet how nutrients influence parasite physiology and life history strategies within its host are unknown. We performed microscopy and transcriptomic analyses of Ca. A. rohweri populations during a 6-week nutrient exposure experiment. Microscopy showed that this parasite was abundant in coral tissue and densely packed in mucocytes prior to nutrient enrichment. Ca. A. rohweri energy scavenging genes and those potentially involved in this habitat transition are significantly upregulated during enrichment. Specifically, transcripts involved in signaling, virulence, two-component systems, and nutrient import genes are elevated under higher nutrients. These data support the predicted role of Ca. A. rohweri as a highly active nutrient-responsive A. cervicornis parasite, and provide a glimpse at the mechanism of induced disease susceptibility while implicating nutrient exposure in its horizontal transmission. SignificanceThe coral disease crisis has contributed to global declines in coral abundance and diversity and is exacerbated by environmental stressors like eutrophication. Thus, identifying factors that influence coral disease resistance and resilience is a top priority. The Rickettsiales-like bacterium, Candidatus Aquarickettsia rohweri is ubiquitous coral symbiont that is strongly linked to coral disease susceptibility in staghorn coral, and is undergoing positive selection across the Caribbean. Although Ca. A. rohweri is a putative parasite, little is known about the activity of this bacterium in coral tissue. This work supports the role of Ca. A. rohweri as a highly active, nutrient-responsive parasite and proposes a mechanism for how Ca. A. rohweri contributes to coral disease susceptibility, parasite expansion, and horizontal transmission.

microbiology↗

bioRxiv2022

Cultivation and Fluorescent in situ hybridization suggest that some shipworm species acquire endosymbiotic bacteria through indirect horizontal transmission

Beneficial microbial symbionts provide essential functions for their host from nutrients to defense against disease. Whether hosts acquire their symbionts directly from parents (vertical transmission) or by sampling from the environment (horizontal transmission) can have dramatic impacts on host adaptability and, in the case of ecosystem engineers, ecosystem health. Wood-boring bivalve mollusks (Teredinidae shipworms) act as ecosystem engineers in marine environments, creating habitat out of submerged wood for fish and invertebrates. Essential to shipworm success is their community of endosymbiotic gill bacteria that produce the enzymes necessary for wood digestion. How shipworms acquire their symbionts, however, remains largely unexplored. Using culturing, fluorescence in-situ hybridization, confocal microscopy, and tank experiments, we provide evidence suggesting the mode of symbiont transmission the shipworms for either the shipworm, Lyrodus pedicellatus or Teredo bartschi or both. Symbiotic bacteria were not detected by cultivation or microscopy in brooding larvae within gravid adults or as veliger larvae collected from the water column, but were observed in adult specimens and juveniles that had begun burrowing into wood. These data suggest that the specimens examined have both aposymbiotic and symbiotic life phases and acquire their symbionts through indirect horizontal transmission. Our findings reveal how the long-term brooders L. pedicellatus and/or T. bartschi acquire their gill endosymbionts. IMPORTANCEHow eukaryotic hosts acquire their microbial symbionts can have significant consequences for their ability to adapt to varied environments. Although wood-boring bivalve shipworms have diverse reproductive strategies and are found in unique environments across the globe, little is known about how they transmit their essential gill endosymbionts. We used the closely related shipworms, Lyrodus pedicellatus and/or Teredo bartschi to study how these long-term brooding shipworms acquire their gill endosymbionts. Our work, unlike previous claims for the broadcast spawning species Bankia setacae which reportedly transmits its symbionts directly from parent to offspring, suggests that juvenile L. pedicellatus and/or T. bartschi acquire their symbionts through horizontal transmission rather than directly from their parents. This work reveals the mechanism by which some brooding shipworm species acquire their symbionts, adding to our limited understanding of intracellular symbiont transmission of Teredinidae.

microbiology↗

bioRxiv2021

A large lipoprotein mediates target specificity for T6SS-dependent killing

Interbacterial competition is prevalent in host-associated microbiota, where it can shape community structure and function, impacting host health in both positive and negative ways. However, the factors that permit bacteria to discriminate among their various neighbors for targeted elimination of competitors remain elusive. We identified a specificity factor in Vibrio species that is used to target specific competitors for elimination. Here, we describe this specificity factor, which is associated with the broadly-distributed type VI secretion system (T6SS), by studying symbiotic Vibrio fischeri, which use the T6SS to compete for colonization sites in their squid host. We demonstrate that a large lipoprotein (TasL) allows V. fischeri cells to restrict T6SS-dependent killing to certain genotypes by selectively integrating competitor cells into aggregates while excluding other cell types. TasL is also required for T6SS-dependent competition within juvenile squid, indicating the adhesion factor is active in the host. Because TasL homologs are found in other host-associated bacterial species, this newly-described specificity factor has the potential to impact microbiome structure within diverse hosts.

microbiology↗