Biology

Event Date: 
Wednesday, March 25, 2015 - 18:15 - 18:30
Institution: 
University of Florida / UWS
Title: 

Candidatus Liberibacter asiaticus encodes a functional salicylic acid hydroxylase which degrades SA and contributes to the suppression of plant defence

Abstract: 

Salicylate (SA) is a plant hormone and plays important roles in plant defence. SA is synthesized in the chloroplast and transmitted in the phloem. SA hydroxylase is a flavoprotein monooxygenase with the enzyme activity of degradation of SA and is a proximal component of the naphthalene degradation pathway in many bacteria. Candidatus Liberibacter asiaticus, the causal agent of the most devastating citrus disease, is phloem limited and encodes a SA hydroxylase. In this study, we have shown that the SA hydroxylase is functional in degrading SA and its analogs. Ca. L. asiaticus infected plants have reduced PR gene (PR1, PR2, and PR5) expression and SA accumulation in Duncan grapefruit and Valencia sweet orange in response to subsequent inoculation with Xanthomonas citri subsp. citri (Xac) Aw, which is nonpathogenic on both citrus varieties. Ca. L. asiaticus also increased citrus susceptibility to subsequent infection by X. citri. The bacterial populations of XacA and XacAw in grapefruit were significantly higher in Ca. L. asiaticus infected plants compared to healthy control. Our data suggest that Ca. L. asiaticus encodes a functional salicylic acid hydroxylase which degrades SA and contributes to the suppression of plant defence. To counteract this virulence mechanism of Ca. L. asiaticus, foliar spray of SA analogs 2, 6-Dichloroisonicotinic acid (INA) and 2,1,3-Benzothiadiazole (BTH) and SA producing bacterial isolates was conducted to control HLB in large scale field trials. Both INA and BTH in combination with selected bacterial strains slowed down the increase of Ca. L. asiaticus titers in planta and HLB disease severity compared to negative control. SA hydroxylase seems to be an ideal target to develop small molecule inhibitors since no human homolog is present and it is not essential for bacterial growth, hence, the possibility of resistance development is minimized.      Salicylate (SA) is a plant hormone and plays important roles in plant defence. SA is synthesized in the chloroplast and transmitted in the phloem. SA hydroxylase is a flavoprotein monooxygenase with the enzyme activity of degradation of SA and is a proximal component of the naphthalene degradation pathway in many bacteria. Candidatus Liberibacter asiaticus, the causal agent of the most devastating citrus disease, is phloem limited and encodes a SA hydroxylase. In this study, we have shown that the SA hydroxylase is functional in degrading SA and its analogs. Ca. L. asiaticus infected plants have reduced PR gene (PR1, PR2, and PR5) expression and SA accumulation in Duncan grapefruit and Valencia sweet orange in response to subsequent inoculation with Xanthomonas citri subsp. citri (Xac) Aw, which is nonpathogenic on both citrus varieties. Ca. L. asiaticus also increased citrus susceptibility to subsequent infection by X. citri. The bacterial populations of XacA and XacAw in grapefruit were significantly higher in Ca. L. asiaticus infected plants compared to healthy control. Our data suggest that Ca. L. asiaticus encodes a functional salicylic acid hydroxylase which degrades SA and contributes to the suppression of plant defence. To counteract this virulence mechanism of Ca. L. asiaticus, foliar spray of SA analogs 2, 6-Dichloroisonicotinic acid (INA) and 2,1,3-Benzothiadiazole (BTH) and SA producing bacterial isolates was conducted to control HLB in large scale field trials. Both INA and BTH in combination with selected bacterial strains slowed down the increase of Ca. L. asiaticus titers in planta and HLB disease severity compared to negative control. SA hydroxylase seems to be an ideal target to develop small molecule inhibitors since no human homolog is present and it is not essential for bacterial growth, hence, the possibility of resistance development is minimized.      

Event Date: 
Wednesday, February 25, 2015 - 18:00 - 18:30
Institution: 
San Diego State University
Title: 

Integrating microbial community dynamics into kelp forest ecosystem models

Abstract: 

Metagenomics has enabled a greater understanding of microbial community dynamics than previously realized and now the challenge is to integrate microbial dynamics into ecological models. My lab takes an ‘omics approach mixed with classical microbiology to identify factors affecting microbial communities and how an altered microbial community will affect macro-organism health and ecosystem functioning. The key habitats are coral reefs and kelp forests. Within the kelp forest, we have started with a culturing approach that has identified novel genomes associated with the giant kelp Macrocystis pyrifera. Phenotypic assessments of these bacteria have identified increase in the microbe’s ability to tolerate copper and resist antibiotics with increasing human activities. We have tested the effects of altered microbial abundance and community composition on survival and development of M. pyrifera gametophytes. Decreasing microbial abundance enhanced M. pyrifera recruitment, increasing zoospore settlement and gametophyte development. Gametophytes reared in microbial communities sampled adjacent to the populated city showed lower survival and growth compared to gametophytes in microbial communities from a remote island. Metagenomics revealed a high abundance of phototrophic and oligotrophic microbes from the island, compared with an abundance of eutrophic microbes adjacent to the city. In addition, microbes adjacent to the city lacked genes that produce quorum signaling molecules, negatively influencing kelp spore settlement. Long term analyses of the microbial communities from the kelp forest have been initiated and we are currently investigating the microbes associated with the water column and kelp surface at two distinct depth. First, at 0.5 m depth where the water is warmer, highly oxygenated and receiving large amounts of carbon from photosynthesis and second, at 15 m depth where the water is under seasonal thermocline, colder, lower in oxygen, and can potentially be exposed to high partial pressure of carbon dioxide. Monthly sampling has revealed microbial number is lower at depth and pCO2 is higher. Metagenomic analysis of these samples is under way. Kelp feeds the ecosystem through degradation and we are currently investigating the effects of microbes on kelp degradation and subsequent nutritional value. We have shown altered microbial communities are detrimental to kelp recruitment and are identifying way of adding these data to ecosystem models.

Event Date: 
Wednesday, February 25, 2015 - 17:30 - 18:00
Institution: 
University of Melbourne
Title: 

Genomic epidemiology of antibiotic resistant bacteria

Abstract: 

Microbial populations contribute to human disease in a variety of ways, both as agents of infection and as healthy components of the microbiome. Genomic approaches can offer deep insights into this hidden microbial world, including revealing the composition of microbial communities, tracking the movement of individual organisms, and illuminating evolutionary changes. Here I will present recent work applying genomic epidemiology to investigate the emergence and spread of antibiotic resistance in a range of important pathogens, including typhoid, dysentery and the emerging hospital superbug Klebsiella.

Event Date: 
Wednesday, February 25, 2015 - 17:00 - 17:30
Institution: 
University of Southern Maine
Title: 

Prochlorococcus: the “invisible forest” in the ocean’s Outback.

Abstract: 

The smallest, most abundant phototroph in the world, Prochlorococcus, dominates the base of the food web in the “Outback” of the world’s oceans, the nutrient-depleted ocean gyres. This unicellular, marine cyanobacterium, unknown only 30 years ago, is an oligotrophic specialist with a streamlined genome and reduced cellular requirement for the limited resources available in this environment. Based on physiological and molecular analyses of isolated strains from different oceans and depths, two broad groupings of Prochlorococcus were characterized: high- and low-light adapted “ecotypes”. Within these broad groupings are many subclades, some of which have been shown to dominate under certain temperature and light conditions. Through additional culture-based studies, my lab has been exploring nutrient physiology and other physiological characteristics that may contribute to the ecology and evolution of other Prochlorococcus subgroups. Some subgroups have the capacity to utilize nitrate, which was not the case for the initial isolates of Prochlorococcus, and others differ in their pigmentation. We have also found that Prochlorococcus regulates its uptake velocity and specific affinity for inorganic and organic phosphorus under P stress conditions. Examining the physiology, ecology and genomics of Prochlorococcus isolates and natural populations is providing insights into how these tiny photosynthesizing cells create a stable, yet invisible forest in the deserts of the world’s oceans.

Event Date: 
Wednesday, January 28, 2015 - 18:00 - 18:15
Institution: 
University of New South Wales
Title: 

Biomining and methanogenesis for resource extraction from asteroids

Abstract: 

As spacecraft fuel is a limited resource, creating a readily available source for hydrocarbon-based fuels in space will reduce launch cost and increase operating time of spacecraft. Biomethanation is viable for Earth-based operations, thus applications in space under controlled conditions have potential. This study proposes a sustainable environment for methanogens on Near-Earth Objects. Vacuum and desiccation effects, at 0.025% Earth atmospheric pressure, are conducted on three bacterial and three Archaea strains to test post-exposure viability. Cell degradation and colony size reduction was quantified for aerobic strains. Adverse effects were exhibited more so in gram-negative than gram-positive strains. Archaea showed limited to no cell degradation, providing evidence that vacuum effects, at these pressures, will have minor effects on in-situ biofuel operations. If successful, a sustainable and cost-effective method of metal extraction and producing methane based fuel reservoirs could revolutionise in-situ resource and fuel resupply of spacecraft, thus enhancing spacefaring capabilities.

Event Date: 
Wednesday, January 28, 2015 - 19:00 - 19:45
Institution: 
University of Sydney
Title: 

The use of genomics in diagnostic and public health microbiology

Abstract: 

Since 2004 technological advances have enabled us to sequence more nucleic acid and generate more data in a shorter amount of time. Decreases in cost per nucleotide sequenced, the initial price of sequencing machines and the complexity of library construction means that whole genome sequencing (WGS) is available in many research labs and an increasing number of public health microbiology labs. I will examine the use of WGS in public health microbiology, particularly the possibility of investigating organisms without culture, the interrogation of genomes where PCR may be unavailable, outbreak investigation, tracking resistance mutations and novel pathogen discovery.

Event Date: 
Wednesday, November 26, 2014 - 19:00 - 19:45
Institution: 
UNSW
Title: 

“The microbial friends and foes of seaweeds”

Abstract: 

 

Seaweeds (macroalgae) form a diverse and ubiquitous group of photosynthetic organisms that play an essential role in many aquatic ecosystems, yet till recently very little was understood with respect to their associated microbiota. We now know that macroalgae are home to a diverse community of microorganisms, that display both temporal and spatial variation yet remain distinct from the surrounding seawater. Symbiotic interactions between marine microorganisms and macroalgae can have both positive (e.g. providing nutrients and morphogenic cues or protection from biofouling) and negative (e.g. disease) outcomes for the host. This talk will give an overview of the microorganisms typically associated with macroalgae with a focus on the bacterial symbionts. Details of how bacteria successfully colonize macroalgal hosts will be discussed with specific examples of the functional role of microbial epiphytes in macroalgal health (including disease) highlighted from a “holobiont” perspective.

 

Event Date: 
Wednesday, November 26, 2014 - 18:15 - 18:30
Institution: 
QAAFI
Title: 

Plant Cell Wall Breakdown in Complex Ecosystems

Abstract: 

Plant cell walls in e.g. whole grains, fruits and vegetables are a major source of dietary fibre (DF) in human diets. Cellulose is a key DF component, and its fermentation in the large intestine also contributes to the extent of nutritional benefits to the host. However our understanding of which microbes actively ferment cellulose in the complex gut environment is minimal. Here we report on the use of isotopically-labelled cellulose as a route to defining microbial fermentation in a complex ecosystem. The ability of the Gram-negative, obligately aerobic, rod-shaped bacteriumGluconacetobacter xylinus, to produce extracellular cellulose in simple fermentation experiments, in the presence of a 13C-labelled carbon source, was exploited to make isotopically labelled cellulose. Scanning electron microscopy (SEM) and nuclear magnetic resonance spectroscopy (NMR) showed no differences in micro-architecture and crystallinity between native and isotopically labelled bacterial cellulose. Fermentability was assessed by an in vitro batch culture system, where anaerobic fermentations with either a pig faecal slurry or minimal medium with a 1: 5 diluted pig faecal inoculum were carried out.  The gas production kinetics was recorded and end-products were analysed. Results indicated that 13C did not alter the fermentability of bacterial cellulose. We are now carrying out DNA-stable isotope probing coupled with high-throughput sequencing, to provide direct information on which microbes from the porcine faecal inoculum actively ferment the substrates. Ultimately, combining such studies will identify mechanisms of plant cell wall breakdown in the human nutritional context and allow for the understanding of gut microbiota responses to molecularly-defined dietary changes.

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