Quorum sensing-related activities of beneficial and pathogenic bacteria have important implications for plant and human health.

Eukaryotic organisms coevolved with microbes from the environment forming holobiotic meta-genomic units. Members of host-associated microbiomes have commensalic, beneficial/symbiotic, or pathogenic phenotypes. More than 100 years ago, Lorenz Hiltner, pioneer of soil microbiology, introduced the term 'Rhizosphere' to characterize the observation that a high density of saprophytic, beneficial, and pathogenic microbes are attracted by root exudates. The balance between these types of microbes decide about the health of the host. Nowadays we know, that for the interaction of microbes with all eukaryotic hosts similar principles and processes of cooperative and competitive functions are in action. Small diffusible molecules like (phyto)hormones, volatiles and quorum sensing signals are examples for mediators of interspecies and cross-kingdom interactions. Quorum sensing of bacteria is mediated by different autoinducible metabolites in a density-dependent manner. In this perspective publication, the role of QS-related activities for the health of hosts will be discussed focussing mostly on N-acyl-homoserine lactones (AHL). It is also considered that in some cases very close phylogenetic relations exist between plant beneficial and opportunistic human pathogenic bacteria. Based on a genome and system-targeted new understanding, sociomicrobiological solutions are possible for the biocontrol of diseases and the health improvement of eukaryotic hosts.

Long-term conservation tillage with reduced nitrogen fertilization intensity can improve winter wheat health via positive plant-microorganism feedback in the rhizosphere.

Microbiome-based solutions are regarded key for sustainable agroecosystems. However, it is unclear how agricultural practices affect the rhizosphere microbiome, plant-microorganism interactions and crop performance under field conditions. Therefore, we installed root observation windows in a winter wheat field cultivated either under long-term mouldboard plough (MP) or cultivator tillage (CT). Each tillage practice was also compared at two nitrogen (N) fertilization intensities, intensive (recommended N-supply with pesticides/growth regulators) or extensive (reduced N-supply, no fungicides/growth regulators). Shoot biomass, root exudates and rhizosphere metabolites, physiological stress indicators, and gene expression were analyzed together with the rhizosphere microbiome (bacterial/archaeal 16S rRNA gene, fungal ITS amplicon, and shotgun metagenome sequencing) shortly before flowering. Compared to MP, the rhizosphere of CT winter wheat contained more primary and secondary metabolites, especially benzoxazinoid derivatives. Potential copiotrophic and plant-beneficial taxa (e.g. Bacillus, Devosia, and Trichoderma) as well as functional genes (e.g. siderophore production, trehalose synthase, and ACC deaminase) were enriched in the CT rhizosphere, suggesting that tillage affected belowground plant-microorganism interactions. In addition, physiological stress markers were suppressed in CT winter wheat compared to MP. In summary, tillage practice was a major driver of crop performance, root deposits, and rhizosphere microbiome interactions, while the N-fertilization intensity was also relevant, but less important.

Long-Term Fertilization Strategy Impacts Rhizoctonia solani-Microbe Interactions in Soil and Rhizosphere and Defense Responses in Lettuce.

The long-term effects of agricultural management such as different fertilization strategies on soil microbiota and soil suppressiveness against plant pathogens are crucial. Therefore, the suppressiveness of soils differing in fertilization history was assessed using two Rhizoctonia solani isolates and their respective host plants (lettuce, sugar beet) in pot experiments. Further, the effects of fertilization history and the pathogen R. solani AG1-IB on the bulk soil, root-associated soil and rhizosphere microbiota of lettuce were analyzed based on amplicon sequencing of the 16S rRNA gene and ITS2 region. Organic fertilization history supported the spread of the soil-borne pathogens compared to long-term mineral fertilization. The fertilization strategy affected bacterial and fungal community composition in the root-associated soil and rhizosphere, respectively, but only the fungal community shifted in response to the inoculated pathogen. The potential plant-beneficial genus Talaromyces was enriched in the rhizosphere by organic fertilization and presence of the pathogen. Moreover, increased expression levels of defense-related genes in shoots of lettuce were observed in the soil with organic fertilization history, both in the absence and presence of the pathogen. This may reflect the enrichment of potential plant-beneficial microorganisms in the rhizosphere, but also pathogen infestation. However, enhanced defense responses resulted in retarded plant growth in the presence of R. solani (plant growth/defense tradeoff).

Novel Pseudomonas sp. SCA7 Promotes Plant Growth in Two Plant Families and Induces Systemic Resistance in Arabidopsis thaliana.

Pseudomonas sp. SCA7, characterized in this study, was isolated from roots of the bread wheat Triticum aestivum. Sequencing and annotation of the complete SCA7 genome revealed that it represents a potential new Pseudomonas sp. with a remarkable repertoire of plant beneficial functions. In vitro and in planta experiments with the reference dicot plant A. thaliana and the original monocot host T. aestivum were conducted to identify the functional properties of SCA7. The isolate was able to colonize roots, modify root architecture, and promote growth in A. thaliana. Moreover, the isolate increased plant fresh weight in T. aestivum under unchallenged conditions. Gene expression analysis of SCA7-inoculated A. thaliana indicated a role of SCA7 in nutrient uptake and priming of plants. Moreover, confrontational assays of SCA7 with fungal and bacterial plant pathogens revealed growth restriction of the pathogens by SCA7 in direct as well as indirect contact. The latter indicated involvement of microbial volatile organic compounds (mVOCs) in this interaction. Gas chromatography-mass spectrometry (GC-MS) analyses revealed 1-undecene as the major mVOC, and octanal and 1,4-undecadiene as minor abundant compounds in the emission pattern of SCA7. Additionally, SCA7 enhanced resistance of A. thaliana against infection with the plant pathogen Pseudomonas syringae pv. tomato DC3000. In line with these results, SA- and JA/ET-related gene expression in A. thaliana during infection with Pst DC3000 was upregulated upon treatment with SCA7, indicating the ability of SCA7 to induce systemic resistance. The thorough characterization of the novel Pseudomonas sp. SCA7 showed a remarkable genomic and functional potential of plant beneficial traits, rendering it a promising candidate for application as a biocontrol or a biostimulation agent.

Impaired microbial N-acyl homoserine lactone signalling increases plant resistance to aphids across variable abiotic and biotic environments.

Beneficial bacteria interact with plants using signalling molecules, such as N-acyl homoserine-lactones (AHLs). Although there is evidence that these molecules affect plant responses to pathogens, few studies have examined their effect on plant-insect and microbiome interactions, especially under variable soil conditions. We investigated the effect of the AHL-producing rhizobacterium Acidovorax radicis and its AHL-negative mutant (does not produce AHLs) on modulating barley (Hordeum vulgare) plant interactions with cereal aphids (Sitobion avenae) and earthworms (Dendrobaena veneta) across variable nutrient soils. Acidovorax radicis inoculation increased plant growth and suppressed aphids, with stronger effects by the AHL-negative mutant. However, effects varied between barley cultivars and the presence of earthworms altered interaction outcomes. Bacteria-induced plant defences differed between cultivars, and aphid exposure, with pathogenesis-related and WRKY pathways partly explaining the ecological effects in the more resistant cultivars. Additionally, we observed few but specific indirect effects via the wider root microbiome where the AHL-mutant strain influenced rare OTU abundances. We conclude that bacterial AHL-signalling disruption affects plant-microbial interactions by inducing different plant pathways, leading to increased insect resistance, also mediated by the surrounding biotic and abiotic environment. Understanding the mechanisms by which beneficial bacteria can reduce insect pests is a key research area for developing effective insect pest management strategies in sustainable agriculture.

Importance of N-Acyl-Homoserine Lactone-Based Quorum Sensing and Quorum Quenching in Pathogen Control and Plant Growth Promotion.

The biological control of plant pathogens is linked to the composition and activity of the plant microbiome. Plant-associated microbiomes co-evolved with land plants, leading to plant holobionts with plant-beneficial microbes but also with plant pathogens. A diverse range of plant-beneficial microbes assists plants to reach their optimal development and growth under both abiotic and biotic stress conditions. Communication within the plant holobiont plays an important role, and besides plant hormonal interactions, quorum-sensing signalling of plant-associated microbes plays a central role. Quorum-sensing (QS) autoinducers, such as N-acyl-homoserine lactones (AHL) of Gram-negative bacteria, cause a pronounced interkingdom signalling effect on plants, provoking priming processes of pathogen defence and insect pest control. However, plant pathogenic bacteria also use QS signalling to optimise their virulence; these QS activities can be controlled by quorum quenching (QQ) and quorum-sensing inhibition (QSI) approaches by accompanying microbes and also by plants. Plant growth-promoting bacteria (PGPB) have also been shown to demonstrate QQ activity. In addition, some PGPB only harbour genes for AHL receptors, so-called luxR-solo genes, which can contribute to plant growth promotion and biological control. The presence of autoinducer solo receptors may reflect ongoing microevolution processes in microbe-plant interactions. Different aspects of QS systems in bacteria-plant interactions of plant-beneficial and pathogenic bacteria will be discussed, and practical applications of bacteria with AHL-producing or -quenching activity; QS signal molecules stimulating pathogen control and plant growth promotion will also be presented.

Genome-Based Characterization of Plant-Associated Rhodococcus qingshengii RL1 Reveals Stress Tolerance and Plant-Microbe Interaction Traits.

Stress tolerant, plant-associated bacteria can play an important role in maintaining a functional plant microbiome and protecting plants against various (a)biotic stresses. Members of the stress tolerant genus Rhodococcus are frequently found in the plant microbiome. Rhodococcus qingshengii RL1 was isolated from Eruca sativa and the complete genome was sequenced, annotated and analyzed using different bioinformatic tools. A special focus was laid on functional analyses of stress tolerance and interactions with plants. The genome annotation of RL1 indicated that it contains a repertoire of genes which could enable it to survive under different abiotic stress conditions for e.g., elevated mercury concentrations, to interact with plants via root colonization, to produce phytohormones and siderophores, to fix nitrogen and to interact with bacterial signaling via a LuxR-solo and quorum quenching. Based on the identified genes, functional analyses were performed in vitro with RL1 under different growth conditions. The R. qingshengii type strain djl6 and a closely related Rhodococcus erythropolis BG43 were included in the experiments to find common and distinct traits between the strains. Genome based phylogenetic analysis of 15 available and complete R. erythropolis and R. qingshengii genome sequences revealed a separation of the R. erythropolis clade in two subgroups. First one harbors only R. erythropolis strains including the R. erythropolis type strain. The second group consisted of the R. qingshengii type strain and a mix of R. qingshengii and R. erythropolis strains indicating that some strains of the second group should be considered for taxonomic re-assignment. However, BG43 was clearly identified as R. erythropolis and RL1 clearly as R. qingshengii and the strains had most tested traits in common, indicating a close functional overlap of traits between the two species.

Adsorption-desorption and leaching behavior of diuron on selected Kenyan agricultural soils.

In this work, the adsorption-desorption dynamics of diuron in three typical Kenyan agricultural soils, Nzoia (NZ), Thika (TH) and Machakos (MK) was investigated. The equilibrium adsorption data, tested against three classical nonlinear adsorption isotherms, was best described by the Freundlich model. The Freundlich adsorption constant, (KF), increased in the order MK > TH > NZ soil. Additionally, the negative Gibb's free energy values indicate the adsorption processes were thermodynamically spontaneous and physical. Multiple linear regression analysis indicated that the adsorption-desorption behavior was controlled by the clay and phosphorus contents of the soil. Phosphorus negatively affected the adsorption of diuron and promoted desorption. The groundwater ubiquity score (GUS) indicated that diuron movement rating in MK soil was 'moderate' while the movement in TH and NZ soils was 'high'.

Distinct rhizomicrobiota assemblages and plant performance in lettuce grown in soils with different agricultural management histories.

A better understanding of factors shaping the rhizosphere microbiota is important for sustainable crop production. We hypothesized that the effect of agricultural management on the soil microbiota is reflected in the assemblage of the rhizosphere microbiota with implications for plant performance. We designed a growth chamber experiment growing the model plant lettuce under controlled conditions in soils of a long-term field experiment with contrasting histories of tillage (mouldboard plough vs cultivator tillage), fertilization intensity (intensive standard nitrogen (N) + pesticides/growth regulators vs extensive reduced N without fungicides/growth regulators), and last standing field crop (rapeseed vs winter wheat). High-throughput sequencing of bacterial and archaeal 16S rRNA genes and fungal ITS2 regions amplified from total community DNA showed that these factors shaped the soil and rhizosphere microbiota of lettuce, however, to different extents among the microbial domains. Pseudomonas and Olpidium were identified as major indicators for agricultural management in the rhizosphere of lettuce. Long-term extensive fertilization history of soils resulted in higher lettuce growth and increased expression of genes involved in plant stress responses compared to intensive fertilization. Our work adds to the increasing knowledge on how soil microbiota can be manipulated by agricultural management practices which could be harnessed for sustainable crop production.

Microbial-Mediated Plant Growth Promotion and Pest Suppression Varies Under Climate Change.

Climate change is altering the dynamics of crop pests and diseases resulting in reduced crop yields. Using beneficial soil bacterial to increase crop health is a quickly developing area in sustainable agriculture, but it is unknown if climate change or interactions with other species could alter their effect. The plant growth-promoting rhizobacterium Acidovorax radicis N35 is known to increase barley (Hordeum vulgare) plant growth under laboratory conditions, and we tested the stability of the plant-bacterial interactions when exposed to elevated carbon dioxide (CO2) and ozone (O3) levels while infesting the aboveground leaves with cereal aphids (Sitobion avenae) and the soil with beneficial earthworms. Acidovorax radicis N35 increased plant growth and reduced insect growth - with greatest effect in a high-stress elevated O3 environment, but reduced effects under elevated CO2. Earthworms promoted both plant and insect growth, but inoculation with A. radicis N35 alleviated some of the earthworm-mediated increase in pest abundance, particularly in the ambient environment. The consistency of these beneficial effects highlights the potential of exploiting local species interactions for predicting and mitigating climate change effects in managed systems. We conclude that microbial bioprotectants have high potential for benefiting agriculture via plant-growth promotion and pest suppression.

Genome-based reclassification of Azospirillum brasilense Sp245 as the type strain of Azospirillum baldaniorum sp. nov.

Azospirillum sp. strain Sp245T, originally identified as belonging to Azospirillum brasilense, is recognized as a plant-growth-promoting rhizobacterium due to its ability to fix atmospheric nitrogen and to produce plant-beneficial compounds. Azospirillum sp. Sp245T and other related strains were isolated from the root surfaces of different plants in Brazil. Cells are Gram-negative, curved or slightly curved rods, and motile with polar and lateral flagella. Their growth temperature varies between 20 to 38 °C and their carbon source utilization is similar to other Azospirillum species. A preliminary 16S rRNA sequence analysis showed that the new species is closely related to A. brasilense Sp7T and A. formosense CC-Nfb-7T. Housekeeping genes revealed that Azospirillum sp. Sp245T, BR 12001 and Vi22 form a separate cluster from strain A. formosense CC-Nfb-7T, and a group of strains closely related to A. brasilense Sp7T. Overall genome relatedness index (OGRI) analyses estimated based on average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) between Azospirillum sp. Sp245T and its close relatives to other Azospirillum species type strains, such as A. brasilense Sp7T and A. formosense CC-Nfb-7T , revealed values lower than the limit of species circumscription. Moreover, core-proteome phylogeny including 1079 common shared proteins showed the independent clusterization of A. brasilense Sp7T, A. formosense CC-Nfb-7T and Azospirillum sp. Sp245T, a finding that was corroborated by the genome clustering of OGRI values and housekeeping phylogenies. The DNA G+C content of the cluster of Sp245T was 68.4-68.6 %. Based on the phylogenetic, genomic, phenotypical and physiological analysis, we propose that strain Sp245T together with the strains Vi22 and BR12001 represent a novel species of the genus Azospirillum, for which the name Azospirillum baldaniorum sp. nov. is proposed. The type strain is Sp245T (=BR 11005T=IBPPM 219T) (GCF_007827915.1, GCF_000237365.1, and GCF_003119195.2).

Long-Term Fluctuation of Oral Biofilm Microbiota following Different Dietary Phases.

Caries development is associated with shifts in the oral biofilm microbiota and primarily linked to frequent simple carbohydrate consumption. Different nutritional ingredients can either promote or prevent caries development. To investigate the effects of selected ingredients on the oral biofilm microbiota in situ, 11 study participants underwent 3-month-long dietary phases with intake of a regular diet (PI), additional frequent sucrose (PII), milk and yoghurt (PIII), and a diet rich in dietary fiber (PIV) and then returned to their regular diet (PV). Oral biofilm was sampled and analyzed applying 16S rRNA Illumina MiSeq sequencing. Additionally, the effect on the enamel was analyzed by measuring enamel surface roughness with laser scanning microscopy. The beta-diversity results showed that the microbiota in all the following phases differed significantly from PI and that the microbial community in PII was significantly different from all other phases. The abundance of the genus Streptococcus fluctuated over the course of the five phases, with a significant increase in PII (P = 0.01), decreasing in PIII and PIV (PIII and PIV versus PII: P < 0.00001) and increasing again toward PV. Other taxa showed various fluctuations of their abundances, with PV returning approximately to the levels of PI. In conclusion, while elevated sucrose consumption favored caries-promoting non-mutans streptococci, frequent milk and yoghurt intake caused a significant decrease in the abundance of these microbial taxa and in addition reduced enamel surface roughness. These results indicate that modulations of the oral biofilm microbiota can be attained even in adults through dietary changes and corresponding recommendations can be made for the prevention of caries development.IMPORTANCE Caries affects a large proportion of the population worldwide, resulting in high treatment costs. Its etiology can be ascribed to shifts of the microbiota in dental biofilms primarily driven by dietary factors. It is unclear how diet affects the microbial community of plaque biofilm in situ and whether it can be modulated to help prevent caries development. To address these issues, we analyzed changes of the in situ plaque microbiota following 3-month-long dietary changes involving elevated sucrose, dairy, and dietary fiber consumption over a period of 15 months. Applying high-throughput sequencing, we found non-mutans streptococci, a taxonomic group involved in the beginning stages toward microbial dysbiosis, in decreased abundance with elevated dairy and dietary fiber intake. Through analysis of the enamel surface roughness, these effects were confirmed. Therefore, correspondent dietary measures can be recommended for children as well as adults for caries prevention.

Impact of Long-Term Organic and Mineral Fertilization on Rhizosphere Metabolites, Root-Microbial Interactions and Plant Health of Lettuce.

Fertilization management can affect plant performance and soil microbiota, involving still poorly understood rhizosphere interactions. We hypothesized that fertilization practice exerts specific effects on rhizodeposition with consequences for recruitment of rhizosphere microbiota and plant performance. To address this hypothesis, we conducted a minirhizotron experiment using lettuce as model plant and field soils with contrasting properties from two long-term field experiments (HUB-LTE: loamy sand, DOK-LTE: silty loam) with organic and mineral fertilization history. Increased relative abundance of plant-beneficial arbuscular mycorrhizal fungi and fungal pathotrophs were characteristic of the rhizospheres in the organically managed soils (HU-org; BIODYN2). Accordingly, defense-related genes were systemically expressed in shoot tissues of the respective plants. As a site-specific effect, high relative occurrence of the fungal lettuce pathogen Olpidium sp. (76-90%) was recorded in the rhizosphere, both under long-term organic and mineral fertilization at the DOK-LTE site, likely supporting Olpidium infection due to a lower water drainage potential compared to the sandy HUB-LTE soils. However, plant growth depressions and Olpidium infection were exclusively recorded in the BIODYN2 soil with organic fertilization history. This was associated with a drastic (87-97%) reduction in rhizosphere abundance of potentially plant-beneficial microbiota (Pseudomonadaceae, Mortierella elongata) and reduced concentrations of the antifungal root exudate benzoate, known to be increased in presence of Pseudomonas spp. In contrast, high relative abundance of Pseudomonadaceae (Gammaproteobacteria) in the rhizosphere of plants grown in soils with long-term mineral fertilization (61-74%) coincided with high rhizosphere concentrations of chemotactic dicarboxylates (succinate, malate) and a high C (sugar)/N (amino acid) ratio, known to support the growth of Gammaproteobacteria. This was related with generally lower systemic expression of plant defense genes as compared with organic fertilization history. Our results suggest a complex network of belowground interactions among root exudates, site-specific factors and rhizosphere microbiota, modulating the impact of fertilization management with consequences for plant health and performance.

De Novo Genome Assembly of a Plant-Associated Rhodococcus qingshengii Strain (RL1) Isolated from Eruca sativa Mill. and Showing Plant Growth-Promoting Properties.

Rhodococcus qingshengii RL1 was isolated from surface-sterilized leaves of Eruca sativa Mill. and shows plant growth-promoting (PGP) properties. The de novo genome assembly consists of one chromosome with 6,253,838 bp and two plasmids with 144,038 bp and 448,745 bp. Many genes could be identified reflecting its PGP potential.

Assessment of the structural and functional diversities of plant microbiota: Achievements and challenges - A review.

Analyses of the spatial localization and the functions of bacteria in host plant habitats through in situ identification by immunological and molecular genetic techniques combined with high resolving microscopic tools and 3D-image analysis contributed substantially to a better understanding of the functional interplay of the microbiota in plants. Among the molecular genetic methods, 16S-rRNA genes were of central importance to reconstruct the phylogeny of newly isolated bacteria and to localize them in situ. However, they usually do not allow resolution for phylogenetic affiliations below genus level. Especially, the separation of opportunistic human pathogens from plant beneficial strains, currently allocated to the same species, needs genome-based resolving techniques. Whole bacterial genome sequences allow to discriminate phylogenetically closely related strains. In addition, complete genome sequences enable strain-specific monitoring for biotechnologically relevant strains. In this mini-review we present high resolving approaches for analysis of the composition and key functions of plant microbiota, focusing on interactions of diazotrophic plant growth promoting bacteria, like Azospirillum brasilense, with non-legume host plants. Combining high resolving microscopic analyses with specific immunological detection methods and molecular genetic tools, including especially transcriptome analyses of both the bacterial and plant partners, enables new insights into key traits of beneficial bacteria-plant interactions in holobiontic systems.

Systems Biology of Plant-Microbiome Interactions.

In natural environments, plants are exposed to diverse microbiota that they interact with in complex ways. While plant-pathogen interactions have been intensely studied to understand defense mechanisms in plants, many microbes and microbial communities can have substantial beneficial effects on their plant host. Such beneficial effects include improved acquisition of nutrients, accelerated growth, resilience against pathogens, and improved resistance against abiotic stress conditions such as heat, drought, and salinity. However, the beneficial effects of bacterial strains or consortia on their host are often cultivar and species specific, posing an obstacle to their general application. Remarkably, many of the signals that trigger plant immune responses are molecularly highly similar and often identical in pathogenic and beneficial microbes. Thus, it is unclear what determines the outcome of a particular microbe-host interaction and which factors enable plants to distinguish beneficials from pathogens. To unravel the complex network of genetic, microbial, and metabolic interactions, including the signaling events mediating microbe-host interactions, comprehensive quantitative systems biology approaches will be needed.

Effect of long-term organic and mineral fertilization strategies on rhizosphere microbiota assemblage and performance of lettuce.

Long-term agricultural fertilization strategies gradually change soil properties including the associated microbial communities. Cultivated crops recruit beneficial microbes from the surrounding soil environment via root exudates. In this study, we aimed to investigate the effects of long-term fertilization strategies across field sites on the rhizosphere prokaryotic (Bacteria and Archaea) community composition and plant performance. We conducted growth chamber experiments with lettuce (Lactuca sativa L.) cultivated in soils from two long-term field experiments, each of which compared organic versus mineral fertilization strategies. 16S rRNA gene amplicon sequencing revealed the assemblage of a rhizosphere core microbiota shared in all lettuce plants across soils, going beyond differences in community composition depending on field site and fertilization strategies. The enhanced expression of several plant genes with roles in oxidative and biotic stress signalling pathways in lettuce grown in soils with organic indicates an induced physiological status in plants. Lettuce plants grown in soils with different fertilization histories were visibly free of stress symptoms and achieved comparable biomass. This suggests a positive aboveground plant response to belowground plant-microbe interactions in the rhizosphere. Besides effects of fertilization strategy and field site, our results demonstrate the crucial role of the plant in driving rhizosphere microbiota assemblage.

In-vivo shift of the microbiota in oral biofilm in response to frequent sucrose consumption.

Caries is associated with shifts of microbiota in dental biofilms and primarily driven by frequent sucrose consumption. Data on environmentally induced in vivo microbiota shifts are scarce therefore we investigated the influence of frequent sucrose consumption on the oral biofilm. Splint systems containing enamel slabs were worn for 3 × 7 days with 7-day intervals to obtain oral biofilm samples. After a three-month dietary change of sucking 10 g of sucrose per day in addition to the regular diet, biofilm was obtained again at the end of the second phase. The microbiota was analysed using Illumina MiSeq amplicon sequencing (v1-v2 region). In addition, roughness of the enamel surface was measured with laser scanning microscopy. The sucrose phase resulted in significant differences in beta-diversity and significantly decreased species richness. It was marked by a significant increase in abundance of streptococci, specifically Streptococcus gordonii, Streptococcus parasanguinis and Streptococcus sanguinis. Enamel surface roughness began to increase, reflecting initial impairment of dental enamel surface. The results showed that frequent sucrose consumption provoked compositional changes in the microbiota, leading to an increase of non-mutans streptococci, hence supporting the extended ecological plaque hypothesis and emphasizing the synergy of multiple bacterial species in the development of caries.

Listeria monocytogenes - Danger for health safety vegetable production.

The microbiologically contaminated vegetables represent a risk for consumers, especially vegetables without thermal processing. It is known that human pathogen bacteria, such as Listeria monocytogenes, could exist on fresh vegetables. The fresh vegetables could become Listeria-contaminated if they come in touch with contaminated soil, manure, irrigation water. The aim of this work was to investigate the presence of Listeria spp. and L. monocytogenes in different kind of vegetables grown in field and greenhouse condition as well as surface and endophytic colonization plant roots of different vegetables species by L. monocytogenes in laboratory conditions. The detection of Listeria spp. and L. monocytogenes in vegetable samples was done using ISO and PCR methods. The investigation of colonization vegetable roots and detection Listeria-cells inside plant root tissue was done using Fluorescence in situ hybridization (FISH) method in combination with confocal laser scanning microscopy (CLSM). The results showed that 25.58% vegetable samples were positive for Listeria spp. and only one sample (carrot) was positive for L. monocytogenes out of 43 samples in total collected from field and greenhouse. The strain L. monocytogenes EGD-E surface and endophytic colonized carrot root in highest degree while strain L. monocytogenes SV4B was the most represented at leafy vegetable plants, such at lettuce (1.68 × 106 cells/mm3 absolutely dry root) and spinach (1.39 × 106 cells/mm3 absolutely dry root) root surface. The cells of L. monocytogenes SV4B were visible as single cells in interior tissue of plant roots (celery and sweet corn roots) as well as in the interior of the plant root cell at sweet corn root. The cells of L. monocytogenes EGD-E bind to the surface of the plant root and they were less commonly found out on root hair. In the inner layers of the root, those bacterial cells were inhabited intercellular spaces mainly as single cells very close to the larval vessels of root. Our results suggest that L. monocytogenes is very good endophytic colonizer of vegetable plant roots.