Development of techniques for the genetic manipulation of the gliding bacteria Lysobacter enzymogenes and Lysobacter brunescens

Endophytic Bacterium Lysobacter firmicutimachus Strain 5-7 Is a Promising Biocontrol Agent Against Rice Seedling Disease Caused by Pythium arrhenomanes in Nursery Trays

Rice root rot disease caused by Pythium spp. is a highly destructive disease in rice nurseries. Biocontrol with endophytic bacteria was developed in this study to control rice seedling diseases. An in planta screening assay revealed that two bacterial endophytes, strains 5-7 and 6-4, displayed strong protection of rice seedlings from attack by Pythium arrhenomanes. Phylogenetic analysis indicated that strain 5-7 is Lysobacter firmicutimachus, while strain 6-4 belongs to the Kitasatospora genus. To quickly evaluate the disease severity of the root system damaged by Pythium spp. in nursery trays, a root surface area measurement assay was developed. By using this measurement, the control efficacy in nursery trays was evaluated, and L. firmicutimachus 5-7 showed promising biocontrol activity against Pythium disease. In a field trial, the two endophytes exhibited significant disease control efficacy on rice brown spot disease caused by Bipolaris oryzae naturally occurring in a commercial nursery field. The two endophytes exhibited multiple enzymatic activities and broad-spectrum antagonistic activities against multiple rice pathogens. The two endophytes colonized the root surface and inside of the root. L. firmicutimachus 5-7 primarily colonized the intercellular space and aerenchyma. Antibiosis is the major mechanism used by strain 5-7 to cause Bipolaris hyphal swelling and inhibit Pythium zoospore germination and sporangium formation, while a hyperparasitism-like phenomenon was found in the interaction of strain 6-4 with Pythium and Bipolaris hyphae. In conclusion, we report the promising biocontrol agent L. firmicutimachus 5-7 and the potential biocontrol agent Kitasatospora sp. 6-4 for disease control of rice seedlings in commercial nursery trays and their possible mechanisms of action.

Deletion of alpB Gene Influences Outer Membrane Vesicles Biogenesis of Lysobacter sp. XL1

Outer membrane vesicles (OMVs) produced by Gram-negative bacteria constitute important factors in defining interactions with the extracellular milieu. Lysobacter sp. XL1 produces OMVs capable of lysing microbial cells due to the presence in their cargo of bacteriolytic protease L5 (AlpB). Although protein L5 has been functionally and biochemically characterized (including aspects of its packing into OMVs), its role in vesicle biogenesis through genetic deletion of alpB had not been studied previously. Here, we have successfully deleted alpB by allelic replacement and show that the alpB deletion mutant produces a significantly lower amount of OMVs that lack bacteriolytic activity and display altered ultrastructural characteristics in relation to the OMVs produced by the wild-type strain. These results confirm that, as previously proposed, protein L5 participates in OMV production through a mechanism that is not yet fully understood.

Novel Method Reveals a Narrow Phylogenetic Distribution of Bacterial Dispersers in Environmental Communities Exposed to Low-Hydration Conditions

In this study, we developed a method that provides profiles of community-level surface dispersal from environmental samples under controlled hydration conditions and enables us to isolate and uncover the diversity of the fastest bacterial dispersers. The method expands on the porous surface model (PSM), previously used to monitor the dispersal of individual bacterial strains in liquid films at the surface of a porous ceramic disc. The novel procedure targets complex communities and captures the dispersed bacteria on a solid medium for growth and detection. The method was first validated by distinguishing motile Pseudomonas putida and Flavobacterium johnsoniae strains from their nonmotile mutants. Applying the method to soil and lake water bacterial communities showed that community-scale dispersal declined as conditions became drier. However, for both communities, dispersal was detected even under low-hydration conditions (matric potential, -3.1 kPa) previously proven too dry for P. putida strain KT2440 motility. We were then able to specifically recover and characterize the fastest dispersers from the inoculated communities. For both soil and lake samples, 16S rRNA gene amplicon sequencing revealed that the fastest dispersers were substantially less diverse than the total communities. The dispersing fraction of the soil microbial community was dominated by Pseudomonas species cells, which increased in abundance under low-hydration conditions, while the dispersing fraction of the lake community was dominated by Aeromonas species cells and, under wet conditions (-0.5 kPa), also by Exiguobacterium species cells. The results gained in this study bring us a step closer to assessing the dispersal ability within complex communities under environmentally relevant conditions.IMPORTANCE Dispersal is a key process of bacterial community assembly, and yet, very few attempts have been made to assess bacterial dispersal at the community level, as the focus has previously been on pure-culture studies. A crucial factor for dispersal in habitats where hydration conditions vary, such as soils, is the thickness of the liquid films surrounding solid surfaces, but little is known about how the ability to disperse in such films varies within bacterial communities. Therefore, we developed a method to profile community dispersal and identify fast dispersers on a rough surface resembling soil surfaces. Our results suggest that within the motile fraction of a bacterial community, only a minority of the bacterial types are able to disperse in the thinnest liquid films. During dry periods, these efficient dispersers can gain a significant fitness advantage through their ability to colonize new habitats ahead of the rest of the community.

Antagonistic Microbial Interactions: Contributions and Potential Applications for Controlling Pathogens in the Aquatic Systems

Despite the active and intense treatment of wastewater, pathogenic microorganisms and viruses are frequently introduced into the aquatic environment. For most human pathogens, however, this is a rather hostile place, where starvation, continuous inactivation, and decay generally occur, rather than successful reproduction. Nevertheless, a great diversity of the pathogenic microorganisms can be detected, in particular, in the surface waters receiving wastewater. Pathogen survival depends majorly on abiotic factors such as irradiation, changes in water ionic strength, temperature, and redox state. In addition, inactivation is enhanced by the biotic interactions in the environment. Although knowledge of the antagonistic biotic interactions has been available since a long time, certain underlying processes and mechanisms still remain unclear. Others are well-appreciated and increasingly are applied to the present research. Our review compiles and discusses the presently known biotic interactions between autochthonous microbes and pathogens introduced into the aquatic environment, including protozoan grazing, virus-induced bacterial cell lysis, antimicrobial substances, and predatory bacteria. An overview is provided on the present knowledge, as well as on the obvious research gaps. Individual processes that appear promising for future applications in the aquatic environment are presented and discussed.

Myxococcus cells respond to elastic forces in their substrate

Elasticotaxis describes the ability of Myxococcus xanthus cells to sense and to respond to elastic forces within an agar gel on which they rest. Within 5 min of the application of stress, each cell begins to reorient its long axis perpendicular to the stress force. The cells then glide in that direction, and the swarm becomes asymmetric. A quantifiable assay for the strength of elasticotaxis is based on the change in swarm shape from circular to elliptic. By using a collection of isogenic motility mutants, it has been found that the ability to respond to stress in agar depends totally on adventurous (A) motility, but not at all on social (S) motility or on the frz genes. In fact, S- mutants (which are moving only by means of A motility) respond to the applied stress more strongly than does the wild type, despite the fact that their spreading rates are slower than that of the wt strain. Based on the swarming and elasticotactic phenotypes of isogenic frizzy strains that were also defective either in A or S motility, frz behaves as if part of the S motility system.