Role of multicellular aggregates in biofilm formation

In traditional models of in vitro biofilm development, individual bacterial cells seed a surface, multiply, and mature into multicellular, three-dimensional structures. Much research has been devoted to elucidating the mechanisms governing the initial attachment of single cells to surfaces. However, in natural environments and during infection, bacterial cells tend to clump as multicellular aggregates, and biofilms can also slough off aggregates as a part of the dispersal process. This makes it likely that biofilms are often seeded by aggregates and single cells, yet how these aggregates impact biofilm initiation and development is not known. Here we use a combination of experimental and computational approaches to determine the relative fitness of single cells and preformed aggregates during early development of Pseudomonas aeruginosa biofilms. We find that the relative fitness of aggregates depends markedly on the density of surrounding single cells, i.e., the level of competition for growth resources. When competition between aggregates and single cells is low, an aggregate has a growth disadvantage because the aggregate interior has poor access to growth resources. However, if competition is high, aggregates exhibit higher fitness, because extending vertically above the surface gives cells at the top of aggregates better access to growth resources. Other advantages of seeding by aggregates, such as earlier switching to a biofilm-like phenotype and enhanced resilience toward antibiotics and immune response, may add to this ecological benefit. Our findings suggest that current models of biofilm formation should be reconsidered to incorporate the role of aggregates in biofilm initiation

Surface sensing for biofilm formation in Pseudomonas aeruginosa

YesAggregating and forming biofilms on biotic or abiotic surfaces are ubiquitous bacterial behaviors under various conditions. In clinical settings, persistent presence of biofilms increases the risks of healthcare-associated infections and imposes huge healthcare and economic burdens. Bacteria within biofilms are protected from external damage and attacks from the host immune system and can exchange genomic information including antibiotic-resistance genes. Dispersed bacterial cells from attached biofilms on medical devices or host tissues may also serve as the origin of further infections. Understanding how bacteria develop biofilms is pertinent to tackle biofilm-associated infections and transmission. Biofilms have been suggested as a continuum of growth modes for adapting to different environments, initiating from bacterial cells sensing their attachment to a surface and then switching cellular physiological status for mature biofilm development. It is crucial to understand bacterial gene regulatory networks and decision-making processes for biofilm formation upon initial surface attachment. Pseudomonas aeruginosa is one of the model microorganisms for studying bacterial population behaviors. Several hypotheses and studies have suggested that extracellular macromolecules and appendages play important roles in bacterial responses to the surface attachment. Here, I review recent studies on potential molecular mechanisms and signal transduction pathways for P. aeruginosa surface sensing.This work is supported by University of Bradfor

Mechanosensing of shear by Pseudomonas aeruginosa leads to increased levels of the cyclic-di-GMP signal initiating biofilm development

Biofilms are communities of sessile microbes that are phenotypically distinct from their genetically identical, free-swimming counterparts. Biofilms initiate when bacteria attach to a solid surface. Attachment triggers intracellular signaling to change gene expression from the planktonic to the biofilm phenotype. For Pseudomonas aeruginosa, it has long been known that intracellular levels of the signal cyclic-di-GMP increase upon surface adhesion and that this is required to begin biofilm development. However, what cue is sensed to notify bacteria that they are attached to the surface has not been known. Here, we show that mechanical shear acts as a cue for surface adhesion and activates cyclic-di-GMP signaling. The magnitude of the shear force, and thereby the corresponding activation of cyclic-di-GMP signaling, can be adjusted both by varying the strength of the adhesion that binds bacteria to the surface and by varying the rate of fluid flow over surface-bound bacteria. We show that the envelope protein PilY1 and functional type IV pili are required mechanosensory elements. An analytic model that accounts for the feedback between mechanosensors, cyclic-di-GMP signaling, and production of adhesive polysaccharides describes our data well

Mechanosensing and early biofilm development in Pseudomonas aeruginosa

Biofilms are communities of sessile microbes that are phenotypically distinct from their genetically-identical, free-swimming counterparts. Biofilms initiate when bacteria attach to a solid surface, as this attachment triggers intracellular signaling to change gene expression of individual bacteria from the planktonic to the biofilm phenotype. However the initial cues leading allowing bacteria to sense a surface, as well as the role of spatial structure in biofilm development, are not well known. This dissertation has two main parts, the first presenting a method for growing biofilms from initiating cells whose positions are controlled with single-cell precision using laser trapping. Biofilm infections are notoriously intractable, in part due to changes in the bacterial phenotype that result from spatial structure. Understanding the role of structure in biofilm development requires methods to control the spatial structure of biofilms. The native growth, motility, and surface adhesion of positioned microbes are preserved, as we show for model organisms Pseudomonas aeruginosa and Staphylococcus aureus. We demonstrate that laser-trapping and placing bacteria on surfaces can reveal the e↵ects of spatial structure on bacterial growth in early biofilm development. In the second part we show that mechanical shear acts as a cue for surface adhesion in P. aeruginosa. For P. aeruginosa, it has long been known that intracellular levels of the signaling molecule cyclic-di-GMP increase upon surface adhesion and that increased cyclic-di-GMP is required to begin biofilm development. The magnitude of the shear force, and thereby the corresponding activation of cyclic-di-GMP signaling, can be adjusted both by varying the strength of the adhesion that binds bacteria to the surface and by varying the rate of fluid flow over surface-bound bacteria. We show that the envelope protein PilY1 and functional Type IV pili are required mechanosensory elements. Finally, we propose an analytic model that accounts for the feedback between mechanosensors, cyclic-di-GMP signaling, and production of adhesive polysaccharides, describing our dataPhysic

Provenance and Composition of Impactite Sands; AU Drill Core #09-04, Wetumpka Impact Structure, Alabama

The Wetumpka impact structure is a Late Cretaceous shallow-marine impact crater about 6 km in diameter located in central Alabama. The target consisted of Upper Cretaceous sediments that were unconformably overlying Piedmont schists and gneisses. An arcuate crystalline crater rim is surrounded on the east and northeast by Upper Cretaceous sedimentary units, on the north by Piedmont basement, and on the west by Quaternary alluvium. There are several shallow drill cores at Wetumpka, including Auburn University drill core #09-04, which penetrated a depth of 217.7 m (715 feet) near the southeastern portion of the rim. The upper ~ 60 m (197 feet) of core is interpreted as a segment of slumped, overturned sedimentary section of megablocks that was formerly on the rim. Below this overturned section are 152 m (500 feet) of impactite sands with sedimentary blocks. The objective of the present project is to determine the provenance of the nearly 152 m (500 feet) of impactite sand in the lower part of drill core #09-04. Thin-sections were made from 43 samples taken from impactite sand intervals in the lower portion of the drill core. Fining-upward trends were detected in eight intervals and this pattern is interpreted as the result of an aqueous settling process. Point-counting and statistical analysis of framework grain characteristics within the loosely consolidated sands indicate that the grains do not originate from a single target unit and provide reasonable evidence that they are derived from a mixture of the sedimentary and metamorphic target units