Pore Constriction Research Articles

Influence of relative density of the granular base soil on filter performance

Perpendicular contact erosion due to poorly designed filters is a frequent hazard for water-retaining structures serving as lifeblood to the community. This phenomenon occurs when the fine particles of a base soil at the contact interface with a coarser material are detached and transported through pores formed by the coarse particles. Therefore, most filter design criteria focus on the gradation of coarse particles or the gradation of pore constrictions. Meanwhile, the parameters of the base soil, such as relative density, are often overlooked. On the one hand, some experts neglect the impact of relative density because perpendicular contact erosion occurs at the interface, where fine particles expose themselves to larger pores. On the other hand, it is a general belief that the more compacted a base soil is, the less susceptible it will be to erosion as the seepage is reduced. This paper discusses this dilemma from a mutual perspective which assesses the influence of relative density from experimental, numerical, and analytical standpoints. The experimental study reveals that there is an optimal relative density which will release the least eroded mass. The influence is crucial as it can change the status of stability to unstable. The physical essence of the phenomenon is expressed by a numerical study at the micro-scale, which investigates the redistribution of flow lines and stress resulting from a particle detachment. The discovery at the micro-scale is confirmed by an analytical evaluation at the macro-scale, which assesses the redistribution of pore constrictions.

Analytical model of flow-through-screen pressure drop for metal wire screens considering the effects of pore structures

The study of pressure flow through wire screens has attracted considerable attention as these screens are employed in various engineering applications. This paper presents an analytical model for a flow-through-screen (FTS) friction factor considering the effects of the pore structures. The pressure drop is quantified using an integrated form of the Forchheimer equation, where permeability is expressed as a function of screen parameters, including pore diameter, tortuosity, constriction factor, and porosity. Each parameter is determined solely from the pore-scale morphology of various weaves, and hence, the accurate expression for each screen is derived without introducing additional fitting constants. The results obtained using the proposed model are corroborated by 1500 + data points from existing literature with a mean absolute relative error of 23.0%, focusing on four types of typical wire screens—Plain Square, Twilled Square, Plain Dutch, and Dutch Twill—and both room-temperature and cryogenic fluids in the Reynolds number range of 0.1<Re<1000.

Self-assembled nanoparticle-coated interfaces: Capillary pressure, shell formation and buckling

HypothesisParticle accumulation at liquid-liquid or liquid-gas interfaces can significantly alter capillary behavior and give rise to unusual interfacial phenomena including the asymmetric macroscopic mechanical response of the interface. ExperimentsThis study explores the accumulation of cetyltrimethylammonium bromide-modified nanoparticles at fluid interfaces and the subsequent mechanical response of nanoparticle-coated droplets during contraction and expansion. Droplet tests involve the simultaneous recording of the droplet shape and the capillary pressure. Complementary single-pore experiments examine the response of particle-coated interfaces as they traverse a pore constriction. FindingsInterfaces promote order. The time-dependent nanoparticle accumulation at the interface is diffusion-controlled. The nanoparticle coated droplets can sustain negative capillary pressure before they buckle. Buckling patterns strongly depend on the boundary conditions: non-slip boundary conditions lead to crumples while slip boundary conditions result in just a few depressions. The particle-coated interface exhibits asymmetric behavior in response to particle-level capillary forces: an “oil droplet in a nanofluid bath” withstands a significantly higher capillary pressure difference than a “nanofluid droplet in an oil bath”. A first-order equilibrium analysis of interaction forces explains the asymmetric response. Single-constriction experiments show that the formation of particle-coated interfaces has a pronounced effect on fluid displacement in porous media.

Water and humic acid transport in graphene-derived membrane: Mechanisms and implications to functional membrane design

Graphene-derived membranes (GDM) have drawn broad attentions due to their superb water permeation and contaminant retention capabilities. In this paper, the transport of model humic acid (HA) and water through GDM is studied using the molecular dynamics simulation approach. By systematically varying the positions of functional groups and membrane pore dimensions, we find that the introduction of functional groups at the slit edge, the basal plane or the interlayer of bilayer graphene nanosheets reduces water permeation through the membranes to significantly different extents. Comparatively, -COO- at the slit edge of GE nanosheet produces the strongest electrostatic interaction for enhanced HA retention and greater water permeability than graphene oxide membranes. Furthermore, HA adsorbed inside GDM may cause membrane fouling due to pore blocking at the slit entrance or pore constriction inside the slit or the interlayer. Meanwhile, the reversibility of membrane fouling depends upon HA desorption, which is easier for HA anchored via functional groups than stacked via π-π interaction onto graphene surfaces. Our findings emphasize that the performance of functional GDM for water treatment is highly dependent upon the specific position of chemical modification, which has been commonly overlooked in previous studies.

New insights into the structure of membrane fouling by biomolecules using comparison with isotherms and ATR-FTIR local quantification

ABSTRACT The objective of this paper was to propose a deepened analyze of a microfiltration membrane fouling by two biomolecules: a protein (Bovine Serum Albumin) and a peptide (Glutathione). In addition to an analysis of flux decline, the mass of biomolecules accumulated on the membrane during filtration was quantified and compared to adsorption experiments, using Fourier Transform Infra Red spectroscopy in Attenuated Total Reflection mode (ATR-FTIR). It was demonstrated that the same quantity of accumulated biomolecules on the apparent membrane area can generate totally different flux declines because of different fouling mechanisms. On the one hand, Glutathione can adsorb in the whole porous media of the membrane, penetrating through the pores, modifying the hydrophilicity at low concentrations and generating pore constriction at high concentrations. On the other hand, BSA organize a dense irreversible fouling in the first minutes of filtration containing a quantity equivalent to more than 45 monolayers, leading to pore blocking and pore constriction. This structure is resistant to rinsing and NaOH cleaning. Then a reversible fouling, containing a quantity equivalent to more than 90 monolayers is settled. The above structure can be removed with an intensive water rinsing and corresponds to a rather porous cake leading to a low resistance to water permeation, whereas the intermediate structure can only be removed with chemical cleaning and has a higher impact on water permeation. The original approach detailed in this paper allowed to go deeper in the understanding of the membrane fouling by soft matter, not detailed in previous papers.

Toward a better understanding of low-frequency electrical relaxation — An enhanced pore space characterization

The relaxation phenomena observed in the electrical low-frequency range (approximately 1 MHz–10 kHz) of natural porous media such as sandstones are often assumed to be directly related to the dominant (modal) pore throat sizes measured, for instance, with mercury intrusion porosimetry. Attempts to establish a universally valid relationship between pore size and peak spectral induced polarization (SIP) relaxation time have failed, considering sandstones from very different origins and featuring great variations in textural and chemical compositions as well as in geometric pore space properties. In addition working with characteristic relaxation times determined in Cole-Cole or Debye decomposition fits to build the relationship have not been successful. In particular, samples with narrow pore throats are often characterized by long SIP relaxation times corresponding to long “characteristic length scales” in these media, assuming that the diffusion coefficients along the electrical double layer were constant. Based on these observations, three different types of SIP relaxation can be distinguished. We have developed a new way of assessing complex pore spaces of very different sandstones in a multimethodical approach to combine the benefits of mercury intrusion porosimetry, micro-computed tomography, and nuclear magnetic resonance. In this way, we achieve much deeper insight into the pore space due to the different resolutions and sensitivities of the applied methods to pore constrictions (throats) and wide pores (pore bodies). We experimentally quantify pore aspect ratios and volume distributions within the two pore regions. We clearly observe systematic differences between three SIP relaxation types identified previously, and we can attribute the SIP peak relaxation times to measured characteristic length scales within our materials. We highlight selected results for a total of nine sandstones. It seems that SIP relaxation behavior depends on the size difference of the narrow pore throats to the wide pore bodies, which increases from SIP type 1 to type 3.

Cryo-EM structure of the volume-regulated anion channel LRRC8D isoform identifies features important for substrate permeation

Members of the leucine-rich repeat-containing 8 (LRRC8) protein family, composed of the five LRRC8A-E isoforms, are pore-forming components of the volume-regulated anion channel (VRAC). LRRC8A and at least one of the other LRRC8 isoforms assemble into heteromers to generate VRAC transport activities. Despite the availability of the LRRC8A structures, the structural basis of how LRRC8 isoforms other than LRRC8A contribute to the functional diversity of VRAC has remained elusive. Here, we present the structure of the human LRRC8D isoform, which enables the permeation of organic substrates through VRAC. The LRRC8D homo-hexamer structure displays a two-fold symmetric arrangement, and together with a structure-based electrophysiological analysis, revealed two key features. The pore constriction on the extracellular side is wider than that in the LRRC8A structures, which may explain the increased permeability of organic substrates. Furthermore, an N-terminal helix protrudes into the pore from the intracellular side and may be critical for gating.

Efficient hexane isomers separation in isoreticular bipyrazolate metal-organic frameworks: The role of pore functionalization

Hydrocarbons separation in petrochemical industries is a key, energy-consuming stage in the manufacture of high-quality added-value products—hence the need for more efficient materials and environmentally friendly methodologies to improve this process. In this context, we have studied the effect of metal-organic frameworks (MOFs) pore functionalization in hexane isomers separation, isolating the robust isoreticular zinc(II) bipyrazolates Zn(BPZ), showing no pore decoration, Zn(Me2BPZ), the pores of which are decorated with apolar methyl groups, and Zn(BPZ(NH2)2), the spacers of which possess polar Lewis-basic functions (H2BPZ = 1H,1′H-4,4′-bipyrazole; H2Me2BPZ = 3,3′-dimethyl-1H,1′H-4,4′-bipyrazole; H2BPZ(NH2)2 = 3,5-diamino-1′H,1′H-4,4′-bipyrazole; DMF = dimethylformamide). After characterizing Zn(BPZ(NH2)2) as per its crystal structure and thermal behaviour, and all the three MOFs as per their textural properties, we investigated, from the experimental and computational points of view, the impact of the square one-dimensional channels decoration on the separation of the hexane isomers, demonstrating the relevance of pore constrictions in the resolution of the title alkanes mixture.

3D-printed surface-patterned ceramic membrane with enhanced performance in crossflow filtration

Appropriate surface patterns on polymer membranes have been demonstrated to have a strong effect on improving the permeability and anti-fouling ability. Herein, for the first time we have developed a surface-patterned alumina ceramic membrane with gradient porous structure using a novel 3D-printing technology. The well-controllable surface-patterned alumina membrane was fabricated by use of 3D-printable ceramic ink through a layer-by-layer coating on a porous substrate, after which the designed patterns were made on the membrane surface by 3D printing the same ink. The 3D-printed line-patterned membrane showed a notable increase in steady flux and a significantly enhanced antifouling ability, especially when the printed lines were perpendicular to the feed flow direction, as evaluated by our experimental and computational fluid dynamics simulation results. Fouling mechanisms were similar between different feed flow directions and evolved from the pore constriction or intermediate pore blocking towards cake filtration. The distance and height of the 3D-printed lines was also revealed to have influences on the fouling performance. Together with the advantages of easily designed structures and a rapid design-to-fabrication process using 3D-printing technology, this study provides an effective new approach to fabricate the advanced ceramic membranes with fouling mitigation for wastewater treatment.

Bistability in the unstable flow of polymer solutions through pore constriction arrays

Polymer solutions are often injected in porous media for applications such as oil recovery and groundwater remediation. As the fluid navigates the tortuous pore space, elastic stresses build up, causing the flow to become unstable at sufficiently large injection rates. However, it is poorly understood how the spatial and temporal characteristics of this unstable flow depend on pore space geometry, which can vary widely between different porous media. We investigate this dependence by systematically varying the spacing between pore constrictions in a one-dimensional ordered array. We find that when the pore spacing is large, unstable eddies form upstream of each constriction, similar to observations of an isolated constriction. By contrast, when the pore spacing is sufficiently small, the flow in the different pores exhibits a surprising bistability, stochastically switching between two distinct unstable flow states. We hypothesize that this unusual behaviour arises from the interplay between elongation and relaxation of polymers as they are advected through the pore space. Consistent with this idea, we find that the flow state in a given pore persists for long times; moreover, flow states are strongly correlated between neighbouring pores. Thus, the characteristics of unstable flow are not determined just by injection conditions and the geometry of the individual pores, but also depend on the spacing between pores. Ultimately, these results help to elucidate the rich array of behaviours that can arise in polymer solution flow through porous media.

Molecular Insights into Distinct Detection Properties of α-Hemolysin, MspA, CsgG, and Aerolysin Nanopore Sensors.

Protein nanopores have been widely used as single-molecule sensors for the detection and characterization of biological polymers such as DNA, RNA, and polypeptides. A variety of protein nanopores with various geometries have been exploited for this purpose, which usually exhibit distinct sensing capabilities, but the underlying molecular mechanism remains elusive. Here, we systematically characterize the molecular transport properties of four widely studied protein nanopores, α-hemolysin, MspA, CsgG, and aerolysin, by extensive molecular dynamics simulations. It is found that a sudden drop in electrostatic potentials occurs at the sole constriction in MspA and CsgG nanopores in contrast to the gradual potential change inside α-hemolysin and aerolysin pores, indicating the crucial role of pore geometry in ionic and molecular transport. We further demonstrate that these protein nanopores exhibit open-pore currents and ssDNA-induced current blockades both in the order MspA > α-hemolysin > CsgG > aerolysin, but an equivalent blockade percentage around 80%. In addition, the substitution of key amino acids at the pore constriction, especially by charged ones, provides an efficient way to modulate the pore electrostatic potential and ionic current. This work sheds new light on the search for high-performance nanopores, engineering of protein nanopores, and design of bioinspired solid-state nanopores.

Internal fouling during microfiltration with foulants of different surface charges

In view of the inevitability of membrane fouling, it is critically important to understand the underlying mechanisms to alleviate fouling and thereby improve the feasibility of membrane-filtration for more applications. This study was targeted at understanding the internal membrane fouling mechanisms by three types of polystyrene (PS) particles (sized at ~ 0.45 μm; different surface charges) during microfiltration with the same polycarbonate track-etched (PCTE) membrane (nominal pore size of 2 μm). Optical coherence tomography (OCT) technique, which was employed to monitor membrane fouling, indicated greater extents of particle deposition in the case of the positive and less negative PS particles, which is consistent with the more severe flux declines relative to the more negative PS particles. The new fouling model developed that additionally accounts for internal cake filtration, on top of the well-reported pore constriction and pore blockage, revealed the fouling mechanisms at play. For the positive PS particles, the attractive particle-membrane interaction led to a more homogeneous layer-by-layer cake growth, which resulted in denser packing in the pore and higher internal cake resistance, and thereby the worst flux decline. In contrast, for the negative PS particles, the repulsive particle-particle and particle-membrane interactions resulted in more random deposition of the particles inside the pores, resulting in more extensive pore constriction and blockage, as well as lower resistance of the internal cake. This study underscored the importance of surface charge in internal fouling and provided insights on the lesser reported internal fouling mechanisms.

Effect of supercritical-CO2 interaction time on the alterations in coal pore structure

The supercritical CO2 (S-CO2) interaction time is a vital parameter in CO2 geo-sequestration in coal seams, which is under-evaluated up to date. Through this study, we provide an experimental insight into this aspect by evaluating the effect of S-CO2 interaction time on the coal pore structural alterations. The results revealed that the S-CO2 pre-exposure has a notable impact on the adsorption capacity of coal, in which short-term (order of hours) and long-term (order of weeks) S-CO2-treated coal show less and higher adsorption capacities, respectively, compared to untreated coal. The pore characterisation results infer that both surface area and pore volume are decreased by about 20% after short-term S-CO2 treatment and then show an approximate linear increment with further longer treatment time. Based on the experimental observations, we propose a conceptual model to explain the temporal alterations, in which we elucidate that the complex temporal alterations in coal pore morphology occur due to the combined effect of multiple factors: 1) S-CO2 adsorption-induced coal matrix swelling and consequent pore shrinkage, that causes physical constriction of pores, reducing the surface area and the pore volume, 2) S-CO2-induced plasticization that extracts and mobilizes the low-molecular-weight hydrocarbons, which are entrapped in pore mouths or necks, increasing the number of accessible pores. The resultant overall alteration at a given S-CO2 pre-exposure time depends on the competitive behaviour and the significance of the multiple processes. The proposed model is supported by 1) measuring the S-CO2 adsorption-induced volumetric swelling of the coal matrix, which concludes that 70% of volumetric swelling has already been completed within 24 h of coal-S-CO2 interaction, causing the pore shrinkage and, 2) analysing the FT-IR spectrums, which confirms the S-CO2-induced bond dissociation and substitution, confirming the re-arrangement of coal macromolecular structure that possibly leads to hydrocarbon mobilization and pore structural alterations.

Dynamic surface tension of xylem sap lipids.

The surface tension of xylem sap has been traditionally assumed to be close to that of the pure water because decreasing surface tension is thought to increase vulnerability to air seeding and embolism. However, xylem sap contains insoluble lipid-based surfactants, which also coat vessel and pit membrane surfaces, where gas bubbles can enter xylem under negative pressure in the process known as air seeding. Because of the insolubility of amphiphilic lipids, the surface tension influencing air seeding in pit pores is not the equilibrium surface tension of extracted bulk sap but the local surface tension at gas-liquid interfaces, which depends dynamically on the local concentration of lipids per surface area. To estimate the dynamic surface tension in lipid layers that line surfaces in the xylem apoplast, we studied the time-dependent and surface area-regulated surface tensions of apoplastic lipids extracted from xylem sap of four woody angiosperm plants using constrained drop surfactometry. Xylem lipids were found to demonstrate potent surface activity, with surface tensions reaching an equilibrium at ~25 mN m-1 and varying between a minimum of 19 mN m-1 and a maximum of 68 mN m-1 when changing the surface area between 50 and 160% around the equilibrium surface area. It is concluded that xylem lipid films in natural conditions most likely range from nonequilibrium metastable conditions of a supersaturated compression state to an undersaturated expansion state, depending on the local surface areas of gas-liquid interfaces. Together with findings that maximum pore constrictions in angiosperm pit membranes are much smaller than previously assumed, low dynamic surface tension in xylem turns out to be entirely compatible with the cohesion-tension and air-seeding theories, as well as with the existence of lipid-coated nanobubbles in xylem sap, and with the range of vulnerabilities to embolism observed in plants.

Function and three-dimensional structure of intervessel pit membranes in angiosperms: a review

ABSTRACT Pit membranes in bordered pits of tracheary elements of angiosperm xylem represent primary cell walls that undergo structural and chemical modifications, not only during cell death but also during and after their role as safety valves for water transport between conduits. Cellulose microfibrils, which are typically grouped in aggregates with a diameter between 20 to 30 nm, make up their main component. While it is clear that pectins and hemicellulose are removed from immature pit membranes during hydrolysis, recent observations of amphiphilic lipids and proteins associated with pit membranes raise important questions about drought-induced embolism formation and spread via air-seeding from gas-filled conduits. Indeed, mechanisms behind air-seeding remain poorly understood, which is due in part to little attention paid to the three-dimensional structure of pit membranes in earlier studies. Based on perfusion experiments and modelling, pore constrictions in fibrous pit membranes are estimated to be well below 50 nm, and typically smaller than 20 nm. Together with the low dynamic surface tensions of amphiphilic lipids at air-water interfaces in pit membranes, 5 to 20 nm pore constrictions are in line with the observed xylem water potentials values that generally induce spread of embolism. Moreover, pit membranes appear to show ideal porous medium properties for sap flow to promote hydraulic efficiency and safety due to their very high porosity (pore volume fraction), with highly interconnected, non-tortuous pore pathways, and the occurrence of multiple pore constrictions within a single pore. This three-dimensional view of pit membranes as mesoporous media may explain the relationship between pit membrane thickness and embolism resistance, but is largely incompatible with earlier, two-dimensional views on air-seeding. It is hypothesised that pit membranes enable water transport under negative pressure by producing stable, surfactant coated nanobubbles while preventing the entry of large bubbles that would cause embolism.

High porosity with tiny pore constrictions and unbending pathways characterize the 3D structure of intervessel pit membranes in angiosperm xylem.

Pit membranes between xylem vessels play a major role in angiosperm water transport. Yet, their three-dimensional (3D) structure as fibrous porous media remains unknown, largely due to technical challenges and sample preparation artefacts. Here, we applied a modelling approach based on thickness measurements of fresh and fully shrunken pit membranes of seven species. Pore constrictions were also investigated visually by perfusing fresh material with colloidal gold particles of known sizes. Based on a shrinkage model, fresh pit membranes showed tiny pore constrictions of ca. 20nm, but a very high porosity (i.e. pore volume fraction) of on average 0.81. Perfusion experiments showed similar pore constrictions in fresh samples, well below 50nm based on transmission electron microscopy. Drying caused a 50% shrinkage of pit membranes, resulting in much smaller pore constrictions. These findings suggest that pit membranes represent a mesoporous medium, with the pore space characterized by multiple constrictions. Constrictions are much smaller than previously assumed, but the pore volume is large and highly interconnected. Pores do not form highly tortuous, bent, or zigzagging pathways. These insights provide a novel view on pit membranes, which is essential to develop a mechanistic, 3D understanding of air-seeding through this porous medium.

Poly (ether sulfone) membrane surface modification using norepinephrine to mitigate fouling

Here we report on the efficacy of poly(norepinephrine) (PNE) coatings to mitigate membrane fouling. We modified 30 kDa poly(ether sulfone) (PES) ultrafiltration (UF) membranes using a facile dip-coating method. Norepinephrine (NE) is a dopamine (DA) analog, having an additional OH group on the ethyl chain. Like dopamine, it polymerizes in mildly oxidizing conditions, and creates robust coatings. PES membranes were modified with 2 g L−1 NE up to 24 h, and then characterized by contact angle measurement, AFM, FTIR, SEM, and XPS. The coating was confirmed by XPS, showing an increase in N1s and decrease in S2p signals. AFM results showed that the PNE coatings were quite smooth, and the contact angles of modified membranes decreased significantly compared to the as-received (AR) membrane, confirming that membrane wettability increased. The ultrapure water permeability of modified membranes increased relative to the AR membrane, and the modification enhanced anti-fouling properties against model foulants (BSA, SA, and SRNOM), and two treated municipal wastewaters. The modified membranes exhibited less energy consumption and lower fouling resistance as compared to the AR membrane. For both treated municipal wastewaters, optimum performance was exhibited with the 3-hr modification. The PNE coatings did not have a significant impact on foulant rejection. Both the two- and the three-mechanism fouling models were able to provide accurate descriptions of measured flux decline; however, the parameters of the three-mechanism model have greater physical significance because pore constriction is considered explicitly. The PNE layer was stable in 0.1 M HCl and 100 ppm NaOCl (3-day treatments) but was removed by 0.01 M NaOH.

Packed bed compression visualisation and flow simulation using an erosion-dilation approach

X-ray computed tomography has been demonstrated to be capable of imaging 1 mL (5 mm diameter, 50 mm height) chromatography packed beds under compression, visualising the 3D structure and measuring changes to geometry of the packing. 1 mL pre-packed columns did not exhibit any structural changes at vendor specified flow rate limits, however cellulose beds did compress at higher flow rates that were imaged before, during and after flow. This was used to visualise and quantitate changes to porosity, tortuosity and permeability based on simulation of flow through the packed bed structure using the imaging data. When using a high flow rate it was found that a decrease in porosity could be measured during compression before reverting after flow had ceased, with corresponding changes to tortuosity and permeability also occurring.X-ray CT imaging of packed beds and individual beads exposed to foulant-rich process streams resulted in considerable image quality loss, associated with residual biological material. In order to address this, digital processing using an erosion-dilation method was applied at bead and bed scales to computationally alter the porosity by adding or removing material from the existing surface to calculate the impact upon tortuosity factor. The eroded and dilated bead volumes of agarose, cellulose and ceramic materials were used to simulate diffusivity whilst mimicking internal bead pore constriction and blocking mechanisms.

Combined Three Mechanisms Models for Membrane Fouling during Microfiltration

Five new mathematical triple fouling models were developed to explore the flux decline behavior during the microfiltration. The first model was developed by the assumption of the successive effects of standard mechanism, intermediate pore blockage and cake formation by using the standard blocking flux expression in the model calculations. The second and third models also obtained by the successive effects of pore constriction, pore blockage and cake formation mechanisms but in the calculation of these models, the Hagen-Poisseuille law for the filtrate flux has been used. For the fourth and fifth models, the classical standard mechanism has been modified by the assumption of zero order time dependent equation for the particle deposition inside the pores. In these models, the zero-order standard mechanism has been used instead of the classical standard mechanism to combine with the pore blockage and then the cake formation mechanism and the Hagen-Poisseuille law. The ability of developed models for the prediction of experimental data for the bovine serum albumin (BSA) filtration was examined. The zero-order standard complete pore blockage-cake formation and the zero-order standard intermediate pore blockage-cake formation models provide fit experimental data, and predict well.

OmpK36-mediated Carbapenem resistance attenuates ST258 Klebsiella pneumoniae in vivo

Carbapenem-resistance in Klebsiella pneumoniae (KP) sequence type ST258 is mediated by carbapenemases (e.g. KPC-2) and loss or modification of the major non-selective porins OmpK35 and OmpK36. However, the mechanism underpinning OmpK36-mediated resistance and consequences of these changes on pathogenicity remain unknown. By solving the crystal structure of a clinical ST258 OmpK36 variant we provide direct structural evidence of pore constriction, mediated by a di-amino acid (Gly115-Asp116) insertion into loop 3, restricting diffusion of both nutrients (e.g. lactose) and Carbapenems. In the presence of KPC-2 this results in a 16-fold increase in MIC to Meropenem. Additionally, the Gly-Asp insertion impairs bacterial growth in lactose-containing medium and confers a significant in vivo fitness cost in a murine model of ventilator-associated pneumonia. Our data suggests that the continuous selective pressure imposed by widespread Carbapenem utilisation in hospital settings drives the expansion of KP expressing Gly-Asp insertion mutants, despite an associated fitness cost.