Oxygen governs gonococcal microcolony stability by enhancing the interaction force between type IV pili

Collective polarization dynamics in bacterial colonies signify the occurrence of distinct subpopulations

Membrane potential in bacterial systems has been shown to be dynamic and tightly related to survivability at the single-cell level. However, little is known about spatiotemporal patterns of membrane potential in bacterial colonies and biofilms. Here, we discovered a transition from uncorrelated to collective dynamics within colonies formed by the human pathogen Neisseria gonorrhoeae. In freshly assembled colonies, polarization is heterogeneous with instances of transient and uncorrelated hyper- or depolarization of individual cells. As colonies reach a critical size, the polarization behavior transitions to collective dynamics: A hyperpolarized shell forms at the center, travels radially outward, and halts several micrometers from the colony periphery. Once the shell has passed, we detect an influx of potassium correlated with depolarization. Transient hyperpolarization also demarks the transition from volume to surface growth. By combining simulations and the use of an alternative electron acceptor for the respiratory chain, we provide strong evidence that local oxygen gradients shape the collective polarization dynamics. Finally, we show that within the hyperpolarized shell, tolerance against aminoglycoside antibiotics increases. These findings highlight that the polarization pattern can signify the differentiation into distinct subpopulations with different growth rates and antibiotic tolerance.

Mobility of extracellular DNA within gonococcal colonies

Transformation enables bacteria to acquire genetic information from extracellular DNA (eDNA). Close proximity between bacteria in colonies and biofilms may inhibit escape of eDNA from the colony but it also hinders its diffusion between donor and recipient. In this study, we investigate the mobility of DNA within colonies formed by Neisseria gonorrhoeae, and relate it to transformation efficiency. We characterize the penetration dynamics of fluorescent DNA into the colony at a time scale of hours and find that 300 bp fragments diffuse through the colony without hindrance. For DNA length exceeding 3 kbp, a concentration gradient between the edge and the center of the colony develops, indicating hindered diffusion. Accumulation of DNA within the colony increases with increasing DNA length. The presence of the gonococcal DNA uptake sequence (DUS), which mediates specific binding to type 4 pili (T4P) and uptake into the cell, steepens the radial concentration gradient within the colony, suggesting that the DUS reduces DNA mobility. In particular, DNA of N. gonorrhoeae containing multiple DUS is trapped at the periphery. Under conditions, where DUS containing DNA fragments readily enter the colony center, we investigate the efficiency of transformation. We show that despite rapid diffusion of DNA, the transformation is limited to the edge of young colonies. We conclude that DNA mobility depends on DNA length and specific binding mediated by the DUS, resulting in restricted mobility of gonococcal DNA. Yet gonococcal colonies accumulate DNA, and may therefore act as a reservoir for eDNA.

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DNA fragments encompassing the length of a typical operon efficiently penetrate bacterial colonies.

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Bacterial colonies accumulate eDNA with an efficiency that depends on the length and the DNA uptake sequence.

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Genomic DNA from a distinct species spreads efficiently through gonococcal colonies, while gonococcal DNA and DNA from a closely related species are trapped.

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Transformation is most efficient at the periphery of freshly assembled gonococcal colonies.

External Stresses Affect Gonococcal Type 4 Pilus Dynamics

Bacterial type 4 pili (T4P) are extracellular polymers that serve both as adhesins and molecular motors. Functionally, they are involved in adhesion, colony formation, twitching motility, and horizontal gene transfer. T4P of the human pathogen Neisseria gonorrhoeae have been shown to enhance survivability under treatment with antibiotics or hydrogen peroxide. However, little is known about the effect of external stresses on T4P production and motor properties. Here, we address this question by directly visualizing gonococcal T4P dynamics. We show that in the absence of stress gonococci produce T4P at a remarkably high rate of ∼200 T4P min–1. T4P retraction succeeds elongation without detectable time delay. Treatment with azithromycin or ceftriaxone reduces the T4P production rate. RNA sequencing results suggest that reduced piliation is caused by combined downregulation of the complexes required for T4P extrusion from the cell envelope and cellular energy depletion. Various other stresses including inhibitors of cell wall synthesis and DNA replication, as well as hydrogen peroxide and lactic acid, inhibit T4P production. Moreover, hydrogen peroxide and acidic pH strongly affect pilus length and motor function. In summary, we show that gonococcal T4P are highly dynamic and diverse external stresses reduce piliation despite the protective effect of T4P against some of these stresses.

Lactate-induced dispersal of Neisseria meningitidis microcolonies is mediated by changes in cell density and pilus retraction and is influenced by temperature change

Neisseria meningitidis is the etiologic agent of meningococcal meningitis and sepsis. Initial colonization of meningococci to the upper respiratory tract epithelium is crucial for disease development. The colonization occurs in several steps and expression of type IV pili (Tfp) is essential for both attachment and microcolony formation of encapsulated bacteria. Previously, we have shown that host-derived lactate induces synchronized dispersal of meningococcal microcolonies. In this study, we demonstrated that lactate-induced dispersal is dependent on bacterial concentration but not on the quorum sensing system autoinducer-2 or the two-component systems NarP/NarQ, PilR/PilS, NtrY/NtrX, and MisR/MisS. Further, there were no changes in expression of genes related to assembly, elongation, retraction, and modification of Tfp throughout the time course of lactate induction. By using pilT and pptB mutants, however, we found that lactate-induced dispersal was dependent on PilT-retraction but not on phosphoglycerol-modification of Tfp even though the PptB activity was important for preventing re-aggregation post-dispersal. Furthermore, protein synthesis was required for lactate-induced dispersal. Finally, we found that at a lower temperature, lactate-induced dispersal was delayed and unsynchronized, and bacteria reformed microcolonies. We conclude that lactate-induced microcolony dispersal is dependent on bacterial concentration, PilT-dependent Tfp retraction, and protein synthesis and influenced by environmental temperature.

Type IV pilus retraction enables sustained bacteremia and plays a key role in the outcome of meningococcal sepsis in a humanized mouse model

Neisseria meningitidis (the meningococcus) remains a major cause of bacterial meningitis and fatal sepsis. This commensal bacterium of the human nasopharynx can cause invasive diseases when it leaves its niche and reaches the bloodstream. Blood-borne meningococci have the ability to adhere to human endothelial cells and rapidly colonize microvessels. This crucial step enables dissemination into tissues and promotes deregulated inflammation and coagulation, leading to extensive necrotic purpura in the most severe cases. Adhesion to blood vessels relies on type IV pili (TFP). These long filamentous structures are highly dynamic as they can rapidly elongate and retract by the antagonistic action of two ATPases, PilF and PilT. However, the consequences of TFP dynamics on the pathophysiology and the outcome of meningococcal sepsis in vivo have been poorly studied. Here, we show that human graft microvessels are replicative niches for meningococci, that seed the bloodstream and promote sustained bacteremia and lethality in a humanized mouse model. Intriguingly, although pilus-retraction deficient N. meningitidis strain (ΔpilT) efficiently colonizes human graft tissue, this mutant did not promote sustained bacteremia nor induce mouse lethality. This effect was not due to a decreased inflammatory response, nor defects in bacterial clearance by the innate immune system. Rather, TFP-retraction was necessary to promote the release of TFP-dependent contacts between bacteria and, in turn, the detachment from colonized microvessels. The resulting sustained bacteremia was directly correlated with lethality. Altogether, these results demonstrate that pilus retraction plays a key role in the occurrence and outcome of meningococcal sepsis by supporting sustained bacteremia. These findings open new perspectives on the role of circulating bacteria in the pathological alterations leading to lethal sepsis.

Characterization of Actinobacillus seminis biofilm formation

Actinobacillus seminis is an autochthonous gram-negative bacterium that affects reproductive organs, causing epididymitis, low fertility, and occasional abortions in ovine and goats. The virulence factors and the pathogenicity mechanisms of A. seminis have not been clearly elucidated yet. In this work, biofilm production by A. seminis in in vitro assays is described and characterized. After 48-h incubation at 37 °C in trypticase soy broth, A. seminis formed biofilms containing an extracellular matrix comprised mainly of fibrillar material. Microaerophilia or the presence of calcium diminished biofilm formation in approximately 50% and 70%, respectively, but low iron concentrations increased it 40%. Through enzymatic digestion, it was found that proteins were the main component of these biofilms. Structural observations through scanning electron microscopy indicated the presence of a high amount of fibrillar material in which bacteria were immersed. Antibodies against different bacterial surface proteins, such as anti-biofilm matrix and anti-adhesin, diminished biofilm formation in 70% and 25%, respectively; whereas furanone C-30 and LED-209, compounds described as quorum-sensing inhibitors, completely inhibited biofilm formation. In conclusion, environmental conditions can influence strongly biofilm formation in A. seminis, and this could be an advantageous strategy that allows bacteria to persist inside a host.

Bridging Bacteria and the Gut: Functional Aspects of Type IV Pili

Cell-surface-located proteinaceous appendages, such as flagella and fimbriae or pili, are ubiquitous in bacterial communities. Here, we focus on conserved type IV pili (T4P) produced by bacteria in the intestinal tract, one of the most densely populated human ecosystems. Computational analysis revealed that approximately 30% of known intestinal bacteria are predicted to produce T4P. To rationalize how T4P allow intestinal bacteria to interact with their environment, other microbiota members, and host cells, we review their established role in gut commensals and pathogens with respect to adherence, motility, and biofilm formation, as well as protein secretion and DNA uptake. This work indicates that T4P are widely spread among the known members of the intestinal microbiota and that their contribution to human health might be underestimated.

Targeting Type IV pili as an antivirulence strategy against invasive meningococcal disease

Bacterial virulence factors are attractive targets for the development of therapeutics. Type IV pili, which are associated with a remarkable array of properties including motility, the interaction between bacteria and attachment to biotic and abiotic surfaces, represent particularly appealing virulence factor targets. Type IV pili are present in numerous bacterial species and are critical for their pathogenesis. In this study, we report that trifluoperazine and related phenothiazines block functions associated with Type IV pili in different bacterial pathogens, by affecting piliation within minutes. Using Neisseria meningitidis as a paradigm of Gram-negative bacterial pathogens that require Type IV pili for pathogenesis, we show that piliation is sensitive to altered activity of the Na⁺ pumping NADH-ubiquinone oxidoreductase (Na⁺-NQR) complex and that these compounds probably altered the establishment of the sodium gradient. In vivo, these compounds exert a strong protective effect. They reduce meningococcal colonization of the human vessels and prevent subsequent vascular dysfunctions, intravascular coagulation and overwhelming inflammation, the hallmarks of invasive meningococcal infections. Finally, they reduce lethality. This work provides a proof of concept that compounds with activity against bacterial Type IV pili could beneficially participate in the treatment of infections caused by Type IV pilus-expressing bacteria.

Type IV Pilin Post-Translational Modifications Modulate Material Properties of Bacterial Colonies

Bacterial type 4 pili (T4P) are extracellular polymers that initiate the formation of microcolonies and biofilms. T4P continuously elongate and retract. These pilus dynamics crucially affect the local order, shape, and fluidity of microcolonies. The major pilin subunit of the T4P bears multiple post-translational modifications. By interfering with different steps of the pilin glycosylation and phosphoform modification pathways, we investigated the effect of pilin post-translational modification on the shape and dynamics of microcolonies formed by Neisseria gonorrhoeae. Deleting the phosphotransferase responsible for phosphoethanolamine modification at residue serine 68 inhibits shape relaxations of microcolonies after perturbation and causes bacteria carrying the phosphoform modification to segregate to the surface of mixed colonies. We relate these mesoscopic phenotypes to increased attractive forces generated by T4P between cells. Moreover, by deleting genes responsible for the pilin glycan structure, we show that the number of saccharides attached at residue serine 63 affects the ratio between surface tension and viscosity and cause sorting between bacteria carrying different pilin glycoforms. We conclude that different pilin post-translational modifications moderately affect the attractive forces between bacteria but have severe effects on the material properties of microcolonies.

Molecular Motors Govern Liquidlike Ordering and Fusion Dynamics of Bacterial Colonies

Bacteria can adjust the structure of colonies and biofilms to enhance their survival rate under external stress. Here, we explore the link between bacterial interaction forces and colony structure. We show that the activity of extracellular pilus motors enhances local ordering and accelerates fusion dynamics of bacterial colonies. The radial distribution function of mature colonies shows local fluidlike order. The degree and dynamics of ordering are dependent on motor activity. At a larger scale, the fusion dynamics of two colonies shows liquidlike behavior whereby motor activity strongly affects surface tension and viscosity.

Phase and antigenic variation govern competition dynamics through positioning in bacterial colonies

Cellular positioning towards the surface of bacterial colonies and biofilms can enhance dispersal, provide a selective advantage due to increased nutrient and space availability, or shield interior cells from external stresses. Little is known about the molecular mechanisms that govern bacterial positioning. Using the type IV pilus (T4P) of Neisseria gonorrhoeae, we tested the hypothesis that the processes of phase and antigenic variation govern positioning and thus enhance bacterial fitness in expanding gonococcal colonies. By independently tuning growth rate and T4P-mediated interaction forces, we show that the loss of T4P and the subsequent segregation to the front confers a strong selective advantage. Sequencing of the major pilin gene of the spatially segregated sub-populations and an investigation of the spatio-temporal population dynamics was carried out. Our findings indicate that pilin phase and antigenic variation generate a standing variation of pilin sequences within the inoculation zone, while variants associated with a non-piliated phenotype segregate to the front of the growing colony. We conclude that tuning of attractive forces by phase and antigenic variation is a powerful mechanism for governing the dynamics of bacterial colonies.

Host cell-derived lactate functions as an effector molecule in Neisseria meningitidis microcolony dispersal

The development of meningococcal disease, caused by the human pathogen Neisseria meningitidis, is preceded by the colonization of the epithelial layer in the nasopharynx. After initial adhesion to host cells meningococci form aggregates, through pilus-pilus interactions, termed microcolonies from which the bacteria later detach. Dispersal from microcolonies enables access to new colonization sites and facilitates the crossing of the cell barrier; however, this process is poorly understood. In this study, we used live-cell imaging to investigate the process of N. meningitidis microcolony dispersal. We show that direct contact with host cells is not required for microcolony dispersal, instead accumulation of a host-derived effector molecule induces microcolony dispersal. By using a host-cell free approach, we demonstrated that lactate, secreted from host cells, initiate rapid dispersal of microcolonies. Interestingly, metabolic utilization of lactate by the bacteria was not required for induction of dispersal, suggesting that lactate plays a role as a signaling molecule. Furthermore, Neisseria gonorrhoeae microcolony dispersal could also be induced by lactate. These findings reveal a role of host-secreted lactate in microcolony dispersal and virulence of pathogenic Neisseria.

The human restricted pathogen Neisseria meningitidis is a major cause of bacterial meningitis and sepsis worldwide. Colonization of the mucosal layer in the upper respiratory tract is essential to establish invasive disease. The initial interaction with host cells is characterized by bacterial proliferation and adhesion as aggregates, called microcolonies. Detachment from microcolonies in the nasopharyngeal epithelium facilitates crossing of the cell barrier that can result in invasive disease, yet this process is poorly understood. Here we demonstrate that lactate, an abundant molecule in host mucosal environments, induces N. meningitidis microcolony dispersal. Interestingly, metabolic utilization of lactate by the bacteria was not required for the process, suggesting that lactate play a role as a signaling molecule in pathogenic Neisseria. We propose that the microcolony dispersal in pathogenic Neisseria is influenced by environmental concentrations of lactate. These findings will assist in better understanding the transition from asymptomatic carriage to invasive disease.

Role of Cyclic Di-GMP and Exopolysaccharide in Type IV Pilus Dynamics

For Pseudomonas aeruginosa, levels of cyclic di-GMP (c-di-GMP) govern the transition from the planktonic state to biofilm formation. Type IV pili (T4P) are crucial determinants of biofilm structure and dynamics, but it is unknown how levels of c-di-GMP affect pilus dynamics. Here, we scrutinized how c-di-GMP affects molecular motor properties and adhesive behavior of T4P. By means of retraction, T4P generated forces of ∼30 pN. Deletion mutants in the proteins with known roles in biofilm formation, swarming motility, and exopolysaccharide (EPS) production (specifically, the diguanylate cyclases sadC and roeA or the c-di-GMP phosphodiesterase bifA) showed only modest effects on velocity or force of T4P retraction. At high levels of c-di-GMP, the production of exopolysaccharides, particularly of Pel, is upregulated. We found that Pel production strongly enhances T4P-mediated surface adhesion of P. aeruginosa, suggesting that T4P-matrix interactions may be involved in biofilm formation by P. aeruginosa Finally, our data support the previously proposed model of slingshot-like "twitching" motility of P. aeruginosaIMPORTANCE Type IV pili (T4P) play various important roles in the transition of bacteria from the planktonic state to the biofilm state, including surface attachment and surface sensing. Here, we investigate adhesion, dynamics, and force generation of T4P after bacteria engage a surface. Our studies showed that two critical components of biofilm formation by Pseudomonas aeruginosa, T4P and exopolysaccharides, contribute to enhanced T4P-mediated force generation by attached bacteria. These data indicate a crucial role for the coordinated impact of multiple biofilm-promoting factors during the early stages of attachment to a surface. Our data are also consistent with a previous model explaining why pilus-mediated motility in P. aeruginosa results in characteristic "twitching" behavior.

Mutations targeting the plug-domain of the Shewanella oneidensis proton-driven stator allow swimming at increased viscosity and under anaerobic conditions

Shewanella oneidensis MR-1 possesses two different stator units to drive flagellar rotation, the Na⁺ -dependent PomAB stator and the H⁺ -driven MotAB stator, the latter possibly acquired by lateral gene transfer. Although either stator can independently drive swimming through liquid, MotAB-driven motors cannot support efficient motility in structured environments or swimming under anaerobic conditions. Using ΔpomAB cells we isolated spontaneous mutants able to move through soft agar. We show that a mutation that alters the structure of the plug domain in MotB affects motor functions and allows cells to swim through media of increased viscosity and under anaerobic conditions. The number and exchange rates of the mutant stator around the rotor were not significantly different from wild-type stators, suggesting that the number of stators engaged is not the cause of increased swimming efficiency. The swimming speeds of planktonic mutant MotAB-driven cells was reduced, and overexpression of some of these stators caused reduced growth rates, implying that mutant stators not engaged with the rotor allow some proton leakage. The results suggest that the mutations in the MotB plug domain alter the proton interactions with the stator ion channel in a way that both increases torque output and allows swimming at decreased pmf values.

Seminal Plasma Promotes Neisseria gonorrhoeae Aggregation and Biofilm Formation

Neisseria gonorrhoeae causes the human-specific disease gonorrhea and is transmitted from person to person primarily via sexual contact. During transmission, N. gonorrhoeae is often exposed to seminal fluid and must adapt to this change in environment. Previous work demonstrated that seminal fluid facilitates N. gonorrhoeae motility and alters epithelial cell interactions. In this study, exposure to seminal fluid was found to decrease surface adherence of gonococci in a manner that was independent of Opa adhesin proteins or type IV pilus retraction. Semen was also shown to cause dispersal of bacteria that had previously established surface adherence. Although surface adherence decreased, interbacterial interactions were increased by seminal plasma both in long-term static culture and on a cell-to-cell basis over shorter time periods. The result of increased bacterium-bacterium interactions resulted in the formation of microcolonies, an important step in the N. gonorrhoeae infectious process. Seminal fluid also facilitated increased bacterial aggregation in the form of shear-resistant three-dimensional biofilms. These results emphasize the importance of the gonococcal response to the influx of seminal fluid within the genital niche. Further characterization of the N. gonorrhoeae response to semen will advance our understanding of the mechanisms behind the establishment of infection in naive hosts and the process of transmission.

IMPORTANCE

N. gonorrhoeae is the causative agent of the globally prevalent sexually transmitted infection gonorrhea. An understudied aspect of this human-adapted pathogen is the change in bacterial physiology that occurs during sexual transmission. N. gonorrhoeae encounters semen when transmitted from host to host, and it is known that, when N. gonorrhoeae is exposed to seminal fluid, alterations in bacterial motility and type IV pilus arrangement occur. This work extends our previous observations on this modulation of gonococcal physiology by seminal fluid and demonstrates that seminal plasma decreases surface adherence, promotes interbacterial interactions, and enhances biofilm formation.

How Bacteria Use Type IV Pili Machinery on Surfaces

The bacterial type IV pilus (T4P) is a versatile molecular machine with a broad range of functions. Recent advances revealed that the molecular components and the biophysical properties of the machine are well conserved among phylogenetically distant bacterial species. However, its functions are diverse, and include adhesion, motility, and horizontal gene transfer. This review focusses on the role of T4P in surface motility and bacterial interactions. Different species have evolved distinct mechanisms for intracellular coordination of multiple pili and of pili with other motility machines, ranging from physical coordination to biochemical clocks. Coordinated behavior between multiple bacteria on a surface is achieved by active manipulation of surfaces and modulation of pilus-pilus interactions. An emerging picture is that the T4P actively senses and responds to environmental conditions.

Differential interaction forces govern bacterial sorting in early biofilms

Bacterial biofilms can generate micro-heterogeneity in terms of surface structures. However, little is known about the associated changes in the physics of cell–cell interaction and its impact on the architecture of biofilms. In this study, we used the type IV pilus of Neisseria gonorrhoeae to test whether variation of surface structures induces cell-sorting. We show that the rupture forces between pili are fine-tuned by post-translational modification. Bacterial sorting was dependent on pilus post-translational modification and pilus density. Active force generation was necessary for defined morphologies of mixed microcolonies. The observed morphotypes were in remarkable agreement with the differential strength of adhesion hypothesis proposing that a tug-of-war among surface structures of different cells governs cell sorting. We conclude that in early biofilms the density and rupture force of bacterial surface structures can trigger cell sorting based on similar physical principles as in developing embryos.

DOI:http://dx.doi.org/10.7554/eLife.10811.001

Communities of bacterial cells can live together embedded within a slime-like molecular matrix as a biofilm. This allows the bacteria to hide from external stresses. A single bacterium can replicate itself and develop into a biofilm, and over time the bacterial cells in specific regions of the biofilm will start to interact with their neighbors in different ways. These interactions occur via structures on the surface of the bacterial cells, and the differences in these interactions resemble those that occur as cells specialize during the development of animal embryos. Previous research into embryonic development has shown how differences in the physical interactions between embryonic cells are essential for sorting the cells into their correct locations and shaping the embryo. However, little is known about which processes govern the development of biofilms.

Now, Oldewurtel et al. have asked whether differences in the physical interactions between bacteria trigger cell sorting during the early stages of biofilm development. The experiments involved measuring the force required to break the cell–cell connections (called the ‘rupture force’) in biofilms of a bacterium called Neisseria gonorrhoeae. Oldewurtel et al. found that, in agreement with previous predictions, physical interactions were important for sorting bacterial cells into clusters based on the structures on their surfaces. Bacterial cells actively pull on the surface structures of their neighbors, which allows the cells to sort themselves in a tug-of-war fashion. This means that a cell will move in the direction where it can pull the strongest (i.e., in the direction where the rupture force is highest).

While bacteria and embryos use different molecules to generate these pulling forces, these findings indicate that the basic physical principles are similar in both systems. One of the next challenges will be to evaluate how biofilms might benefit from the structures that develop due to cell sorting.

DOI:http://dx.doi.org/10.7554/eLife.10811.002

Pili-Induced Clustering of N. gonorrhoeae Bacteria

Type IV pili (Tfp) are prokaryotic retractable appendages known to mediate surface attachment, motility, and subsequent clustering of cells. Tfp are the main means of motility for Neisseria gonorrhoeae, the causative agent of gonorrhea. Tfp are also involved in formation of the microcolonies, which play a crucial role in the progression of the disease. While motility of individual cells is relatively well understood, little is known about the dynamics of N. gonorrhoeae aggregation. We investigate how individual N. gonorrhoeae cells, initially uniformly dispersed on flat plastic or glass surfaces, agglomerate into spherical microcolonies within hours. We quantify the clustering process by measuring the area fraction covered by the cells, number of cell aggregates, and their average size as a function of time. We observe that the microcolonies are also able to move but their mobility rapidly vanishes as the size of the colony increases. After a certain critical size they become immobile. We propose a simple theoretical model which assumes a pili-pili interaction of cells as the main clustering mechanism. Numerical simulations of the model quantitatively reproduce the experimental data on clustering and thus suggest that the agglomeration process can be entirely explained by the Tfp-mediated interactions. In agreement with this hypothesis mutants lacking pili are not able to form colonies. Moreover, cells with deficient quorum sensing mechanism show similar aggregation as the wild-type bacteria. Therefore, our results demonstrate that pili provide an essential mechanism for colony formation, while additional chemical cues, for example quorum sensing, might be of secondary importance.

Formation and dissolution of bacterial colonies

Many organisms form colonies for a transient period of time to withstand environmental pressure. Bacterial biofilms are a prototypical example of such behavior. Despite significant interest across disciplines, physical mechanisms governing the formation and dissolution of bacterial colonies are still poorly understood. Starting from a kinetic description of motile and interacting cells we derive a hydrodynamic equation for their density on a surface, where most of the kinetic coefficients are estimated from experimental data for N. gonorrhoeae bacteria. We use it to describe the formation of multiple colonies with sizes consistent with experimental observations. Finally, we show how the changes in the cell-to-cell interactions lead to the dissolution of the bacterial colonies. The successful application of kinetic theory to a complex far from equilibrium system such as formation and dissolution of living bacterial colonies potentially paves the way for the physical quantification of the initial stages of biofilm formation.

Multiple Functions of Glutamate Uptake via Meningococcal GltT-GltM L-Glutamate ABC Transporter in Neisseria meningitidis Internalization into Human Brain Microvascular Endothelial Cells

We previously reported that Neisseria meningitidis internalization into human brain microvasocular endothelial cells (HBMEC) was triggered by the influx of extracellular L-glutamate via the GltT-GltM L-glutamate ABC transporter, but the underlying mechanism remained unclear. We found that the ΔgltT ΔgltM invasion defect in assay medium (AM) was alleviated in AM without 10% fetal bovine serum (FBS) [AM(-S)]. The alleviation disappeared again in AM(-S) supplemented with 500 μM glutamate. Glutamate uptake by the ΔgltT ΔgltM mutant was less efficient than that by the wild-type strain, but only upon HBMEC infection. We also observed that both GltT-GltM-dependent invasion and accumulation of ezrin, a key membrane-cytoskeleton linker, were more pronounced when N. meningitidis formed larger colonies on HBMEC under physiological glutamate conditions. These results suggested that GltT-GltM-dependent meningococcal internalization into HBMEC might be induced by the reduced environmental glutamate concentration upon infection. Furthermore, we found that the amount of glutathione within the ΔgltT ΔgltM mutant was much lower than that within the wild-type N. meningitidis strain only upon HBMEC infection and was correlated with intracellular survival. Considering that the L-glutamate obtained via GltT-GltM is utilized as a nutrient in host cells, l-glutamate uptake via GltT-GltM plays multiple roles in N. meningitidis internalization into HBMEC.

Gene Transfer Efficiency in Gonococcal Biofilms: Role of Biofilm Age, Architecture, and Pilin Antigenic Variation

Extracellular DNA is an important structural component of many bacterial biofilms. It is unknown, however, to which extent external DNA is used to transfer genes by means of transformation. Here, we quantified the acquisition of multidrug resistance and visualized its spread under selective and nonselective conditions in biofilms formed by Neisseria gonorrhoeae. The density and architecture of the biofilms were controlled by microstructuring the substratum for bacterial adhesion. Horizontal transfer of antibiotic resistance genes between cocultured strains, each carrying a single resistance, occurred efficiently in early biofilms. The efficiency of gene transfer was higher in early biofilms than between planktonic cells. It was strongly reduced after 24 h and independent of biofilm density. Pilin antigenic variation caused a high fraction of nonpiliated bacteria but was not responsible for the reduced gene transfer at later stages. When selective pressure was applied to dense biofilms using antibiotics at their MIC, the double-resistant bacteria did not show a significant growth advantage. In loosely connected biofilms, the spreading of double-resistant clones was prominent. We conclude that multidrug resistance readily develops in early gonococcal biofilms through horizontal gene transfer. However, selection and spreading of the multiresistant clones are heavily suppressed in dense biofilms.

IMPORTANCE

Biofilms are considered ideal reaction chambers for horizontal gene transfer and development of multidrug resistances. The rate at which genes are exchanged within biofilms is unknown. Here, we quantified the acquisition of double-drug resistance by gene transfer between gonococci with single resistances. At early biofilm stages, the transfer efficiency was higher than for planktonic cells but then decreased with biofilm age. The surface topography affected the architecture of the biofilm. While the efficiency of gene transfer was independent of the architecture, spreading of double-resistant bacteria under selective conditions was strongly enhanced in loose biofilms. We propose that while biofilms help generating multiresistant strains, selection takes place mostly after dispersal from the biofilm.