Interactive forces between co-aggregating and non-co-aggregating oral bacterial pairs

Evaluation of extracellular polymeric substances matrix volume, surface roughness and bacterial adhesion property of oral biofilm

Background/purpose

Oral biofilms are highly structured bacterial colonies embedded in a highly hydrated extracellular polymeric substances (EPS) matrix. This study aimed to investigate the characteristics of oral biofilm at different stages of maturation.

Materials and methods

Oral multispecies biofilms were grown anaerobically from plaque bacteria on collagen coated hydroxyapatite discs in brain heart infusion broth for one and three weeks. The volume of live bacteria and EPS matrix of the biofilms were determined by using corresponding fluorescent probes and confocal laser scanning microscopy. Atomic force microscopy (AFM) was used to quantitatively probe and correlate cell surface adhesion force of biofilms. The surface roughness was quantified in terms of the root mean square average of the height deviations. Adhesion was measured from force-distance data for the retraction of the cell from the surface.

Results

The volume of live bacteria and EPS of 3-week-old biofilms was higher than 1-week-old biofilms. The surface roughness value in 1-week-old biofilms was significantly higher than that in 3-week-old biofilms. AFM force-distance curve results showed that the adhesion force at the cell-cell interface was significantly more at-tractive than those at bacterial cells surface of both stages biofilms. Adhesion forces between the AFM tip and the surface of bacterial cell were fairly constant, whereas the cell-cell interface experienced greater adhesion forces in the biofilm's development.

Conclusion

As oral biofilms become mature, EPS volume and cell-cell adhesion forces increase while the surface roughness decreases.

Bacterial coaggregation in aquatic systems

The establishment of a sessile community is believed to occur in a sequence of steps where genetically distinct bacteria can become attached to partner cells via specific molecules, in a process known as coaggregation. The presence of bacteria with the ability to autoaggregate and coaggregate has been described for diverse aquatic systems, particularly freshwater, drinking water, wastewater, and marine water. In these aquatic systems, coaggregation already demonstrated a role in the development of complex multispecies sessile communities, including biofilms. While specific molecular aspects on coaggregation in aquatic systems remain to be understood, clear evidence exist on the impact of this mechanism in multispecies biofilm resilience and homeostasis. The identification of bridging bacteria among coaggregating consortia has potential to improve the performance of wastewater treatment plants and/or to contribute for the development of strategies to control undesirable biofilms. This study provides a comprehensive analysis on the occurrence and role of bacterial coaggregation in diverse aquatic systems. The potential of this mechanism in water-related biotechnology is further described, with particular emphasis on the role of bridging bacteria.

Antibiotic Resistance Crisis: An Update on Antagonistic Interactions between Probiotics and Methicillin-Resistant Staphylococcus aureus (MRSA)

Antimicrobial resistance (AMR) havoc is a global multifaceted crisis endowing a significant challenge for the successful eradication of devastating pathogens. Methicillin-Resistant Staphylococcus aureus (MRSA) is an enduring superbug involved in causing devastating infections. Although MRSA is a frequent colonizer of human skin, wound, and anterior nares, the intestinal colonization of MRSA has greatly increased the risk of inducing MRSA-associated colitis besides creating a conducive environment for horizontal transfer of resistant genes to commensal microbes. On the other hand, staphylococcal resistance to last-resort antibiotics has urged the development of novel antimicrobial agents for the effective decolonization of MRSA. In response, probiotics and their metabolites (postbiotics) have been proposed as the adjunct therapeutic avenues. Probiotics exhibit a multitude of anti-MRSA actions (anti-bacterial, anti-biofilm, anti-virulence, anti-drug resistance, co-aggregation, and anti-quorum sensing) through the production of numerous antagonistic compounds such as organic acids, hydrogen peroxide, low molecular weight compounds, biosurfactants, bacteriocins, and bacteriocins like inhibitory substances. Besides, probiotics stabilize the epithelial barrier function and positively modulate the host immune system via regulating various signal transduction mechanisms. Preclinical and human intervention studies have suggested that probiotics outcompete with MRSA by exhibiting anti-colonization mechanisms via protective, competitive, and displacement mode. In this review, we aim to highlight the dynamics of MRSA associated virulence and drug resistance properties, and how probiotics antagonize MRSA through various mechanism of action.

No man's land: Species-specific formation of exclusion zones bordering Actinomyces graevenitzii microcolonies in nanoliter cultures

To survive within complex environmental niches, including the human host, bacteria have evolved intricate interspecies communities driven by competition for limited nutrients, cooperation via complementary metabolic proficiencies, and establishment of homeostatic relationships with the host immune system. The study of such complex, interdependent relationships is often hampered by the challenges of culturing many bacterial strains in research settings and the limited set of tools available for studying the dynamic behavior of multiple bacterial species at the microscale. Here, we utilize a microfluidic‐based co‐culture system and time‐lapse imaging to characterize dynamic interactions between Streptococcus species, Staphylococcus aureus, and Actinomyces species. Co‐culture of Streptococcus cristatus or S. salivarius in nanoliter compartments with Actinomyces graevenitzii revealed localized exclusion of Streptococcus and Staphylococcus from media immediately surrounding A. graevenitzii microcolonies. This community structure did not occur with S. mitis or S. oralis strains or in co‐cultures containing other Actinomycetaceae species such as S. odontolyticus or A. naeslundii. Moreover, fewer neutrophils were attracted to compartments containing both A. graevenitzii and Staphylococcus aureus than to an equal number of either species alone, suggesting a possible survival benefit together during immune responses.

Transmission of Monospecies and Dual-Species Biofilms from Smooth to Nanopillared Surfaces

The transmission of bacteria in biofilms from donor to receiver surfaces precedes the formation of biofilms in many applications. Biofilm transmission is different from bacterial adhesion, because it involves biofilm compression in between two surfaces, followed by a separation force leading to the detachment of the biofilm from the donor surface and subsequent adhesion to the receiver surface. Therewith, the transmission depends on a balance between donor and receiver surface properties and the cohesiveness of the biofilm itself. Here, we compare bacterial transmission from biofilms of an extracellular-polymeric-substance (EPS)-producing and a non-EPS-producing staphylococcal strain and a dual-species oral biofilm from smooth silicon (Si) donor surfaces to smooth and nanopillared Si receiver surfaces. Biofilms were fully covering the donor surface before transmission. However, after transmission, the biofilms only partly covered the donor and receiver surfaces regardless of nanopillaring, indicating bacterial transmission through adhesive failure at the interface between biofilms and donor surfaces as well as through cohesive failure in the biofilms. The numbers of bacteria per unit volume in EPS-producing staphylococcal biofilms before transmission were 2-fold smaller than in biofilms of the non-EPS-producing strain and of dual species. This difference increased after transmission in the biofilm left behind on the donor surfaces due to an increased bacterial density for the non-EPS-producing strain and a dual-species biofilm. This suggests that biofilms of the non-EPS-producing strain and dual species remained compressed after transmission, while biofilms of the EPS-producing strain were induced to produce more EPS during transmission and relaxed toward their initial state after transmission due to the viscoelasticity conferred to the biofilm by its EPS.IMPORTANCE Bacterial transmission from biofilm-covered surfaces to surfaces is mechanistically different from bacterial adhesion to surfaces and involves detachment from the donor and adhesion to the receiver surfaces under pressure. Bacterial transmission occurs, for instance, in food processing or packaging, in household situations, or between surfaces in hospitals. Patients admitted to a hospital room previously occupied by a patient with antibiotic-resistant pathogens are at elevated infection risk by the same pathogens through transmission. Nanopillared receiver surfaces did not collect less biofilm from a smooth donor than a smooth receiver, likely because the pressure applied during transmission negated the smaller contact area between bacteria and nanopillared surfaces, generally held responsible for reduced adhesion. Biofilm left behind on smooth donor surfaces of a non-extracellular-polymeric-substance (EPS)-producing strain and dual species had undergone different structural changes than an EPS-producing strain, which is important for their possible further treatment by antimicrobials or disinfectants.

Influence of Fimbriae on Bacterial Adhesion and Viscoelasticity and Correlations of the Two Properties with Biofilm Formation

The surface polymers of bacteria determine the ability of bacteria to adhere to a substrate for colonization, which is an essential step for a variety of microbial processes, such as biofilm formation and biofouling. Capsular polysaccharides and fimbriae are two major components on a bacterial surface, which are critical for mediating cell-surface interactions. Adhesion and viscoelasticity of bacteria are two major physical properties related to bacteria-surface interactions. In this study, we employed atomic force microscopy (AFM) to interrogate how the adhesion work and the viscoelasticity of a bacterial pathogen, Klebsiella pneumoniae, influence biofilm formation. To do this, the wild-type, type 3 fimbriae-deficient, and type 3 fimbriae-overexpressed K. pneumoniae strains have been investigated in an aqueous environment. The results show that the measured adhesion work is positively correlated to biofilm formation; however, the viscoelasticity is not correlated to biofilm formation. This study indicates that AFM-based adhesion measurements of bacteria can be used to evaluate the function of bacterial surface polymers in biofilm formation and to predict the ability of bacterial biofilm formation.

Biofilm in endodontics: A review

Endodontic disease is a biofilm-mediated infection, and primary aim in the management of endodontic disease is the elimination of bacterial biofilm from the root canal system. The most common endodontic infection is caused by the surface-associated growth of microorganisms. It is important to apply the biofilm concept to endodontic microbiology to understand the pathogenic potential of the root canal microbiota as well as to form the basis for new approaches for disinfection. It is foremost to understand how the biofilm formed by root canal bacteria resists endodontic treatment measures. Bacterial etiology has been confirmed for common oral diseases such as caries and periodontal and endodontic infections. Bacteria causing these diseases are organized in biofilm structures, which are complex microbial communities composed of a great variety of bacteria with different ecological requirements and pathogenic potential. The biofilm community not only gives bacteria effective protection against the host's defense system but also makes them more resistant to a variety of disinfecting agents used as oral hygiene products or in the treatment of infections. Successful treatment of these diseases depends on biofilm removal as well as effective killing of biofilm bacteria. So, the fundamental to maintain oral health and prevent dental caries, gingivitis, and periodontitis is to control the oral biofilms. From these aspects, the formation of biofilms carries particular clinical significance because not only host defense mechanisms but also therapeutic efforts including chemical and mechanical antimicrobial treatment measures have the most difficult task of dealing with organisms that are gathered in a biofilm. The aim of this article was to review the mechanisms of biofilms’ formation, their roles in pulpal and periapical pathosis, the different types of biofilms, the factors influencing biofilm formation, the mechanisms of their antimicrobial resistance, techniques to identify biofilms.

Single-bacterium nanomechanics in biomedicine: unravelling the dynamics of bacterial cells

The use of the atomic force microscope (AFM) in microbiology has progressed significantly throughout the years since its first application as a high-resolution imaging instrument. Modern AFM setups are capable of characterizing the nanomechanical behaviour of bacterial cells at both the cellular and molecular levels, where elastic properties and adhesion forces of single bacterium cells can be examined under different experimental conditions. Considering that bacterial and biofilm-mediated infections continue to challenge the biomedical field, it is important to understand the biophysical events leading towards bacterial adhesion and colonization on both biological and non-biological substrates. The purpose of this review is to present the latest findings concerning the field of single-bacterium nanomechanics, and discuss future trends and applications of nanoindentation and single-cell force spectroscopy techniques in biomedicine.

Nanoscale adhesion forces between enamel pellicle proteins and hydroxyapatite

The acquired enamel pellicle (AEP) is important for minimizing the abrasion caused by parafunctional conditions as they occur, for instance, during bruxism. It is a remarkable feature of the AEP that a protein/peptide film can provide enough protection in normofunction to prevent teeth from abrasion and wear. Despite its obvious critical role in the protection of tooth surfaces, the essential adhesion features of AEP proteins on the enamel surface are poorly characterized. The objective of this study was to measure the adhesion force between histatin 5, a primary AEP component, and hydroxyapatite (HA) surfaces. Both biotinylated histatin 5 and biotinylated human serum albumin were allowed to adsorb to streptavidin-coated silica microspheres attached to atomic force microscope (AFM) cantilevers. A multimode AFM with a Nanoscope IIIa controller was used to measure the adhesion force between protein-functionalized silica microspheres attached to cantilever tips and the HA surface. The imaging was performed in tapping mode with a Si3N4 AFM cantilever, while the adhesion forces were measured in AFM contact mode. A collection of force-distance curves (~3,000/replicate) was obtained to generate histograms from which the adhesion forces between histatin 5 or albumin and the HA surface were measured. We found that histatin 5 exhibited stronger adhesion forces (90% >1.830 nN) to the HA surface than did albumin (90% > 0.282 nN). This study presents an objective approach to adhesion force measurements between histatin 5 and HA, and provides the experimental basis for measuring the same parameters for other AEP constituents. Such knowledge will help in the design of synthetic proteins and peptides with preventive and therapeutic benefits for tooth enamel.

Adhesion forces and composition of planktonic and adhering oral microbiomes

The oral microbiome consists of a planktonic microbiome residing in saliva and an adhering microbiome (the biofilm adhering to oral hard and soft tissues). Here we hypothesized that possible differences in microbial composition of the planktonic and adhering oral microbiome on teeth can be related to the forces by which different bacterial species are attracted to the tooth surface. The relative presence of 7 oral bacterial species in saliva and biofilm collected from 10 healthy human volunteers was determined twice in each volunteer by denaturing-gradient-gel electrophoresis. Analysis of both microbiomes showed complete separation of the planktonic from the adhering oral microbiome. Next, adhesion forces of corresponding bacterial strains with saliva-coated enamel surfaces were measured by atomic force microscopy. Species that were found predominantly in the adhering microbiome had significantly higher adhesion forces to saliva-coated enamel (-0.60 to -1.05 nN) than did species mostly present in the planktonic microbiome (-0.40 to -0.55 nN). It is concluded that differences in composition of the planktonic and the adhering oral microbiome are due to small differences in the forces by which strains adhere to saliva-coated enamel, providing an important step in understanding site- and material-specific differences in the composition of biofilms in the oral cavity.

Persistence of the oral probiotic Streptococcus salivarius M18 is dose dependent and megaplasmid transfer can augment their bacteriocin production and adhesion characteristics

Bacteriocin-producing probiotic Streptococcus salivarius M18 offers beneficial modulatory capabilities within the oral microbiome, apparently through potent inhibitory activity against potentially deleterious bacteria, such as Streptococcus pyogenes. The oral cavity persistence of S. salivarius M18 was investigated in 75 subjects receiving four different doses for 28 days. Sixty per cent of the subjects already had some inhibitor-producing S. salivarius in their saliva prior to probiotic intervention. Strain M18's persistence was dependent upon the dose, but not the period of administration. Culture analysis indicated that in some individuals the introduced strain had almost entirely replaced the indigenous S. salivarius, though the total numbers of the species did not increase. Selected subjects showing either high or low probiotic persistence had their salivary populations profiled using Illumina sequencing of the V6 region of the 16S rRNA gene. Analysis indicated that while certain bacterial phenotypes were markedly modulated, the overall composition of the oral microbiome was not modified by the probiotic treatment. Megaplasmids encoding bacteriocins and adhesion factors were transferred in vitro to generate a transconjugant S. salivarius exhibiting enhanced antimicrobial production and binding capabilities to HEp-2 cells. Since no widespread perturbation of the existing indigenous microbiota was associated with oral instillation and given its antimicrobial activity against potentially pathogenic streptococci, it appears that application of probiotic strain M18 offers potential low impact alternative to classical antibiotic prophylaxis. For candidate probiotic strains having relatively poor antimicrobial or adhesive properties, unique derivatives displaying improved probiotic performance may be engineered in vitro by megaplasmid transfer.

Surface thermodynamic and adhesion force evaluation of the role of chitin-binding protein in the physical interaction between Pseudomonas aeruginosa and Candida albicans

Candida albicans and Pseudomonas aeruginosa are able to form pathogenic polymicrobial communities. P. aeruginosa colonizes and kills hyphae but is unable to attach to yeast. It is unknown why the interaction of P. aeruginosa is different with yeast than with hyphae. Here we aim to evaluate the role of P. aeruginosa chitin-binding protein (CbpD) in its physical interaction with C. albicans hyphae or yeast, based on surface thermodynamic and atomic force microscopic analyses. A P. aeruginosa mutant lacking CbpD was unable to express strong adhesion forces with hyphae (-2.9 nN) as compared with the parent strain P. aeruginosa PAO1 (-4.8 nN) and showed less adhesion to hyphae. Also blocking of CbpD using N-acetyl-glucosamine yielded a lower adhesion force (-4.3 nN) with hyphae. Strong adhesion forces were restored after complementing the expression of CbpD in P. aeruginosa PAO1 ΔcbpD yielding an adhesion force of -5.1 nN. These observations were confirmed with microscopic evaluation of adhesion tests. Regardless of the absence or presence of CbpD on the bacterial cell surfaces, or their blocking, P. aeruginosa experienced favorable thermodynamic conditions for adhesion with hyphae, which were absent with yeast. In addition, adhesion forces with yeast were less than 0.5 nN in all cases. Concluding, CbpD in P. aeruginosa is responsible for strong physical interactions with C. albicans hyphae. The development of this interaction requires time due to the fact that CbpDs have to invade the outermost mannoprotein layer on the hyphal cell surfaces. In order to do this, thermodynamic conditions at the outermost cell surfaces have to be favorable.

Atomic force and super-resolution microscopy support a role for LapA as a cell-surface biofilm adhesin of Pseudomonas fluorescens

Pseudomonas fluorescence Pf0-1 requires the large repeat protein LapA for stable surface attachment. This study presents direct evidence that LapA is a cell-surface-localized adhesin. Atomic force microscopy (AFM) revealed a significant 2-fold reduction in adhesion force for mutants lacking the LapA protein on the cell surface compared to the wild-type strain. Deletion of lapG, a gene encoding a periplasmic cysteine protease that functions to release LapA from the cell surface, resulted in a 2-fold increase in the force of adhesion. Three-dimensional structured illumination microscopy (3D-SIM) revealed the presence of the LapA protein on the cell surface, consistent with its role as an adhesin. The protein is only visualized in the cytoplasm for a mutant of the ABC transporter responsible for translocating LapA to the cell surface. Together, these data highlight the power of combining the use of AFM and 3D-SIM with genetic studies to demonstrate that LapA, a member of a large group of RTX-like repeat proteins, is a cell-surface adhesin.

Staphylococcus aureus adherence to Candida albicans hyphae is mediated by the hyphal adhesin Als3p

The bacterium Staphylococcus (St.) aureus and the opportunistic fungus Candida albicans are currently among the leading nosocomial pathogens, often co-infecting critically ill patients, with high morbidity and mortality. Previous investigations have demonstrated preferential adherence of St. aureus to C. albicans hyphae during mixed biofilm growth. In this study, we aimed to characterize the mechanism behind this observed interaction. C. albicans adhesin-deficient mutant strains were screened by microscopy to identify the specific receptor on C. albicans hyphae recognized by St. aureus. Furthermore, an immunoassay was developed to validate and quantify staphylococcal binding to fungal biofilms. The findings from these experiments implicated the C. albicans adhesin agglutinin-like sequence 3 (Als3p) in playing a major role in the adherence process. This association was quantitatively established using atomic force microscopy, in which the adhesion force between single cells of the two species was significantly reduced for a C. albicans mutant strain lacking als3. Confocal microscopy further confirmed these observations, as St. aureus overlaid with a purified recombinant Als3 N-terminal domain fragment (rAls3p) exhibited robust binding. Importantly, a strain of Saccharomyces cerevisiae heterologously expressing Als3p was utilized to further confirm this adhesin as a receptor for St. aureus. Although the parental strain does not bind bacteria, expression of Als3p on the cell surface conferred upon the yeast the ability to strongly bind St. aureus. To elucidate the implications of these in vitro findings in a clinically relevant setting, an ex vivo murine model of co-infection was designed using murine tongue explants. Fluorescent microscopic images revealed extensive hyphal penetration of the epithelium typical of C. albicans mucosal infection. Interestingly, St. aureus bacterial cells were only seen within the epithelial tissue when associated with the invasive hyphae. This differed from tongues infected with St. aureus alone or in conjunction with the als3 mutant strain of C. albicans, where bacterial presence was limited to the outer layers of the oral tissue. Collectively, the findings generated from this study identified a key role for C. albicans Als3p in mediating this clinically relevant fungal-bacterial interaction.

Adhesion forces and coaggregation between vaginal staphylococci and lactobacilli

Urogenital infections are the most common ailments afflicting women. They are treated with dated antimicrobials whose efficacy is diminishing. The process of infection involves pathogen adhesion and displacement of indigenous Lactobacillus crispatus and Lactobacillus jensenii. An alternative therapeutic approach to antimicrobial therapy is to reestablish lactobacilli in this microbiome through probiotic administration. We hypothesized that lactobacilli displaying strong adhesion forces with pathogens would facilitate coaggregation between the two strains, ultimately explaining the elimination of pathogens seen in vivo. Using atomic force microscopy, we found that adhesion forces between lactobacilli and three virulent toxic shock syndrome toxin 1-producing Staphylococcus aureus strains, were significantly stronger (2.2–6.4 nN) than between staphylococcal pairs (2.2–3.4 nN), especially for the probiotic Lactobacillus reuteri RC-14 (4.0–6.4 nN) after 120 s of bond-strengthening. Moreover, stronger adhesion forces resulted in significantly larger coaggregates. Adhesion between the bacteria occurred instantly upon contact and matured within one to two minutes, demonstrating the potential for rapid anti-pathogen effects using a probiotic. Coaggregation is one of the recognized mechanisms through which lactobacilli can exert their probiotic effects to create a hostile micro-environment around a pathogen. With antimicrobial options fading, it therewith becomes increasingly important to identify lactobacilli that bind strongly with pathogens.

Atomic force microscopy investigations of heterogeneities in the adhesion energies measured between pathogenic and non-pathogenic Listeria species and silicon nitride as they correlate to virulence and adherence

Atomic force microscopy (AFM) was used to probe heterogeneities in adhesion energies measured between pathogenic and non-pathogenic species of Listeria and silicon nitride in water at four levels. Adhesion energies were quantified on individual bacterial cells (cell level), bacterial cells that belonged to an individual Listeria strain but varied in their cultures (strain level), bacterial cells that belonged to an individual Listeria species but varied in their strain type (species level) and on bacterial cells that belonged to the Listeria genus but varied in their species type (genus level). To quantify heterogeneities in the adhesion energies, a heterogeneity index (HI) was defined based on quantified standard errors of mean. At the cell level, spatial variations in the adhesion energies were not observed. For the strain, species, and genus levels, the HI increased with increased adhesion energies. At the species level, the HI increased with strain virulence.

Determination of adhesion between single Aspergillus niger spores in aqueous solutions using an atomic force microscope

The interaction force between single cells in contact is of high interest in various interdisciplinary fields of biotechnology, for instance, in cultivation or biofilm formation. A method for the determination of adhesion forces between two single Aspergillus niger spores in different aqueous solutions was established in this study. Adhesion force distributions were determined at three different sodium chloride concentrations and two different pH values using an atomic force microscope (AFM). It was pointed out that adhesion data can be described by log-normal density functions, of which corresponding parameters have been estimated. Using the knowledge of distribution shape, the influence of the environmental condition on the mean values of adhesion force could be studied quantitatively. The highest value of 0.95 nN was observed at pH 2.5 and an ionic strength of 0.5 mol L(-1). Decreasing the ionic strength to 0.05 mol L(-1) decreases the adhesion force mean for about 25%. Increasing the pH value to pH 5 at a sodium chloride concentration of 0.154 mol L(-1) entails a decrease of adhesion from 0.88 to 0.56 nN. These results qualitatively agree with the absolute value of the expected surface potential of Aspergillus niger spores, which is much higher at pH 5 and should take more effect at lower concentrations of counterions.

Towards nanomicrobiology using atomic force microscopy

At the cross-roads of nanoscience and microbiology, the nanoscale analysis of microbial cells using atomic force microscopy (AFM) is an exciting, rapidly evolving research field. Over the past decade, there has been tremendous progress in our use of AFM to observe membrane proteins and live cells at high resolution. Remarkable advances have also been made in applying force spectroscopy to manipulate single membrane proteins, to map surface properties and receptor sites on cells and to measure cellular interactions at the single-cell and single-molecule levels. In addition, recent developments in cantilever nanosensors have opened up new avenues for the label-free detection of microorganisms and bioanalytes.

Efficient immobilization and patterning of live bacterial cells

A monolayer of live bacterial cells has been patterned onto substrates through the interaction between CFA/I fimbriae and the corresponding antibody. Patterns of live bacteria have been prepared with cellular resolution on silicon and gold substrates for Salmonella enterica serovar Typhimurium as a model with high specificity and efficiency. The immobilized cells are capable of dividing in growth medium to form a self-sustaining bacterial monolayer on the patterned areas. Interestingly, the immobilized cells can alter their orientation on the substrate, from lying-down to standing-up, as a response to the cell density increase during incubation. This method was successfully used to sort a targeted bacterial species from a mixed culture within 2 h.

Interaction forces between salivary proteins and Streptococcus mutans with and without antigen I/II

The antigen I/II family of surface proteins is expressed by oral streptococci, including Streptococcus mutans, and mediates specific binding to, among others, salivary films. The aim of this study was to investigate the interaction forces between salivary proteins and S. mutans with (LT11) and without (IB03987) antigen I/II through atomic force microscopy (AFM) and to relate these interaction forces with the adhesion of the strains to saliva-coated glass in a parallel plate flow chamber. Upon approach of the bacteria toward a saliva-coated AFM tip, both strains experienced a similar repulsive force that was significantly smaller at pH 6.8 (median 3.0 and 3.1 nN for LT11 and IB03987, respectively) than at pH 5.8 (median 4.6 and 4.7 nN). The decay length of these repulsive forces was between 19 and 37 nm. Upon retraction at pH 6.8, the combined specific and nonspecific adhesion forces were significantly stronger for the parent strain LT11 (median -0.4 nN) than for the mutant strain IB03987 (median 0.0 nN), whereas at pH 5.8 the median of the adhesion forces measured was 0.0 nN for both strains. Moreover, at pH 6.8, the parent strain LT11 adhered in significantly higher numbers (9.6 x 106 cm-2) to a salivary coating than the mutant strain IB03987 (2.5 x 106 cm-2). Similar to the difference in adhesion forces between both strains at pH 5.8, the difference in adhesion between both strains also disappeared at pH 5.8, which suggests the involvement of attractive electrostatic forces in the interaction between antigen I/II and salivary coatings. In summary, this study shows that antigen I/II at the surface of S. mutans LT11 is responsible for its increased adhesion to salivary coatings under flow through an additional attractive electrostatic force.

Intermolecular forces and enthalpies in the adhesion of Streptococcus mutans and an antigen I/II-deficient mutant to laminin films

The antigen I/II family of surface proteins is expressed by most oral streptococci, including Streptococcus mutans, and mediates specific adhesion to, among other things, salivary films and extracellular matrix proteins. In this study we showed that antigen I/II-deficient S. mutans isogenic mutant IB03987 was nearly unable to adhere to laminin films under flow conditions due to a lack of specific interactions (0.8 x 10(6) and 1.1 x 10(6) cells cm(-2) at pH 5.8 and 6.8, respectively) compared with parent strain LT11 (21.8 x 10(6) and 26.1 x 10(6) cells cm(-2)). The adhesion of both the parent and mutant strains was slightly greater at pH 6.8 than at pH 5.8. In addition, atomic force microscopy (AFM) experiments demonstrated that the parent strain experienced less repulsion when it approached a laminin film than the mutant experienced. Upon retraction, combined specific and nonspecific adhesion forces were stronger for the parent strain (up to -5.0 and -4.9 nN at pH 5.8 and 6.8, respectively) than for the mutant (up to -1.5 and -2.1 nN), which was able to interact only through nonspecific interactions. Enthalpy was released upon adsorption of laminin to the surface of the parent strain but not upon adsorption of laminin to the surface of IB03987. A comparison of the adhesion forces in AFM with the adhesion forces reported for specific ligand-receptor complexes resulted in the conclusion that the number of antigen I/II binding sites for laminin on S. mutans LT11 is on the order of 6 x 10(4) sites per organism and that the sites are probably arranged along exterior surface structures, as visualized here by immunoelectron microscopy.