Physiological properties of Streptococcus mutans UA159 biofilm-detached cells

Application of biofilm dispersion-based nanoparticles in cutting off reinfection

Bacterial biofilms commonly cause chronic and persistent infections in humans. Bacterial biofilms consist of an inner layer of bacteria and an autocrine extracellular polymeric substance (EPS). Biofilm dispersants (abbreviated as dispersants) have proven effective in removing the bacterial physical protection barrier EPS. Dispersants are generally weak or have no bactericidal effect. Bacteria dispersed from within biofilms (abbreviated as dispersed bacteria) may be more invasive, adhesive, and motile than planktonic bacteria, characteristics that increase the probability that dispersed bacteria will recolonize and cause reinfection. The dispersants should be combined with antimicrobials to avoid the risk of severe reinfection. Dispersant-based nanoparticles have the advantage of specific release and intense penetration, providing the prerequisite for further antibacterial agent efficacy and achieving the eradication of biofilms. Dispersant-based nanoparticles delivered antimicrobial agents for the treatment of diseases associated with bacterial biofilm infections are expected to be an effective measure to prevent reinfection caused by dispersed bacteria. KEY POINTS: • Dispersed bacteria harm and the dispersant's dispersion mechanisms are discussed. • The advantages of dispersant-based nanoparticles in bacteria biofilms are discussed. • Dispersant-based nanoparticles for cutting off reinfection in vivo are highlighted.

Efficacy of Indolicidin, Cecropin A (1-7)-Melittin (CAMA) and Their Combination Against Biofilm-Forming Multidrug-Resistant Enteroaggregative Escherichia coli

The present study examined the anti-biofilm efficacy of two short-chain antimicrobial peptides (AMPs), namely, indolicidin and cecropin A (1-7)-melittin (CAMA) against biofilm-forming multidrug-resistant enteroaggregative Escherichia coli (MDR-EAEC) isolates. The typical EAEC isolates re-validated by PCR and confirmed using HEp-2 cell adherence assay was subjected to antibiotic susceptibility testing to confirm its MDR status. The biofilm-forming ability of MDR-EAEC isolates was assessed by Congo red binding, microtitre plate assays and hydrophobicity index; broth microdilution technique was employed to determine minimum inhibitory concentrations (MICs) and minimum biofilm eradication concentrations (MBECs). The obtained MIC and MBEC values for both AMPs were evaluated alone and in combination against MDR-EAEC biofilms using crystal violet (CV) staining and confocal microscopy-based live/dead cell quantification methods. All the three MDR-EAEC strains revealed weak to strong biofilm-forming ability and were found to be electron-donating and weakly electron-accepting (hydrophobicity index). Also, highly significant (P < 0.001) time-dependent hydrodynamic growth of the three MDR-EAEC strains was observed at 48 h of incubation in Dulbecco's modified Eagle's medium (DMEM) containing 0.45% D-glucose. AMPs and their combination were able to inhibit the initial biofilm formation at 24 h and 48 h as evidenced by CV staining and confocal quantification. Further, the application of AMPs (individually and combination) against the preformed MDR-EAEC biofilms resulted in highly significant eradication (P < 0.001) at 24 h post treatment. However, significant differences were not observed between AMP treatments (individually or in combination). The AMPs seem to be an effective candidates for further investigations such as safety, stability and appropriate biofilm-forming MDR-EAEC animal models.

Virulence Factors in Coagulase-Negative Staphylococci

Coagulase-negative staphylococci (CoNS) have emerged as major pathogens in healthcare-associated facilities, being S. epidermidis, S. haemolyticus and, more recently, S. lugdunensis, the most clinically relevant species. Despite being less virulent than the well-studied pathogen S. aureus, the number of CoNS strains sequenced is constantly increasing and, with that, the number of virulence factors identified in those strains. In this regard, biofilm formation is considered the most important. Besides virulence factors, the presence of several antibiotic-resistance genes identified in CoNS is worrisome and makes treatment very challenging. In this review, we analyzed the different aspects involved in CoNS virulence and their impact on health and food.

Biofilm dispersion

The formation of microbial biofilms enables single planktonic cells to assume a multicellular mode of growth. During dispersion, the final step of the biofilm life cycle, single cells egress from the biofilm to resume a planktonic lifestyle. As the planktonic state is considered to be more vulnerable to antimicrobial agents and immune responses, dispersion is being considered a promising avenue for biofilm control. In this Review, we discuss conditions that lead to dispersion and the mechanisms by which native and environmental cues contribute to dispersion. We also explore recent findings on the role of matrix degradation in the dispersion process, and the distinct phenotype of dispersed cells. Last, we discuss the translational and therapeutic potential of dispersing bacteria during infection.

Emergent Properties in Streptococcus mutans Biofilms Are Controlled through Adhesion Force Sensing by Initial Colonizers

Bacterial adhesion is accompanied by altered gene expression, leading to "emergent" properties of biofilm bacteria that are alien to planktonic ones. With the aim of revealing the role of environmental adhesion forces in emergent biofilm properties, genes in Streptococcus mutans UA159 and a quorum-sensing-deficient mutant were identified that become expressed after adhesion to substratum surfaces. Using atomic force microscopy, adhesion forces of initial S. mutans colonizers on four different substrata were determined and related to gene expression. Adhesion forces upon initial contact were similarly low across different substrata, ranging between 0.2 and 1.2 nN regardless of the strain considered. Bond maturation required up to 21 s, depending on the strain and substratum surface involved, but stationary adhesion forces also were similar in the parent and in the mutant strain. However, stationary adhesion forces were largest on hydrophobic silicone rubber (19 to 20 nN), while being smallest on hydrophilic glass (3 to 4 nN). brpA gene expression in thin (34 to 48 μm) 5-h S. mutans UA159 biofilms was most sensitive to adhesion forces, while expression of gbpB and comDE expressions was weakly sensitive. ftf, gtfB, vicR, and relA expression was insensitive to adhesion forces. In thicker (98 to 151 μm) 24-h biofilms, adhesion-force-induced gene expression and emergent extracellular polymeric substance (EPS) production were limited to the first 20 to 30 μm above a substratum surface. In the quorum-sensing-deficient S. mutans, adhesion-force-controlled gene expression was absent in both 5- and 24-h biofilms. Thus, initial colonizers of substratum surfaces sense adhesion forces that externally trigger emergent biofilm properties over a limited distance above a substratum surface through quorum sensing.IMPORTANCE A new concept in biofilm science is introduced: "adhesion force sensitivity of genes," defining the degree up to which expression of different genes in adhering bacteria is controlled by the environmental adhesion forces they experience. Analysis of gene expression as a function of height in a biofilm showed that the information about the substratum surface to which initially adhering bacteria adhere is passed up to a biofilm height of 20 to 30 μm above a substratum surface, highlighting the importance and limitations of cell-to-cell communication in a biofilm. Bacteria in a biofilm mode of growth, as opposed to planktonic growth, are responsible for the great majority of human infections, predicted to become the number one cause of death in 2050. The concept of adhesion force sensitivity of genes provides better understanding of bacterial adaptation in biofilms, direly needed for the design of improved therapeutic measures that evade the recalcitrance of biofilm bacteria to antimicrobials.

Comparative Study on the Impact of Growth Conditions on the Physiology and the Virulence of Pseudomonas aeruginosa Biofilm and Planktonic Cells

The aim of the present work was to study and compare the effect of growth temperature (20, 30, and 37°C) and surface type (stainless steel and polycarbonate) on the production of virulence factors, such as proteases and siderophores, and the risk of surface contamination associated with Pseudomonas aeruginosa biofilm and planktonic cells. The increase of growth temperature from 20 to 37°C increased (approximately twofold) the electronegative charge and the hydrophobicity of the P. aeruginosa biofilm cell surface. P. aeruginosa biofilm cell adhesion to stainless steel and polycarbonate was 5- and 1.5-fold higher than their planktonic counterparts at 20 and 30°C, respectively. The increase of growth temperature from 20 to 37°C increased the production of proteases (twofold) and siderophores (twofold) and the cytotoxicity (up to 30-fold) against the HeLa cell line in the supernatants of P. aeruginosa planktonic and biofilm cultures. This study also highlighted that biofilm and planktonic P. aeruginosa cells exhibited distinct physiological properties with respect to the production of virulence factors and the cytotoxicity against the Hela cell line. Therefore, effective disinfection procedures should be adapted to inactivate bacteria detached from biofilms.

Purification and Characterization of a Biofilm-Degradable Dextranase from a Marine Bacterium

This study evaluated the ability of a dextranase from a marine bacterium Catenovulum sp. (Cadex) to impede formation of Streptococcus mutans biofilms, a primary pathogen of dental caries, one of the most common human infectious diseases. Cadex was purified 29.6-fold and had a specific activity of 2309 U/mg protein and molecular weight of 75 kDa. Cadex showed maximum activity at pH 8.0 and 40 °C and was stable at temperatures under 30 °C and at pH ranging from 5.0 to 11.0. A metal ion and chemical dependency study showed that Mn²⁺ and Sr²⁺ exerted positive effects on Cadex, whereas Cu²⁺, Fe³⁺, Zn²⁺, Cd²⁺, Ni²⁺, and Co²⁺ functioned as inhibitors. Several teeth rinsing product reagents, including carboxybenzene, ethanol, sodium fluoride, and xylitol were found to have no effects on Cadex activity. A substrate specificity study showed that Cadex specifically cleaved the α-1,6 glycosidic bond. Thin layer chromatogram and high-performance liquid chromatography indicated that the main hydrolysis products were isomaltoogligosaccharides. Crystal violet staining and scanning electron microscopy showed that Cadex impeded the formation of S. mutans biofilm to some extent. In conclusion, Cadex from a marine bacterium was shown to be an alkaline and cold-adapted endo-type dextranase suitable for development of a novel marine agent for the treatment of dental caries.

Dispersal from Microbial Biofilms

One common feature of biofilm development is the active dispersal of cells from the mature biofilm, which completes the biofilm life cycle and allows for the subsequent colonization of new habitats. Dispersal is likely to be critical for species survival and appears to be a precisely regulated process that involves a complex network of genes and signal transduction systems. Sophisticated molecular mechanisms control the transition of sessile biofilm cells into dispersal cells and their coordinated detachment and release in the bulk liquid. Dispersal cells appear to be specialized and exhibit a unique phenotype different from biofilm or planktonic bacteria. Further, the dispersal population is characterized by a high level of heterogeneity, reminiscent of, but distinct from, that in the biofilm, which could potentially allow for improved colonization under various environmental conditions. Here we review recent advances in characterizing the molecular mechanisms that regulate biofilm dispersal events and the impact of dispersal in a broader ecological context. Several strategies that exploit the mechanisms controlling biofilm dispersal to develop as applications for biofilm control are also presented.

Biofilm formation and antibiotic susceptibility in dispersed cells versus planktonic cells from clinical, industry and environmental origins

We examined the cell-surface physicochemical properties, the biofilm formation capability and the antibiotic susceptibility in dispersed cells (from an artificial biofilm of alginate beads) and compared with their planktonic (free-swimming) counterparts. The strains used were from different origins, such as clinical (Acinetobacter baumannii AB4), cosmetic industry (Klebsiella oxytoca EU213, Pseudomonas aeruginosa EU190), and environmental (Halomonas venusta MAT28). In general, dispersed cells adhered better to surfaces (measured as the "biofilm index") and had a greater hydrophobicity [measured as the microbial affinity to solvents (MATS)] than planktonic cells. The susceptibility to two antibiotics (ciprofloxacin and tetracycline) of dispersed cells was higher compared with that of their planktonic counterparts (tested by the "bactericidal index"). Dispersed and planktonic cells exhibited differences in cell permeability, especially in efflux pump activity, which could be related to the differences observed in susceptibility to antibiotics. At 1 h of biofilm formation in microtiter plates, dispersed cells treated with therapeutic concentration of ciprofloxacin yielded a lower biofilm index than the control dispersed cells without ciprofloxacin. With respect to the planktonic cells, the biofilm index was similar with and without the ciprofloxacin treatment. In both cases there were a reduction of the number of bacteria measured as viable count of the supernatant. The lower biofilm formation in dispersed cells with ciprofloxacin treatment may be due to a significant increase of biofilm disruption with respect to the biofilm from planktonic cells. From a clinical point of view, biofilms formed on medical devices such as catheters, cells that can be related to an infection were the dispersed cells. Our results showed that early treatment with ciprofloxacin of dispersed cells could diminishe bacterial dispersion and facilitate the partial elimination of the new biofilm formed.

Inactivation of a putative efflux pump (LmrB) in Streptococcus mutans results in altered biofilm structure and increased exopolysaccharide synthesis: implications for biofilm resistance

Efflux pumps are a mechanism associated with biofilm formation and resistance. There is limited information regarding efflux pumps in Streptococcus mutans, a major pathogen in dental caries. The aim of this study was to investigate potential roles of a putative efflux pump (LmrB) in S. mutans biofilm formation and susceptibility. Upon lmrB inactivation and antimicrobial exposure, the biofilm structure and expression of other efflux pumps were examined using confocal laser scanning microscopy (CLSM) and qRT-PCR. lmrB inactivation resulted in biofilm structural changes, increased EPS formation and EPS-related gene transcription (p < 0.05), but no improvement in susceptibility was observed. The expression of most efflux pump genes increased upon lmrB inactivation when exposed to antimicrobials (p < 0.05), suggesting a feedback mechanism that activated the transcription of other efflux pumps to compensate for the loss of lmrB. These observations imply that sole inactivation of lmrB is not an effective solution to control biofilms.

Living together in biofilms: the microbial cell factory and its biotechnological implications

In nature, bacteria alternate between two modes of growth: a unicellular life phase, in which the cells are free-swimming (planktonic), and a multicellular life phase, in which the cells are sessile and live in a biofilm, that can be defined as surface-associated microbial heterogeneous structures comprising different populations of microorganisms surrounded by a self-produced matrix that allows their attachment to inert or organic surfaces. While a unicellular life phase allows for bacterial dispersion and the colonization of new environments, biofilms allow sessile cells to live in a coordinated, more permanent manner that favors their proliferation. In this alternating cycle, bacteria accomplish two physiological transitions via differential gene expression: (i) from planktonic cells to sessile cells within a biofilm, and (ii) from sessile to detached, newly planktonic cells. Many of the innate characteristics of biofilm bacteria are of biotechnological interest, such as the synthesis of valuable compounds (e.g., surfactants, ethanol) and the enhancement/processing of certain foods (e.g., table olives). Understanding the ecology of biofilm formation will allow the design of systems that will facilitate making products of interest and improve their yields.

Staphylococcus epidermidis Biofilm-Released Cells Induce a Prompt and More Marked In vivo Inflammatory-Type Response than Planktonic or Biofilm Cells

Staphylococcus epidermidis biofilm formation on indwelling medical devices is frequently associated with the development of chronic infections. Nevertheless, it has been suggested that cells released from these biofilms may induce severe acute infections with bacteraemia as one of its major associated clinical manifestations. However, how biofilm-released cells interact with the host remains unclear. Here, using a murine model of hematogenously disseminated infection, we characterized the interaction of cells released from S. epidermidis biofilms with the immune system. Gene expression analysis of mouse splenocytes suggested that biofilm-released cells might be particularly effective at activating inflammatory and antigen presenting cells and inducing cellular apoptosis. Furthermore, biofilm-released cells induced a higher production of pro-inflammatory cytokines, in contrast to mice infected with planktonic cells, even though these had a similar bacterial load in livers and spleens. Overall, these results not only provide insights into the understanding of the role of biofilm-released cells in S. epidermidis biofilm-related infections and pathogenesis, but may also help explain the relapsing character of these infections.

Characterization of an in vitro fed-batch model to obtain cells released from S. epidermidis biofilms

Both dynamic and fed-batch systems have been used for the study of biofilms. Dynamic systems, whose hallmark is the presence of continuous flow, have been considered the most appropriate for the study of the last stage of the biofilm lifecycle: biofilm disassembly. However, fed-batch is still the most used system in the biofilm research field. Hence, we have used a fed-batch system to collect cells released from Staphylococcus epidermidis biofilms, one of the most important etiological agents of medical device-associated biofilm infections. Herein, we showed that using this model it was possible to collect cells released from biofilms formed by 12 different S. epidermidis clinical and commensal isolates. In addition, our data indicated that biofilm disassembly occurred by both passive and active mechanisms, although the last occurred to a lesser extent. Moreover, it was observed that S. epidermidis biofilm-released cells presented higher tolerance to vancomycin and tetracycline, as well as a particular gene expression phenotype when compared with either biofilm or planktonic cells. Using this model, biofilm-released cells phenotype and their interaction with the host immune system could be studied in more detail, which could help providing significant insights into the pathophysiology of biofilm-related infections.

Inactivation of glutamate racemase (MurI) eliminates virulence in Streptococcus mutans

Inhibition of enzymes required for bacterial cell wall synthesis is often lethal or leads to virulence defects. Glutamate racemase (MurI), an essential enzyme in peptidoglycan biosynthesis, has been an attractive target for therapeutic interventions. Streptococcus mutans, one of the many etiological factors of dental caries, possesses a series of virulence factors associated with cariogenicity. However, little is known regarding the mechanism by which MurI influences pathogenesis of S. mutans. In this work, a stable mutant of S. mutans deficient in glutamate racemase (S. mutans FW1718) was constructed to investigate the impact of murI inactivation on cariogenic virulence in S. mutans UA159. Microscopy revealed that the murI mutant exhibited an enlarged cell size, longer cell chains, diminished cell⬜cell aggregation, and altered cell surface ultrastructure compared with the wild-type. Characterization of this mutant revealed that murI deficiency weakened acidogenicity, aciduricity, and biofilm formation ability of S. mutans (P<0.05). Real-time quantitative polymerase chain reaction (qRT-PCR) analysis demonstrated that the deletion of murI reduced the expression of the acidogenesis-related gene ldh by 44-fold (P<0.0001). The expression levels of the gene coding for surface protein antigen P (spaP) and the acid-tolerance related gene (atpD) were down-regulated by 99% (P<0.0001). Expression of comE, comD, gtfB and gtfC, genes related to biofilm formation, were down-regulated 8-, 43-, 85- and 298-fold in the murI mutant compared with the wild-type (P<0.0001), respectively. Taken together, the current study provides the first evidence that MurI deficiency adversely affects S. mutans virulence properties, making MurI a potential target for controlling dental caries.

Pluronics-Formulated Farnesol Promotes Efficient Killing and Demonstrates Novel Interactions with Streptococcus mutans Biofilms

Streptococcus mutans is the primary causative agent of dental caries, one of the most prevalent diseases in the United States. Previously published studies have shown that Pluronic-based tooth-binding micelles carrying hydrophobic antimicrobials are extremely effective at inhibiting S. mutans biofilm growth on hydroxyapatite (HA). Interestingly, these studies also demonstrated that non-binding micelles (NBM) carrying antimicrobial also had an inhibitory effect, leading to the hypothesis that the Pluronic micelles themselves may interact with the biofilm. To explore this potential interaction, three different S. mutans strains were each grown as biofilm in tissue culture plates, either untreated or supplemented with NBM alone (P85), NBM containing farnesol (P85F), or farnesol alone (F). In each tested S. mutans strain, biomass was significantly decreased (SNK test, p < 0.05) in the P85F and F biofilms relative to untreated biofilms. Furthermore, the P85F biofilms formed large towers containing dead cells that were not observed in the other treatment conditions. Tower formation appeared to be specific to formulated farnesol, as this phenomenon was not observed in S. mutans biofilms grown with NBM containing triclosan. Parallel CFU/ml determinations revealed that biofilm growth in the presence of P85F resulted in a 3-log reduction in viability, whereas F decreased viability by less than 1-log. Wild-type biofilms grown in the absence of sucrose or gtfBC mutant biofilms grown in the presence of sucrose did not form towers. However, increased cell killing with P85F was still observed, suggesting that cell killing is independent of tower formation. Finally, repeated treatment of pre-formed biofilms with P85F was able to elicit a 2-log reduction in viability, whereas parallel treatment with F alone only reduced viability by 0.5-log. Collectively, these results suggest that Pluronics-formulated farnesol induces alterations in biofilm architecture, presumably via interaction with the sucrose-dependent biofilm matrix, and may be a viable treatment option in the prevention and treatment of pathogenic plaque biofilms.

Evolution of the rapidly mutating human salivary agglutinin gene (DMBT1) and population subsistence strategy

The dietary change resulting from the domestication of plant and animal species and development of agriculture at different locations across the world was one of the most significant changes in human evolution. An increase in dietary carbohydrates caused an increase in dental caries following the development of agriculture, mediated by the cariogenic oral bacterium Streptococcus mutans. Salivary agglutinin [SAG, encoded by the deleted in malignant brain tumors 1 (DMBT1) gene] is an innate immune receptor glycoprotein that binds a variety of bacteria and viruses, and mediates attachment of S. mutans to hydroxyapatite on the surface of the tooth. In this study we show that multiallelic copy number variation (CNV) within DMBT1 is extensive across all populations and is predicted to result in between 7-20 scavenger-receptor cysteine-rich (SRCR) domains within each SAG molecule. Direct observation of de novo mutation in multigeneration families suggests these CNVs have a very high mutation rate for a protein-coding locus, with a mutation rate of up to 5% per gamete. Given that the SRCR domains bind S. mutans and hydroxyapatite in the tooth, we investigated the association of sequence diversity at the SAG-binding gene of S. mutans, and DMBT1 CNV. Furthermore, we show that DMBT1 CNV is also associated with a history of agriculture across global populations, suggesting that dietary change as a result of agriculture has shaped the pattern of CNV at DMBT1, and that the DMBT1-S. mutans interaction is a promising model of host-pathogen-culture coevolution in humans.

Antimicrobial resistance in clinically important biofilms

A biofilm contains a consortium of cohesive bacterial cells forming a complex structure that is a sedentary, but dynamic, community. Biofilms adhere on biotic and abiotic surfaces, including the surfaces of practically all medical devices. Biofilms are reported to be responsible for approximately 60% of nosocomial infections due to implanted medical devices, such as intravenous catheters, and they also cause other foreign-body infections and chronic infections. The presence of biofilm on a medical device may result in the infection of surrounding tissues and failure of the device, necessitating the removal and replacement of the device. Bacteria from biofilms formed on medical devices may be released and disperse, with the potential for the formation of new biofilms in other locations and the development of a systemic infection. Regardless of their location, bacteria in biofilms are tolerant of the activities of the immune system, antimicrobial agents, and antiseptics. Concentrations of antimicrobial agents sufficient to eradicate planktonic cells have no effect on the same microorganism in a biofilm. Depending on the microbial consortium or component of the biofilm that is involved, various combinations of factors have been suggested to explain the recalcitrant nature of biofilms toward killing by antibiotics. In this mini-review, some of the factors contributing to antimicrobial resistance in biofilms are discussed.