Effect of low-intensity ultrasound upon biofilm structure from confocal scanning laser microscopy observation

Effect of biofilm physical characteristics on their susceptibility to antibiotics: impacts of low-frequency ultrasound

Biofilms are highly resistant to antimicrobials, often causing chronic infections. Combining antimicrobials with low-frequency ultrasound (LFU) enhances antimicrobial efficiency, but little is known about the underlying mechanisms. Biofilm physical characteristics, which depend on factors such as growth conditions and age, can have significant effects on inactivation efficiency. In this study, we investigated the susceptibility of Pseudomonas aeruginosa biofilms to tobramycin, with and without LFU treatment. The biofilms were grown under low and high fluid shear to provide different characteristics. Low-shear biofilms exhibited greater thickness, roughness, and porosity and lower density, compared to high-shear biofilms. The biofilm matrix of the high-shear biofilms had a three times higher protein-to-polysaccharide ratio, suggesting greater biofilm stiffness. This was supported by microrheology measurements of biofilm creep compliance. For the low-shear biofilms without LFU, the viability of the biofilms in their inner regions was largely unaffected by the antibiotic after a 2-hour treatment. However, when tobramycin was combined with LFU, the inactivation for the entire biofilm increased to 80% after 2 h. For the high-shear biofilms without LFU, higher LFU intensities were needed to achieve similar inactivation results. Microrheology measurements revealed that changes in biofilm inactivation profiles were closely related to changes in biofilm mechanical properties. Modeling suggests that LFU changes antibiotic diffusivity within the biofilm, probably due to a "decohesion" effect. Overall, this research suggests that biofilm physical characteristics (e.g., compliance, morphology) are linked to antimicrobial efficiency. LFU weakens the biofilm while increasing its diffusivity for antibiotics.

Nanoparticle ultrasonication outperforms conventional irrigation solutions in eradicating Staphylococcus aureus biofilm from titanium surfaces: an in vitro study

PURPOSE

Bacterial biofilms create a challenge in the treatment of prosthetic joint infection (PJI), and failure to eradicate biofilms is often implicated in the high rates of recurrence. In this study, we aimed to compare the effectiveness of a novel nanoparticle ultrasonication technology on Staphylococcus aureus biofilm eradication compared to commonly used orthopedic irrigation solutions.

METHODS

Twenty-four sterile, titanium alloy discs were inoculated with a standardized concentration of methicillin-resistant S. aureus and cultured for seven days to allow for biofilm formation. Discs were then treated with either ultrasonicated nanoparticle therapy or irrigation with chlorhexidine gluconate, povidone-iodine or normal saline. The remaining bacteria on each surface was subsequently plated for colony-forming units of S. aureus. Bacterial eradication was reported as a decrease in CFUs relative to the control group. Mann-Whitney U tests were used to compare between groups.

RESULTS

Treatment with ultrasonicated nanoparticles resulted in a significant mean decrease in CFUs of 99.3% compared to controls (p < 0.0001). Irrigation with povidone-iodine also resulted in a significant 77.5% reduction in CFUs compared to controls (p < 0.0001). Comparisons between ultrasonicated nanoparticles and povidone-iodine demonstrated a significantly higher reduction in bacterial CFUs in the nanoparticle group (p < 0.0001).

CONCLUSION

Ultrasonicated nanoparticle were superior to commonly used bactericidal irrigation solutions in the eradication of S. aureus from a titanium surface. Future clinical studies are warranted to evaluate this ultrsonication technology in the treatment of PJI.

Molecular and proteomic response of Pseudomonas fluorescens biofilm cultured on lettuce (Lactuca sativa L.) after ultrasound treatment at different intensity levels

Ultrasonic treatment is widely used for surface cleaning of vegetables in the processing of agricultural products. In the present study, the molecular and proteomic response of Pseudomonas fluorescens biofilm cultured on lettuce was investigated after ultrasound treatment at different intensity levels. The results show that the biofilm was efficiently removed after ultrasound treatment with intensity higher than 21.06 W/cm2. However, at an intensity of less than 18.42 W/cm2, P. fluorescens was stimulated by ultrasound leading to promoted bacterial growth, extracellular protease activity, extracellular polysaccharide secretion (EPS), and synthesis of acyl-homoserine lactones (AHLs) as quorum-sensing signaling molecules. The expression of biofilm-related genes, stress response, and dual quorum sensing system was upregulated during post-treatment ultrasound. Proteomic analysis showed that ultrasound activated proteins in the flagellar system, which led to changes in bacterial tendency; meanwhile, a large number of proteins in the dual-component system began to be regulated. ABC transporters accelerated the membrane transport of substances inside and outside the cell membrane and equalized the permeability conditions of the cell membrane. In addition, the expression of proteins related to DNA repair was upregulated, suggesting that bacteria repair damaged DNA after ultrasound exposure.

Overcoming biological barriers to improve treatment of a Staphylococcus aureus wound infection

Chronic wounds frequently become infected with bacterial biofilms which respond poorly to antibiotic therapy. Aminoglycoside antibiotics are ineffective at treating deep-seated wound infections due to poor drug penetration, poor drug uptake into persister cells, and widespread antibiotic resistance. In this study, we combat the two major barriers to successful aminoglycoside treatment against a biofilm-infected wound: limited antibiotic uptake and limited biofilm penetration. To combat the limited antibiotic uptake, we employ palmitoleic acid, a host-produced monounsaturated fatty acid that perturbs the membrane of gram-positive pathogens and induces gentamicin uptake. This novel drug combination overcomes gentamicin tolerance and resistance in multiple gram-positive wound pathogens. To combat biofilm penetration, we examined the ability of sonobactericide, a non-invasive ultrasound-mediated-drug delivery technology to improve antibiotic efficacy using an in vivo biofilm model. This dual approach dramatically improved antibiotic efficacy against a methicillin-resistant Staphylococcus aureus (MRSA) wound infection in diabetic mice.

Ultrasound-Assisted Cavitation Effect on the Biofilm-Forming Ability of Common Dairy Sporeformers

Thermoduric sporeformers survive heat treatment and can form biofilm on contact food surfaces that is difficult to clean and may cause cross contamination to milk products. It was hypothesized that cavitation would influence sporeformers’ ability to attach to contact surfaces and form biofilm. Common dairy sporeformers of Geobacillus stearothermophilus, Bacillus licheniformis, and Bacillus sporothermodurans were individually inoculated in sterile skim milk at the levels of 6.0 log CFU/mL. Inoculated samples were treated by cavitation at 80% amplitudes for 10 min each. Pre and post samples were used to develop biofilms on stainless steel coupons under static conditions. Scanning electron micrograph was used to observe the developed biofilms. All the experiments were conducted in triplicate and were statistically analyzed using a t test. The average counts of spiked milk samples were 7.2, 8.0, and 7.7 logs CFU/mL, respectively, for the three sporeformers. Post-cavitation counts were reduced significantly to 3.4, 4.2, and 3.7 logs CFU/mL, respectively. Pre-cavitation biofilm counts of the three sporeformers were 5.35, 6.42, and 6.5 logs CFU/ cm2, respectively in 72 h. The three sporeformers’ biofilm showed significantly (p < 0.05) lower counts after cavitation of 4.39, 5.44, and lower counts of 4.39 logs CFU/cm2, respectively, for the three organisms. The result showed that G. stearothermophilus formed the least biofilms among others after cavitation. Although the ultrasonication treatment reduced the number of sporeformer bacteria, the survivors still retained the ability to attach to the stainless-steel food contact surfaces.

Drug delivery approaches for enhanced antibiofilm therapy

Biofilms have attracted increasing attention in recent years. Many bacterial infections are associated with biofilm formation. A bacterial biofilm is an aggregated membrane-like substance that is composed of a large number of bacteria and their secreted extracellular polymeric substances. The traditional antibiofilm approaches, such as chemotherapy based on antibiotics, are often ineffective in eradicating biofilms owing to the limited diffusion ability of antibiotics within biofilms and inactivation of antibiotics by biofilms. Moreover, a larger dosage of antibiotics could be effective, but leads to an increased tolerance. Smart drug delivery systems that deliver antibiotics into the biofilm interior is a promising strategy to meet this challenge. In this review, we focus on the methods to improve drug delivery efficiency for enhanced chemotherapy of biofilms. Furthermore, we have summarized chemical approaches for enhanced drug delivery, such as chemical shields, charge reversal, and dual corona enhanced delivery strategies; these methods focus on physicochemical biofilm properties and specific biofilm features. Afterwards, physical approaches are discussed, such as magnetism-mediated drug delivery, electricity-mediated drug delivery, ultrasound-mediated drug delivery, and shock wave-mediated drug delivery. Finally, a perspective on the development of next-generation antibiofilm drug delivery systems is given.

Harnessing ultrasound-stimulated phase change contrast agents to improve antibiotic efficacy against methicillin-resistant Staphylococcus aureus biofilms

Bacterial biofilms, often associated with chronic infections, respond poorly to antibiotic therapy and frequently require surgical intervention. Biofilms harbor persister cells, metabolically indolent cells, which are tolerant to most conventional antibiotics. In addition, the biofilm matrix can act as a physical barrier, impeding diffusion of antibiotics. Novel therapeutic approaches frequently improve biofilm killing, but usually fail to achieve eradication. Failure to eradicate the biofilm leads to chronic and relapsing infection, is associated with major financial healthcare costs and significant morbidity and mortality. We address this problem with a two-pronged strategy using 1) antibiotics that target persister cells and 2) ultrasound-stimulated phase-change contrast agents (US-PCCA), which improve antibiotic penetration.

We previously demonstrated that rhamnolipids, produced by Pseudomonas aeruginosa, could induce aminoglycoside uptake in gram-positive organisms, leading to persister cell death. We have also shown that US-PCCA can transiently disrupt biological barriers to improve penetration of therapeutic macromolecules. We hypothesized that combining antibiotics which target persister cells with US-PCCA to improve drug penetration could improve treatment of methicillin resistant S. aureus (MRSA) biofilms. Aminoglycosides alone or in combination with US-PCCA displayed limited efficacy against MRSA biofilms. In contrast, the anti-persister combination of rhamnolipids and aminoglycosides combined with US-PCCA dramatically improved biofilm killing. This novel treatment strategy has the potential for rapid clinical translation as the PCCA formulation is a variant of FDA-approved ultrasound contrast agents that are already in clinical practice and the low-pressure ultrasound settings used in our study can be achieved with existing ultrasound hardware at pressures below the FDA set limits for diagnostic imaging.

Strategies to Improve the Antimicrobial Efficacy of Photodynamic, Sonodynamic, and Sonophotodynamic Therapies

BACKGROUND AND OBJECTIVES

This work evaluated antimicrobial photodynamic therapy (PDT), sonodynamic therapy (SDT), and the association of both therapies (sonophotodynamic therapy [SPDT]), mediated by curcumin (Cur) against Staphylococcus aureus biofilm. Next, additional strategies for these treatments were assessed.

MATERIALS AND METHODS

S. aureus biofilms received PDT, SDT, and SPDT, mediated by Cur (80 µM), LED light (450 nm), and 1 MHz ultrasound. The same treatments were also performed adding a strategy: Cur with sodium dodecyl sulfate (SDS), Cur with potassium iodide (KI) or a pre-treatment with ultrasound. Cell viability was determined and biofilm architecture was evaluated under confocal microscopy.

RESULTS

SPDT was more effective to inactivate the bacteria than PDT and SDT. SDS achieved the greatest viability reductions, followed by KI and ultrasound pre-treatment. Confocal images revealed biofilm disruption and a reduced number of cells in all treatments. However, SPDT exhibited a pronounced effect and it was greater using SDS.

CONCLUSION

SPDT was more effective and additional strategies potentiated its effectiveness. Lasers Surg. Med. © 2021 Wiley Periodicals LLC.

Improved biofilm removal using cavitation from a dental ultrasonic scaler vibrating in carbonated water

The use of cavitation for improving biofilm cleaning is of great interest. There is no system at present that removes the biofilm from medical implants effectively and specifically from dental implants. Cavitation generated by a vibrating dental ultrasonic scaler tip can clean biomaterials such as dental implants. However, the cleaning process must be significantly accelerated for clinical applications. In this study we investigated whether the cavitation could be increased, by operating the scaler in carbonated water with different CO₂ concentrations. The cavitation around an ultrasonic scaler tip was recorded with high speed imaging. Image analysis was used to calculate the area of cavitation. Bacterial biofilm was grown on surfaces and its removal was imaged with a high speed camera using the ultrasonic scaler in still and carbonated water. Cavitation increases significantly with increasing carbonation. Cavitation also started earlier around the tips when they were in carbonated water compared to non-carbonated water. Significantly more biofilm was removed when the scaler was operated in carbonated water. Our results suggest that using carbonated water could significantly increase and accelerate cavitation around ultrasonic scalers in a clinical situation and thus improve biofilm removal from dental implants and other biomaterials.

Biofilm dispersion: The key to biofilm eradication or opening Pandora's box?

Biofilms are extremely difficult to eradicate due to their decreased antibiotic susceptibility. Inducing biofilm dispersion could be a potential strategy to help combat biofilm-related infections. Mechanisms of biofilm dispersion can basically be divided into two groups, i.e. active and passive dispersion. Active dispersion depends on a decrease in the intracellular c-di-GMP levels, leading to the production of enzymes that degrade the biofilm matrix and promote dispersion. In contrast, passive dispersion relies on triggers that directly release cells from the biofilm. In the present review, several active and passive dispersion strategies are discussed. In addition, the disadvantages and possible consequences of using dispersion as a treatment approach for biofilm-related infections are also reviewed.

Potential biofilm control strategies for extended spaceflight missions

Biofilms, surface-adherent microbial communities, are associated with microbial fouling and corrosion in terrestrial water-distribution systems. Biofilms are also present in human spaceflight, particularly in the Water Recovery System (WRS) on the International Space Station (ISS). The WRS is comprised of the Urine Processor Assembly (UPA) and the Water Processor Assembly (WPA) which together recycles wastewater from human urine and recovered humidity from the ISS atmosphere. These wastewaters and various process streams are continually inoculated with microorganisms primarily arising from the space crew microbiome. Biofilm-related fouling has been encountered and addressed in spacecraft in low Earth orbit, including ISS and the Russian Mir Space Station. However, planned future missions beyond low Earth orbit to the Moon and Mars present additional challenges, as resupplying spare parts or support materials would be impractical and the mission timeline would be in the order of years in the case of a mission to Mars. In addition, future missions are expected to include a period of dormancy in which the WRS would be unused for an extended duration. The concepts developed in this review arose from a workshop including NASA personnel and representatives with biofilm expertise from a wide range of industrial and academic backgrounds. Here, we address current strategies that are employed on Earth for biofilm control, including antifouling coatings and biocides and mechanisms for mitigating biofilm growth and damage. These ideas are presented in the context of their applicability to spaceflight and identify proposed new topics of biofilm control that need to be addressed in order to facilitate future extended, crewed, spaceflight missions.

Which Parameters Affect Biofilm Removal with Acoustic Cavitation? A Review

Bacterial biofilms are a cause of contamination in a wide range of medical and biological areas. Ultrasound is a mechanical energy that can remove these biofilms using cavitation and acoustic streaming, which generate shear forces to disrupt biofilm from a surface. The aim of this narrative review is to investigate the literature on the mechanical removal of biofilm using acoustic cavitation to identify the different operating parameters affecting its removal using this method. The properties of the liquid and the properties of the ultrasound have a large impact on the type of cavitation generated. These include gas content, temperature, surface tension, frequency of ultrasound and acoustic pressure. For many of these parameters, more research is required to understand their mechanisms in the area of ultrasonic biofilm removal, and further research will help to optimise this method for effective removal of biofilms from different surfaces.

Assessment of a low-frequency ultrasound device on prevention of biofilm formation and carbonate deposition in drinking water systems

A device generating low-frequency and low-intensity ultrasound waves was used for mitigating biofilm accumulation and scaling. Two systems were tested: a lab-scale plate heat exchanger operated with continuously recycled water and a continually fed flow-through drinking water pilot used for mimicking water circulation in pipes. Initial deposition of bacterial cells was not prevented by ultrasound wave treatment. However, whatever the tested system, both further calcium carbonate deposition and biofilm growth were markedly inhibited. Biofilms formed in reactors subjected to low-frequency and low-intensity ultrasound waves were weakly attached to the material. Even though the activity of bacteria was affected as shown by their lower cultivability, membrane permeability did not appear compromised. Ultrasound technology sounds very promising in both the mitigation of drinking water biofilm and carbonate accumulation.

Removal of Staphylococcus aureus from skin using a combination antibiofilm approach

Staphylococcus aureus (S. aureus) including methicillin resistant S. aureus (MRSA) is one of the primary microorganisms responsible for surgical site infection (SSI). Since S. aureus contamination is known to originate from the skin, eradicating it on the skin surface at surgical sites is an important intervention to reduce the chance of SSIs. Here we developed and evaluated the efficacy of a combination probiotic/brush sonication strategy for skin preparation at surgical, injection and insertion sites in medicine. A 24 h biofilm on porcine skin explants was used as a worst-case scenario for the evaluation of preparation strategies. Conventional ethanol wipes achieved 0.8~2 log reduction in viable bacteria depending on how many times wiped (x4 or x6). Brush sonication or probiotic supernatant pre-treatment alone achieved a similar reduction as ethanol wipes (1.4 and 0.7~1.4 log reduction, respectively). Notably, combining sonication and probiotic pre-treatment achieved a 4 log reduction in viable bacteria. In addition, probiotic supernatant incubation times as short as 2 h achieved the full effect of this reduction in the combined strategy. These findings suggest the promising potential of combination-format skin preparation strategies that can be developed to more effectively penetrate cracks and folds in the skin to remove biofilms.

Combining brush sonication with secretions from probiotic bacteria cleans skin before surgery more effectively than ethanol wipes. Researchers in the USA, led by K. Scott Phillips at the United States Food and Drug Administration, investigated removal of Staphylococcus aureus biofilm from pig skin as a “worst case” pre-surgical scenario. This bacterium is a major cause of serious and drug-resistant surgical site infections. Brush sonication or treatment with probiotic-derived solutions were individually approximately as effective as ethanol wipes, but in combination they proved substantially more effective. The treatment with the secretions surrounding probiotic bacterial cells requires exposure for several hours, but this could be readily achieved using a pre-surgery ointment. The sonication and probiotic combination could be developed into a highly effective pre-surgical procedure, penetrating cracks and folds in the skin to remove dangerous biofilms.

What Is the Role of Diagnostic and Therapeutic Sonication in Periprosthetic Joint Infections?

BACKGROUND

Periprosthetic joint infection (PJI) is one of the most dreaded complications in joint replacement surgery. Diagnosis and treatment can be difficult and biofilms are of major concern due to their low susceptibility toward antibiotics.

METHODS

This review focuses on the use of sonication as an evolving diagnostic and adjunct treatment modality in the context of PJI. Therapeutic application of sonication is discussed separately for its (i) direct action on bacteria, (ii) synergistic effects with antibiotics, and (iii) effects on release of antibiotics from bone cement.

RESULTS

Used as a diagnostic tool, sonication shows promising results with respect to sensitivity and specificity when compared to conventional methods, notably after previous administration of antibiotics. As an adjunct treatment modality, the chemical, physical, and mechanical effects of sonication are primarily driven by cavitation and recognized as the main cause for bactericidal effects but the exact underlying mechanisms have not been identified yet. Sonication alone does not have the ability to completely eradicate biofilms but synergistic effects when used in conjunction with antibiotics have been reported. There is also evidence for enhanced antibiotic release from bone cement.

CONCLUSION

Sonication is as an evolving modality in the context of PJIs. As a diagnostic tool, it has not been introduced in routine clinical practice and sonication as a treatment modality in PJIs is still in an experimental stage. Factors such as frequency, pressure, chemical activity, intensity, and exposure time need to be evaluated for optimal application of sonication and may also improve study comparison.

A Review of the Combination Therapy of Low Frequency Ultrasound with Antibiotics

Single antimicrobial therapy has been unable to resist the global spread of bacterial resistance. Literatures of available in vitro and in vivo studies were reviewed and the results showed that low frequency ultrasound (LFU) has a promising synergistic bactericidal effect with antibiotics against both planktonic and biofilm bacteria. It also can facilitate the release of antibiotics from medical implants. As a noninvasive and targeted therapy, LFU has great potential in treating bacterial infections. However, more in-depth and detailed studies are still needed before LFU is officially applied as a combination therapy in the field of anti-infective treatment.

Spatial Organization Plasticity as an Adaptive Driver of Surface Microbial Communities

Biofilms are dynamic habitats which constantly evolve in response to environmental fluctuations and thereby constitute remarkable survival strategies for microorganisms. The modulation of biofilm functional properties is largely governed by the active remodeling of their three-dimensional structure and involves an arsenal of microbial self-produced components and interconnected mechanisms. The production of matrix components, the spatial reorganization of ecological interactions, the generation of physiological heterogeneity, the regulation of motility, the production of actives enzymes are for instance some of the processes enabling such spatial organization plasticity. In this contribution, we discussed the foundations of architectural plasticity as an adaptive driver of biofilms through the review of the different microbial strategies involved. Moreover, the possibility to harness such characteristics to sculpt biofilm structure as an attractive approach to control their functional properties, whether beneficial or deleterious, is also discussed.

A quantitative method to measure biofilm removal efficiency from complex biomaterial surfaces using SEM and image analysis

Biofilm accumulation on biomaterial surfaces is a major health concern and significant research efforts are directed towards producing biofilm resistant surfaces and developing biofilm removal techniques. To accurately evaluate biofilm growth and disruption on surfaces, accurate methods which give quantitative information on biofilm area are needed, as current methods are indirect and inaccurate. We demonstrate the use of machine learning algorithms to segment biofilm from scanning electron microscopy images. A case study showing disruption of biofilm from rough dental implant surfaces using cavitation bubbles from an ultrasonic scaler is used to validate the imaging and analysis protocol developed. Streptococcus mutans biofilm was disrupted from sandblasted, acid etched (SLA) Ti discs and polished Ti discs. Significant biofilm removal occurred due to cavitation from ultrasonic scaling (p < 0.001). The mean sensitivity and specificity values for segmentation of the SLA surface images were 0.80 ± 0.18 and 0.62 ± 0.20 respectively and 0.74 ± 0.13 and 0.86 ± 0.09 respectively for polished surfaces. Cavitation has potential to be used as a novel way to clean dental implants. This imaging and analysis method will be of value to other researchers and manufacturers wishing to study biofilm growth and removal.

USMB-induced synergistic enhancement of aminoglycoside antibiotics in biofilms

This study evaluated the effect of combining antibiotics with ultrasound and microbubbles (USMB) toward the eradication of biofilms. Pseudomonas aeruginosa PAO1 biofilms were treated with the antibiotics gentamicin sulfate or streptomycin sulfate, or a combination of USMB with the respective antibiotics. Biofilm structure was quantified using confocal laser scanning microscopy with COMSTAT analysis, while activity was measured as whole-biofilm CO2 production in a continuous-flow biofilm model. The combined antibiotic-USMB treatment significantly impacted biofilm biomass, thickness and surface roughness compared to antibiotics alone (p<0.05). USMB exposure caused the formation of craters (5-20μm in diameter) in the biofilms, and when combined with gentamicin, activity was significantly lower, compared to gentamicin, USMB or untreated controls, respectively. Interestingly, the CO2 production rate following combined streptomycin-USMB treatment was higher than after streptomycin alone, but significantly lower than USMB alone and untreated control. These results show strong evidence of a synergistic effect between antibiotics and USMB, although the varied response to different antibiotics emphasize the need to optimize the USMB exposure conditions to maximize this synergism and ultimately transfer this technology into clinical or industrial practice.

A model of ultrasound-enhanced diffusion in a biofilm

A stochastic model is presented for nanoparticle transport in a biofilm to explain how the combination of acoustic oscillations and intermittent retention due to interaction with the pore walls of the biofilm leads to diffusion enhancement. An expression for the effective diffusion coefficient was derived that varies with the square of the oscillation velocity amplitude. This expression was validated by comparison of an analytical diffusion solution to the stochastic model prediction. The stochastic model was applied to an example problem associated with liposome penetration into a hydrogel, and it was found to yield solutions in which liposome concentration varied exponentially with distance into the biofilm.

Bactericidal Effect of Photolysis of H2O2 in Combination with Sonolysis of Water via Hydroxyl Radical Generation

The bactericidal effect of hydroxyl radical (·OH) generated by combination of photolysis of hydrogen peroxide (H₂O₂) and sonolysis of water was examined under the condition in which the yield of ·OH increased additively when H₂O₂ aqueous solution was concomitantly irradiated with laser and ultrasound. The suspension of Staphylococcus aureus mixed with the different concentrations of H₂O₂ was irradiated simultaneously with a laser light (wavelength: 405 nm, irradiance: 46 and 91 mW/cm²) and ultrasound (power: 30 w, frequency: 1.65 MHz) at 20 ± 1°C of the water bulk temperature for 2 min. The combination of laser and ultrasound irradiation significantly reduced the viable bacterial count in comparison with the laser irradiation of H₂O₂ alone. By contrast, the ultrasound irradiation alone exerted almost no bactericidal effect. These results suggested that the combination effect of photolysis of H₂O₂ and sonolysis of water on bactericidal activity was synergistic. A multi-way analysis of variance also revealed that the interaction of H₂O₂ concentration, laser power and ultrasound irradiation significantly affected the bactericidal activity. Since the result of oxidative DNA damage evaluation demonstrated that the combination of laser and ultrasound irradiation significantly induced oxidative damage of bacterial DNA in comparison with the laser irradiation of H₂O₂ alone, it was suggested that the combination effect of photolysis of H₂O₂ and sonolysis of water on bactericidal activity would be exerted via oxidative damage of cellular components such as DNA.

Biofilms in wounds: a review of present knowledge

Following confirmation of the presence of biofilms in chronic wounds, the term biofilm became a buzzword within the wound healing community. For more than a century pathogens have been successfully isolated and identified from wound specimens using techniques that were devised in the nineteenth century by Louis Pasteur and Robert Koch. Although this approach still provides valuable information with which to help diagnose acute infections and to select appropriate antibiotic therapies, it is evident that those organisms isolated from clinical specimens with the conditions normally used in diagnostic laboratories are mainly in a planktonic form that is unrepresentative of the way in which most microbial species exist naturally. Usually microbial species adhere to each other, as well as to living and non-living surfaces, where they form complex communities surrounded by collectively secreted extracellular polymeric substances (EPS). Cells within such aggregations (or biofilms) display varying physiological and metabolic properties that are distinct from those of planktonic cells, and which contribute to their persistence. There are many factors that influence healing in wounds and the discovery of biofilms in chronic wounds has provided new insight into the reasons why. Increased tolerance of biofilms to antimicrobial agents explains the limited efficacy of antimicrobial agents in chronic wounds and illustrates the need to develop new management strategies. This review aims to explain the nature of biofilms, with a view to explaining their impact on wounds.

Viscoelasticity of biofilms and their recalcitrance to mechanical and chemical challenges

We summarize different studies describing mechanisms through which bacteria in a biofilm mode of growth resist mechanical and chemical challenges. Acknowledging previous microscopic work describing voids and channels in biofilms that govern a biofilms response to such challenges, we advocate a more quantitative approach that builds on the relation between structure and composition of materials with their viscoelastic properties. Biofilms possess features of both viscoelastic solids and liquids, like skin or blood, and stress relaxation of biofilms has been found to be a corollary of their structure and composition, including the EPS matrix and bacterial interactions. Review of the literature on viscoelastic properties of biofilms in ancient and modern environments as well as of infectious biofilms reveals that the viscoelastic properties of a biofilm relate with antimicrobial penetration in a biofilm. In addition, also the removal of biofilm from surfaces appears governed by the viscoelasticity of a biofilm. Herewith, it is established that the viscoelasticity of biofilms, as a corollary of structure and composition, performs a role in their protection against mechanical and chemical challenges. Pathways are discussed to make biofilms more susceptible to antimicrobials by intervening with their viscoelasticity, as a quantifiable expression of their structure and composition.

Recalcitrance of biofilms against mechanical and chemical challenges has been looked at for ages from a microbiological perspective, but an approach based on viscoelastic properties of biofilms yields new insights in this recalcitrance.

Sound waves effectively assist tobramycin in elimination of Pseudomonas aeruginosa biofilms in vitro

Microbial biofilms are highly refractory to antimicrobials. The aim of this study was to investigate the use of low-frequency vibration therapy (20-20 kHz) on antibiotic-mediated Pseudomonas aeruginosa biofilm eradication. In screening studies, low-frequency vibrations were applied on model biofilm compositions to identify conditions in which surface standing waves were observed. Alginate surface tension and viscosity were also measured. The effect of vibration on P. aeruginosa biofilms was studied using a standard biofilm assay. Subminimal inhibitory concentrations (sub-MIC) of tobramycin (5 μg/ml) were added to biofilms 3 h prior, during, and immediately after vibration and quantitatively assessed by (2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) reduction assay (XTT) and, qualitatively, by confocal laser scanning microscopy (CLSM). The standing waves occurred at frequencies <1,000 Hz. Biofilms vibrated without sub-MIC tobramycin showed a significantly reduced metabolism compared to untreated controls (p < 0.05). Biofilms treated with tobramycin and vibrated simultaneously (450, 530, 610, and 650 Hz), or vibrated (450 and 650 Hz) then treated with tobramycin subsequently, or vibrated (610 Hz, 650 Hz) after 3 h of tobramycin treatment showed significantly lower metabolism compared to P. aeruginosa biofilm treated with tobramycin alone (p < 0.05). CLSM imaging further confirmed these findings. Low frequency vibrations assisted tobramycin in killing P. aeruginosa biofilms at sub-MIC. Thus, sound waves together with antibiotics are a promising approach in eliminating pathogenic biofilms.

A new horizon in the treatment of biofilm-associated tonsillitis

Objective:

To demonstrate the efficacy of tonsil brushing in patients with chronic tonsillitis to remove the microbial biofilm on the tonsil surface using an in vitro model.

Design:

Specimens from patients undergoing tonsillectomy were evaluated prior to and following surface cleaning methods, including rinsing and brushing, using scanning electron microscopy (SEM).

Patients:

The study population consisted of 25 randomly selected patients with chronic tonsillitis.

Interventions:

Specimens were collected and divided into four portions. Each portion received distinct surface cleaning methods and was immediately fixed for SEM examination.

Outcome measures:

The biofilm layer on the surface of the tonsils was examined using SEM. The density of the biofilm layer and the degree of persistence of the biofilm after rinsing and brushing were measured.

Results:

The surface biofilm of the tonsils in the first group, which were neither brushed nor rinsed, revealed a thick layer of biofilm on the mucosal surface. The second group of tonsils, which were only rinsed, also showed a thick layer of biofilm. The third group of tonsils, which were rinsed following gentle brushing using a soft toothbrush, showed a reduction in the thickness of the biofilm layer. The fourth group of tonsils, which were brushed with a hard brush, was almost devoid of a biofilm layer.

Conclusion:

Our results demonstrate that rinsing does not effectively remove the biofilm layer on the tonsil surface. The use of a harder brush was identified as a more powerful means of removing biofilm compared with a soft brush.

Application of sub-micrometer vibrations to mitigate bacterial adhesion

As a prominent concern regarding implantable devices, eliminating the threat of opportunistic bacterial infection represents a significant benefit to both patient health and device function. Current treatment options focus on chemical approaches to negate bacterial adhesion, however, these methods are in some ways limited. The scope of this study was to assess the efficacy of a novel means of modulating bacterial adhesion through the application of vibrations using magnetoelastic materials. Magnetoelastic materials possess unique magnetostrictive property that can convert a magnetic field stimulus into a mechanical deformation. In vitro experiments demonstrated that vibrational loads generated by the magnetoelastic materials significantly reduced the number of adherent bacteria on samples exposed to Escherichia coli, Staphylococcus epidermidis and Staphylococcus aureus suspensions. These experiments demonstrate that vibrational loads from magnetoelastic materials can be used as a post-deployment activated means to deter bacterial adhesion and device infection.

Therapeutic ultrasound as a treatment modality for chronic rhinosinusitis

Chronic rhinosinusitis (CRS) is a chronic infective, inflammatory upper respiratory disease. While the current medical treatment of CRS focuses on the systemic and topical use of steroids and/or antibiotics, many bacteria residing on mucosal surfaces of patients with CRS exist in a biofilm state, making them resistant to most systemic antibiotics. Alternative therapeutic strategies that include blocking bacterial molecular communication, inhibiting biofilm matrix production and breaking down bacterial biofilms are all being explored. Physical therapies such as therapeutic ultrasound (US) have been advocated and utilized as a treatment modality for CRS for many years. US may have antiinflammatory actions and can also be used for the local delivery of drugs through the skin. Therapeutic US, which has been shown in clinical studies to be an effective treatment for both acute rhinosinusitis and CRS, offers significant potential in CRS management.

Removal of biofilms by intermittent low-intensity ultrasonication triggered bursting of microbubbles

In this study, a chemical-free cleaning method for biofilms removal is presented, which is based on intermittent low-intensity ultrasonication (US) triggered bursting of microbubbles (MB) in such a sequence that MB were continuously introduced into the reaction vessel for 15 min, while US was activated for 2 s after every 2 min of microbubbling. It was found that the fixed biomass, and the extracellular proteins and polysaccharides of 24-h old biofilms grown on a nylon membrane surface were reduced, respectively, by 75, 79 and 72% after treatment by the US + MB method. Fourier transform infrared (FTIR) analysis further revealed that the chemical composition of the biofilms was not altered by the US + MB treatment, suggesting that biofilms were removed through physical forces due to the generation of a shock wave and a high-speed water jet through US-triggered bursting of the MB. The proposed method can be considered a chemical-free technology for biofilm removal.

Microbial biofilm modulation by ultrasound: current concepts and controversies

Biofilm elimination is often necessary during antimicrobial therapy or industrial medical manufacturing decontamination. In this context, ultrasound treatment has been frequently described in the literature for its antibiofilm effectiveness, but at the same time, various authors have described ultrasound as a formidable enhancer of bacterial viability. This discrepancy has found no solution in the current literature for around 9 years; some works have shown that every time bacteria are exposed to an ultrasonic field, both destruction and stimulation phenomena co-exist. This co-existence proves to have different final effects based on various factors such as: ultrasound frequency and intensity, the bacterial species involved, the material used for ultrasound diffusion, the presence of cavitation effects and the forms of bacterial planktonic or biofilm. The aim of this work is to analyze current concepts regarding ultrasound effect on prokaryotic cells, and in particular ultrasound activity on bacterial biofilm.

Treatment of chronic rhinosinusitis using erythromycin phonophoresis

Bacterial biofilms have been implicated in many chronic infective diseases, including chronic rhinosinusitis (CRS). Therapeutic ultrasound enhances the breakdown of bacterial biofilms and is clinically effective in CRS treatment, while phonophoresis has also been utilized for antibiotic delivery through the skin. The objective of this case report is to describe the results of a novel treatment, erythromycin phonophoresis, in a woman with CRS. A 31-year-old woman with a 7-month history of CRS refractory to conventional medical management was treated with erythromycin phonophoresis to both maxillary sinuses. Individual sinus symptom severity was assessed and sinus CT scans were obtained both pretreatment and posttreatment. After treatment, the total sinusitis symptom score improved from 12 to 0 and the CT scan showed almost complete disease resolution. At 5-month follow-up, the patient reported sustained improvement. Erythromycin phonophoresis has potential as an effective treatment for CRS.

Synergistic bactericidal effects and mechanisms of low intensity ultrasound and antibiotics against bacteria: a review

Low intensity ultrasonic therapy is always an important research area of ultrasonic medicine. This review concentrates on low intensity ultrasound enhancing bactericidal action of antibiotics against bacteria in vitro and in vivo, including planktonic bacteria, bacterial biofilms, Chlamydia, and bacteria in implants. These literatures show that low intensity ultrasound alone is not effective in killing bacteria, while the combination of low intensity ultrasound and antibiotics is promising. Low intensity ultrasound facilitating antibiotic treatment is still in its infancy, and still requires a great deal of research in order to develop the technology on medical treatment scale.

Enhanced hydrogen production by Rhodopseudomonas palustris CQK 01 with ultra-sonication pretreatment in batch culture

In the present study, the photo-hydrogen production performances by Rhodopseudomonas palustris CQK 01 growing from the inoculated cells with ultra-sonication pretreatment (R. palustris CQK 01-USP) were experimentally investigated in batch culture and compared with that without pretreatment (R. palustris CQK 01-NP). It was found that the ultra-sonication pretreatment modified membrane morphology and broke up part of the cells, resulting in improvement of membrane permeability and bacterial activities and hence, helping the improvement of hydrogen production. The hydrogen production rate, hydrogen yield and energy conversion efficiency with R. palustris CQK 01-USP were increased to be nearly 2 times higher than that with R. palustris CQK 01-NP. The parametric study showed that under the conditions of initial glucose concentration 50 mmol/l, inoculum size 12%, illumination wavelength 590 nm, the photobioreactor with R. palustris CQK 01-USP obtained the optimal hydrogen production rate 0.54 mmol/l/h, hydrogen yield 1.2 mol-H2/mol-glucose and energy conversion efficiency 9.03%.

Enhancement of vancomycin activity against biofilms by using ultrasound-targeted microbubble destruction

Treating biofilm infections on implanted medical devices is formidable, even with extensive antibiotic therapy. The aim of this study was to investigate whether ultrasound (US)-targeted microbubble (MB) destruction (UTMD) could enhance vancomycin activity against Staphylococcus epidermidis RP62A biofilms. Twelve-hour biofilms were treated with vancomycin combined with UTMD. The vancomycin and MB (SonoVue) were used at concentrations of 100 μg/ml and 30% (vol/vol), respectively, in studies in vitro. After US exposure (0.08 MHz, 1.0 W/cm(2), 50% duty cycle, and 10-min duration), the biofilms were cultured at 37 °C for another 12 h. The results showed that many micropores were found in biofilms treated with vancomycin combined with UTMD. Biofilm densities (A(570) values) and the viable counts of S. epidermidis recovered from the biofilm were significantly decreased compared with those of any other groups. Furthermore, the highest percentage of dead cells was found, using confocal laser scanning microscopy, in the biofilm treated with vancomycin combined with UTMD. The viable counts of bacteria in biofilms in an in vivo rabbit model also confirmed the enhanced effect of vancomycin combined with UTMD. UTMD may have great potential for improving antibiotic activity against biofilm infections.

Bacterial biofilm disruption using laser generated shockwaves

A system was built to test the efficacy of bacterial biofilm disruption using laser generated shockwaves. The system is based on a Q-switched, ND:YAG pulsed laser operating at a rep rate of 10 Hz with 1500 mJ pulses centered at 1064 nm. The laser pulses were used to create shockwave pulses in Al coated polycarbonate substrates and a resulting peak stress of greater than 50 MPa was measured. These stress pulses were coupled to bacteria grown to confluence on agar plates and cell death as a result of shockwave stress was assessed. The results show a 55% reduction in the number living bacteria between shocked and control samples. This type of biofilm disruption method could prove useful in the treatment of infected wounds where standard treatment methods such as debridement and topical antibiotics have proven to be ineffectual or harmful.

A new small molecule specifically inhibits the cariogenic bacterium Streptococcus mutans in multispecies biofilms

Streptococcus mutans is a major cariogenic bacterium. It has adapted to the biofilm lifestyle, which is essential for pathogenesis of dental caries. We aimed to identify small molecules that can inhibit cariogenic S. mutans and to discover lead structures that could give rise to therapeutics for dental caries. In this study, we screened a focused small-molecule library of 506 compounds. Eight small molecules which inhibited S. mutans at a concentration of 4 μM or less but did not affect cell growth or biofilm formation of commensal bacteria, represented by Streptococcus sanguinis and Streptococcus gordonii, in monospecies biofilms were identified. The active compounds share similar structural properties, which are characterized by a 2-aminoimidazole (2-AI) or 2-aminobenzimidazole (2-ABI) subunit. In multispecies biofilm models, the most active compound also inhibited cell survival and biofilm formation of S. mutans but did not affect commensal streptococci. This inhibitor downregulated the expression of six biofilm-associated genes, ftf, pac, relA, comDE, gbpB, and gtfB, in planktonic S. mutans cells, while it downregulated the expression of only ftf, pac, and relA in the biofilm cells of S. mutans. The most potent compound also inhibited production of two key adhesins of S. mutans, antigen I/II and glucosyltransferase (GTF). However, the compound did not alter the expression of the corresponding genes in both S. sanguinis and S. gordonii, indicating that it possesses a selective inhibitory activity against S. mutans.

Removal of Listeria monocytogenes biofilms from stainless steel by use of ultrasound and ozone

The objective of this study was to determine the efficacy of power ultrasound and ozonation used individually, and in tandem, for the removal of Listeria monocytogenes biofilms from stainless steel chips. Stainless steel chips were inoculated with L. monocytogenes. Power ultrasound (20 kHz, 100% amplitude, 120 W) was applied for 30 or 60 s at a distance of 2.54 cm from a biofilm chip while it was submerged in 250 ml of sterile potassium phosphate buffer (pH 7.0). Ozone was cycled through the 250 ml of potassium phosphate buffer containing the biofilm chip also for 30 or 60 s at concentrations of 0.25, 0.5, or 1.0 ppm. Power ultrasound and ozonation were also used in tandem for testing of their combined effect. Each of the treatments alone resulted in a significant reduction in recoverable cells, with power ultrasound being the most effective (3.8-log CFU/ml reduction after 60 s). For the ozone in combination with power ultrasound treatment, reductions were significantly (P < 0.05) higher than by either treatment alone. There were no recoverable cells after 60 s of this combined treatment when an ozone concentration of 0.5 ppm was used (7.31-log CFU/ml reduction). These results indicated that the combination of power ultrasound and ozonation may be an effective treatment for biofilm removal from stainless steel food contact surfaces.

Advances in microbial biofilm prevention on indwelling medical devices with emphasis on usage of acoustic energy

Microbial biofilms are a major impediment to the use of indwelling medical devices, generating device-related infections with high morbidity and mortality. Major efforts directed towards preventing and eradicating the biofilm problem face difficulties because biofilms protect themselves very effectively by producing a polysaccharide coating, reducing biofilm sensitivity to antimicrobial agents. Techniques applied to combating biofilms have been primarily chemical. These have met with partial and limited success rates, leading to current trends of eradicating biofilms through physico-mechanical strategies. Here we review the different approaches that have been developed to control biofilm formation and removal, focusing on the utilization of acoustic energy to achieve these objectives.

Prevention of biofilm formation with a coating of 2-methacryloyloxyethyl phosphorylcholine polymer

Device-associated infections are serious complications, and their prevention is an issue of considerable importance. Since biofilms are responsible for these refractory infections, effective methods to inhibit biofilm formation are required. In this investigation, stainless steel plates with and without 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer, i.e., poly (MPC-co-n-butyl methacrylate) (PMB) coating, were incubated in a medium containing bacteria. In the course of incubation, half of the specimens received antibiotics. The specimens were stained for nucleic acid and polysaccharides, and then examined with a confocal laser scanning microscope. The numbers of bacteria on the specimen surfaces were evaluated by an ATP assay. On the surfaces of the specimens without PMB coating, the formation of a biofilm enveloping bacteria was confirmed. The addition of antibiotics did not effectively decrease the number of bacteria. On the other hand, on the surfaces of the specimens with PMB coating, no biofilm formation was observed, and the number of bacteria was significantly decreased. The addition of potent antibiotics further decreased the number of bacteria by 1/100 to 1/1000 times. The PMB coating combined with the validated use of antibiotics might provide a method for the simultaneous achievement of biocompatible surfaces of devices and the prevention of device-associated infections.

Ultrasound-induced cavitation: applications in drug and gene delivery

Ultrasound, which has been conventionally used for diagnostics until recently, is now being extensively used for drug and gene delivery. This transformation has come about primarily due to ultrasound-mediated acoustic cavitation - particularly transient cavitation. Acoustic cavitation has been used to facilitate the delivery of small molecules, as well as macromolecules, including proteins and DNA. Controlled generation of cavitation has also been used for targeting drugs to diseased tissues, including skin, brain, eyes and endothelium. Ultrasound has also been employed for the treatment of several diseases, including thromboembolism, arteriosclerosis and cancer. This review provides a detailed account of mechanisms, current status and future prospects of ultrasonic cavitation in drug and gene delivery applications.

Ultrasonic drug delivery--a general review

Ultrasound has an ever-increasing role in the delivery of therapeutic agents, including genetic material, protein and chemotherapeutic agents. Cavitating gas bodies, such as microbubbles, are the mediators through which the energy of relatively non-interactive pressure waves is concentrated to produce forces that permeabilise cell membranes and disrupt the vesicles that carry drugs. Thus, the presence of microbubbles enormously enhances ultrasonic delivery of genetic material, proteins and smaller chemical agents. Numerous reports show that the most efficient delivery of genetic material occurs in the presence of cavitating microbubbles. Attaching the DNA directly to the microbubbles, or to gas-containing liposomes, enhances gene uptake even further. Ultrasonic-enhanced gene delivery has been studied in various tissues, including cardiac, vascular, skeletal muscle, tumour and even fetal tissue. Ultrasonic-assisted delivery of proteins has found most application in transdermal transport of insulin. Cavitation events reversibly disrupt the structure of the stratus corneum to allow transport of these large molecules. Other hormones and small proteins could also be delivered transdermally. Small chemotherapeutic molecules are delivered in research settings from micelles and liposomes exposed to ultrasound. Cavitation appears to play two roles: it disrupts the structure of the carrier vesicle and releases the drug; and makes cell membranes and capillaries more permeable to drugs. There remains a need to better understand the physics of cavitation of microbubbles and the impact that such cavitation has on cells and drug-carrying vesicles.

Ultrasound enhancement of biocide efficiency

In order to take account of the likely increase in costs of biocides in the light of increasing legislation and concern for the environment, there is a need to maximise the efficiency of biocides for the control of biofouling. The use of ultrasound in conjunction with biocides offers such an opportunity. Tests have been carried out using ultrasound generated at 20 kHz in conjunction with the oxidising biocide ozone, in a laboratory pilot plant, to investigate the effects of mutuality. The preliminary results reported in this paper suggest that the combined effect of ultrasound and the biocide is better than either separately employed. Clearly substantially more work is required in order to maximise effectiveness for minimum cost.