Osmotic Compounds Enhance Antibiotic Efficacy against Acinetobacter baumannii Biofilm Communities

Bacterial Hybrid Light-Emitting Diodes

Photon down-converting filters with fluorescent proteins (FPs) are a new frontier in the quest for rare-earth-free and non-toxic color filters for white light-emitting diodes. There are, however, concerns related to the FP purification costs and lack of FP recyclability/reuse. Here, the direct use of bacteria in photon down-converting filters can be of utmost relevance, eliminating purification and allowing in situ production of new FPs. However, their high background autofluorescence/scattering and low stability in polymer coatings have traditionally hampered the application of Engineering Living Materials (ELMs) for photon manipulation. Indeed, there are no examples of ELMs in lighting systems. This work discloses the first protocol to prepare living spheroplasts with > 90% scattering reduction, high FP expression fairly keeping their photoluminescence figures-of-merit, and excellent resilience in polymer films over 1 year under ambient storage. This unlocked the preparation of the first bacteria hybrid light-emitting diodes integrating ELMs for photon conversion. These devices feature similar stabilities to those using purified FPs, while enabling a cost-effective strategy and active FP recycling by the simple recultivation of spheroplasts. Overall, this work introduces a successful case toward bacteria-polymer photon manipulation, in general, and a new living lighting concept, in particular.

Heat shock potentiates aminoglycosides against gram-negative bacteria by enhancing antibiotic uptake, protein aggregation, and ROS

The potentiation of antibiotics is a promising strategy for combatting antibiotic-resistant/tolerant bacteria. Herein, we report that a 5-min sublethal heat shock enhances the bactericidal actions of aminoglycoside antibiotics by six orders of magnitude against both exponential- and stationary-phase Escherichia coli. This combined treatment also effectively kills various E. coli persisters, E. coli clinical isolates, and numerous gram-negative but not gram-positive bacteria and enables aminoglycosides at 5% of minimum inhibitory concentrations to eradicate multidrug-resistant pathogens Acinetobacter baumannii and Klebsiella pneumoniae. Mechanistically, the potentiation is achieved comprehensively by heat shock-enhanced proton motive force that thus promotes the bacterial uptake of aminoglycosides, as well as by increasing irreversible protein aggregation and reactive oxygen species that further augment the downstream lethality of aminoglycosides. Consistently, protonophores, chemical chaperones, antioxidants, and anaerobic culturing abolish heat shock-enhanced aminoglycoside lethality. We also demonstrate as a proof of concept that infrared irradiation- or photothermal nanosphere-induced thermal treatments potentiate aminoglycoside killing of Pseudomonas aeruginosa in a mouse acute skin wound model. Our study advances the understanding of the mechanism of actions of aminoglycosides and demonstrates a high potential for thermal ablation in curing bacterial infections when combined with aminoglycosides.

Interplay between Amoxicillin Resistance and Osmotic Stress in Helicobacter pylori

Rising antibiotic resistance rates are a growing concern for all pathogens, including Helicobacter pylori. We previously examined the association of specific mutations in PBP1 with amoxicillin resistance and fitness in H. pylori and found that V374L and N562Y mutations were associated with resistance, but also resulted in fitness defects. Furthermore, we found that hyperosmotic stress differentially altered the fitness of strains bearing these mutations; survival of the V374L strain was decreased by hyperosmotic stress, but the N562Y strain showed increased cell survival relative to that of wild-type G27. The finding that amoxicillin-resistant strains show environmentally dictated changes in fitness suggests a previously unexplored interaction between amoxicillin resistance and osmotic stress in H. pylori. Here, we further characterized the interaction between osmotic stress and amoxicillin resistance. Wild-type and isogenic PBP1 mutant strains were exposed to amoxicillin, various osmotic stressors, or combined antibiotic and osmotic stress, and viability was monitored. While subinhibitory concentrations of NaCl did not affect H. pylori viability, the combination of NaCl and amoxicillin resulted in synergistic killing; this was true even for the antibiotic-resistant strains. Moreover, similar synergy was found with other beta-lactams, but not with antibiotics that did not target the cell wall. Similar synergistic killing was also demonstrated when KCl was utilized as the osmotic stressor. Conversely, osmolar equivalent concentrations of sucrose antagonized amoxicillin-mediated killing. Taken together, our results support a previously unrecognized interaction between amoxicillin resistance and osmotic stress in H. pylori. These findings have interesting implications for the effectiveness of antibiotic therapy for this pathogen. IMPORTANCE Rising antibiotic resistance rates in H. pylori are associated with increased rates of treatment failure. Understanding how stressors impact antibiotic resistance may shed light on the development of future treatment strategies. Previous studies found that mutations in PBP1 that conferred resistance to amoxicillin were also associated with a decrease in bacterial fitness. The current study demonstrated that osmotic stress can enhance beta lactam-mediated killing of H. pylori. The source of osmotic stress was found to be important for these interactions. Given that relatively little is known about how H. pylori responds to osmotic stress, these findings fill important knowledge gaps on this topic and provide interesting implications for the effectiveness of antibiotic therapy for this pathogen.

Mechanosensitive Channels Mediate Hypoionic Shock-Induced Aminoglycoside Potentiation against Bacterial Persisters by Enhancing Antibiotic Uptake

Improving the efficacy of existing antibiotics is a promising strategy for combating antibiotic-resistant/tolerant bacterial pathogens that have become a severe threat to human health. We previously reported that aminoglycoside antibiotics could be dramatically potentiated against stationary-phase Escherichia coli cells under hypoionic shock conditions (i.e., treatment with ion-free solutions), but the underlying molecular mechanism remains unknown. Here, we show that mechanosensitive (MS) channels, a ubiquitous protein family sensing mechanical forces of cell membrane, mediate such hypoionic shock-induced aminoglycoside potentiation. Two-minute treatment under conditions of hypoionic shock (e.g., in pure water) greatly enhances the bactericidal effects of aminoglycosides against both spontaneous and triggered E. coli persisters, numerous strains of Gram-negative pathogens in vitro, and Pseudomonas aeruginosa in mice. Such potentiation is achieved by hypoionic shock-enhanced bacterial uptake of aminoglycosides and is linked to hypoionic shock-induced destabilization of the cytoplasmic membrane in E. coli. Genetic and biochemical analyses reveal that MscS-family channels directly and redundantly mediate aminoglycoside uptake upon hypoionic shock and thus potentiation, with MscL channel showing reduced effect. Molecular docking and site-directed mutagenesis analyses reveal a putative streptomycin-binding pocket in MscS, critical for streptomycin uptake and potentiation. These results suggest that hypoionic shock treatment destabilizes the cytoplasmic membrane and thus changes the membrane tension, which immediately activates MS channels that are able to effectively transport aminoglycosides into the cytoplasm for downstream killing. Our findings reveal the biological effects of hypoionic shock on bacteria and can help to develop novel adjuvants for aminoglycoside potentiation to combat bacterial pathogens via activating MS channels.

Staphylococcal infection prevention using antibiotic-loaded mannitol-chitosan paste in a rabbit model of implant-associated osteomyelitis

Antibiotic-loaded chitosan pastes have shown advantages in the treatment and coverage of complex musculoskeletal defects. We added mannitol, previously shown to increase antibiotic susceptibility of biofilm, to an injectable chitosan/polyethylene glycol paste for delivery of antibiotics. Ground sponges (0.85% acetic acid solution, 1% chitosan, 0% or 2% mannitol, 1% polyethylene glycol) were hydrated using phosphate-buffered saline with 10 mg/ml amikacin and 10 mg/ml vancomycin added to form pastes. We inoculated rabbit radial defects with 10⁵ colony-forming units of Staphylococcus aureus (UAMS-1) and inserted titanium pins into the cortical bone. Groups compared included mannitol blend pastes, non-mannitol blends, antibiotic-loaded bone cement, vancomycin powder, and no treatment controls. We harvested tissue samples and retrieved the pins retrieved at 3 weeks. All antibiotic-loaded groups lowered bacterial growth and colony-forming unit counts in soft and bone tissue and on titanium pins in in vivo studies. The results indicate this biomaterial is capable of eluting active antibiotics at concentrations that reduce bacterial growth on biomaterials and tissue, which, in turn, may prevent biofilm formation. Blends of chitosan and mannitol may be useful in prevention and treatment of osteomyelitis and implant-associated infections.

Silica Particles Trigger the Exopolysaccharide Production of Harsh Environment Isolates of Growth-Promoting Rhizobacteria and Increase Their Ability to Enhance Wheat Biomass in Drought-Stressed Soils

In coming decades, drought is expected to expand globally owing to increased evaporation and reduced rainfall. Understanding, predicting, and controlling crop plants’ rhizosphere has the potential to manipulate its responses to environmental stress. Our plant growth-promoting rhizobacteria (PGPR) are isolated from a natural laboratory, ‘The Evolution Canyon’, Israel, (EC), from the wild progenitors of cereals, where they have been co-habituating with their hosts for long periods of time. The study revealed that commercial TM50 silica particles (SN) triggered the PGPR production of exopolysaccharides (EPS) containing D-glucuronate (D-GA). The increased EPS content increased the PGPR water-holding capacity (WHC) and osmotic pressure of the biofilm matrix, which led to enhanced plant biomass in drought-stressed growth environments. Light- and cryo-electron- microscopic studies showed that, in the presence of silica (SN) particles, bacterial morphology is changed, indicating that SNs are associated with significant reprogramming in bacteria. The findings encourage the development of large-scale methods for isolate formulation with natural silicas that ensure higher WHC and hyperosmolarity under field conditions. Osmotic pressure involvement of holobiont cohabitation is also discussed.

Development of a Solid Dispersion of Nystatin with Maltodextrin as a Carrier Agent: Improvements in Antifungal Efficacy against Candida spp. Biofilm Infections

The aim of this study was to improve the treatment of Candida albicans biofilms through the use of nystatin solid dispersions developed using maltodextrins as a hyperosmotic carrier. Characterization studies by differential scanning calorimetry, X-ray diffraction, dissolution studies, and particle size analysis were performed to evaluate changes in nystatin crystallinity. Antifungal activity and anti-biofilm efficacy were assessed by microbiological techniques. The results for nystatin solid dispersions showed that the enhancement of antifungal activity may be related to the high proportions of maltodextrins. Anti-biofilm assays showed a significant reduction (more than 80%) on biofilm formation with SD-N:MD [1:6] compared to the nystatin reference suspension. The elaboration process and physicochemical properties of SD-N:MD [1:6] could be a promising strategy for treatment of Candida biofilms.

Sequential Hypertonic-Hypotonic Treatment Enhances Efficacy of Antibiotic against Acinetobacter baumannii Biofilm Communities

Infections with bacterial biofilm communities are highly tolerant of antibiotics. This protection is attributed, in part, to a hydrated extracellular polymeric substance (EPS) that surrounds the bacterial community and that limits antibiotic diffusion. In this study, we evaluated whether it is possible to dehydrate and then re-hydrate a biofilm as a means to increase antibiotic penetration and efficacy. Acinetobacter baumannii biofilms (24 h) were exposed to hypertonic concentrations of maltodextrin, sucrose or polyethylene glycol (PEG) as the dehydration step. These biofilms were then washed with deionized water containing 10 times the concentration of antibiotics needed to kill these bacteria in broth culture (50 µg/mL tobramycin, 300 µg/mL chloramphenicol, 20 µg/mL ciprofloxacin or 100 µg/mL erythromycin) as the rehydration step. Biofilms were then harvested, and the number of viable cells was determined. Sequential treatment with PEG and tobramycin reduced cell counts 4 to 7 log (p < 0.05) relative to combining PEG and tobramycin in a single treatment, and 3 to 7 log relative to tobramycin treatment alone (p < 0.05). Results were variable for other osmotic compounds and antibiotics depending on the concentrations used, likely related to mass and hydrophobicity. Our findings support future clinical evaluation of sequential regimens of hypertonic and hypotonic solutions to enhance antibiotic efficacy against chronic biofilm infections.

In vitro bactericidal activity of a carbohydrate polymer with zinc oxide for the treatment of chronic wounds

Introduction. Biological adhesives and effective topical therapeutic agents that improve wound healing are urgently required for the treatment of chronic ulcers. A biodegradable adhesive based on a carbohydrate polymer with zinc oxide (CPZO) was shown to possess anti-inflammatory activity and enhance wound healing, but its bactericidal activity was unknown.Aim. To investigate the bactericidal activity of CPZO against bacteria commonly present as infectious agents in chronic wounds.Methodology. We examined the bactericidal activity of CPZO against three biofilm-producing bacteria (Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa) through three strategies: bacterial suspension, biofilm disruption and in vitro wound biofilm model.Results. In suspension cultures, CPZO had direct, potent bactericidal action against S. aureus within 24 h, whereas E. coli took 7 days to be eliminated. By contrast, P. aeruginosa survived up to 14 days with CPZO. CPZO had biofilm disruption activity against clinical isolates of S. aureus in the anti-biofilm test. Finally, in the in vitro wound biofilm model, CPZO dramatically reduced the bacterial viability of S. aureus and P. aeruginosa.Conclusions. Together with its previously shown anti-inflammatory properties, the bactericidal activity of CPZO gives it the potential to be a first-line therapeutic option for chronic various ulcers and, possibly, other chronic ulcers, preventing or controlling microbial infections, and leading to the healing of such complicated chronic ulcers.

Rapid Freezing Enables Aminoglycosides To Eradicate Bacterial Persisters via Enhancing Mechanosensitive Channel MscL-Mediated Antibiotic Uptake

Bacterial persisters exhibit noninherited antibiotic tolerance and are linked to the recalcitrance of bacterial infections. It is very urgent but also challenging to develop antipersister strategies. Here, we report that 10-s freezing with liquid nitrogen dramatically enhances the bactericidal action of aminoglycoside antibiotics by 2 to 6 orders of magnitude against many Gram-negative pathogens, with weaker potentiation effects on Gram-positive bacteria. In particular, antibiotic-tolerant Escherichia coli and Pseudomonas aeruginosa persisters-which were prepared by treating exponential-phase cells with ampicillin, ofloxacin, the protonophore cyanide m-chlorophenyl hydrazone (CCCP), or bacteriostatic antibiotics-can be effectively killed. We demonstrated, as a proof of concept, that freezing potentiated the aminoglycosides' killing of P. aeruginosa persisters in a mouse acute skin wound model. Mechanistically, freezing dramatically increased the bacterial uptake of aminoglycosides regardless of the presence of CCCP, indicating that the effects are independent of the proton motive force (PMF). In line with these results, we found that the effects were linked to freezing-induced cell membrane damage and were attributable, at least partly, to the mechanosensitive ion channel MscL, which was able to directly mediate such freezing-enhanced aminoglycoside uptake. In view of these results, we propose that the freezing-induced aminoglycoside potentiation is achieved by freezing-induced cell membrane destabilization, which, in turn, activates the MscL channel, which is able to effectively take up aminoglycosides in a PMF-independent manner. Our work may pave the way for the development of antipersister strategies that utilize the same mechanism as freezing but do so without causing any injury to animal cells.IMPORTANCE Antibiotics have long been used to successfully kill bacterial pathogens, but antibiotic resistance/tolerance usually has led to the failure of antibiotic therapy, and it has become a severe threat to human health. How to improve the efficacy of existing antibiotics is of importance for combating antibiotic-resistant/tolerant pathogens. Here, we report that 10-s rapid freezing with liquid nitrogen dramatically enhanced the bactericidal action of aminoglycoside antibiotics by 2 to 6 orders of magnitude against many bacterial pathogens in vitro and also in a mouse skin wound model. In particular, such combined treatment was able to effectively kill persister cells of Escherichia coli and Pseudomonas aeruginosa, which are per se tolerant of conventional treatment with bactericidal antibiotics for several hours. We also demonstrated that freezing-induced aminoglycoside potentiation was apparently linked to freezing-induced cell membrane damage that may have activated the mechanosensitive ion channel MscL, which, in turn, was able to effectively uptake aminoglycoside antibiotics in a proton motive force-independent manner. Our report sheds light on the development of a new strategy against bacterial pathogens by combining existing antibiotics with a conventional physical treatment or with MscL agonists.

Hypoionic Shock Facilitates Aminoglycoside Killing of Both Nutrient Shift- and Starvation-Induced Bacterial Persister Cells by Rapidly Enhancing Aminoglycoside Uptake

Bacterial persister cells are phenotypic variants that exhibit transient antibiotic tolerance and play a leading role in chronic infections and the development of antibiotic resistance. Determining the mechanism that underlies persister formation and developing anti-persister strategies, therefore, are clinically important goals. Here, we report that many gram-negative and gram-positive bacteria become highly tolerant to typical bactericidal antibiotics when the carbon source for their antibiotic-sensitive exponential growth phase is shifted to fumarate, suggesting a role for fumarate in persister induction. Nutrient shift-induced Escherichia coli but not Staphylococcus aureus persister cells can be killed by aminoglycosides upon hypoionic shock (i.e., the absence of ions), which is achieved by suspending the persisters in aminoglycoside-containing pure water for only 1 or 2 min. Such potentiation can be abolished by inhibitors of the electron transport chain (e.g., NaN₃) or proton motive force (e.g., CCCP). Additionally, we show that hypoionic shock facilitates the eradication of starvation-induced E. coli but not S. aureus persisters by aminoglycosides, and that such potentiation can be significantly suppressed by NaN₃ or CCCP. Mechanistically, hypoionic shock dramatically enhances aminoglycoside uptake by both nutrient shift- and starvation-induced E. coli persisters, whereas CCCP can diminish this uptake. Results of our study illustrate the general role of fumarate in bacterial persistence and may open new avenues for persister eradication and aminoglycoside use.

Responses of Acinetobacter baumannii Bound and Loose Extracellular Polymeric Substances to Hyperosmotic Agents Combined with or without Tobramycin: An Atomic Force Microscopy Study

In this work, contributions of extracellular polymeric substances (EPS) to the nanoscale mechanisms through which the multidrug-resistant Acinetobacter baumannii responds to antimicrobial and hyperosmotic treatments were investigated by atomic force microscopy. Specifically, the adhesion strengths to a control surface of silicon nitride (Si₃N₄) and the lengths of bacterial surface biopolymers of bound and loose EPS extracted from A. baumannii biofilms were quantified after individual or synergistic treatments with hyperosmotic agents (NaCl and maltodextrin) and an antibiotic (tobramycin). In the absence of any treatment, the loose EPS were significantly longer in length and higher in adhesion to Si₃N₄ than the bound EPS. When used individually, the hyperosmotic agents and tobramycin collapsed the A. baumannii bound and loose EPS. The combined treatment of maltodextrin with tobramycin collapsed only the loose EPS and did not alter the adhesion of both bound and loose EPS to Si₃N₄. In addition, the combined treatment was not as effective in collapsing the EPS molecules as when tobramycin was applied alone. Finally, the effects of treatments were dose-dependent. Altogether, our findings suggest that a sequential treatment could be effective in treating A. baumannii biofilms, in which a hyperosmotic agent is used first to collapse the EPS and limit the diffusion of nutrients into the biofilm, followed by the use of an antibiotic to kill the bacterial cells that escape from the biofilm because of starvation.