Antibiofilm efficacy of nitric oxide-releasing alginates against cystic fibrosis bacterial pathogens

Abstract

Colonization of the lungs by biofilm-forming pathogens is a major cause of mortality in cystic fibrosis (CF). In CF patients, these pathogens are difficult to treat due to the additional protection provided by both the biofilm exopolysaccharide matrix and thick, viscous mucus. The antibiofilm efficacy of nitric oxide (NO)-releasing alginates were evaluated against Pseudomonas aeruginosa, Burkholderia cepacia, Staphylococcus aureus, and methicillin-resistant S. aureus biofilms in both aerobic and anaerobic environments. Varying the amine precursor grafted onto alginate oligosaccharides imparted tunable NO storage (~0.1–0.3 µmol/mg) and release kinetics (~4–40 min half-lives) in the artificial sputum media used for biofilm testing. The NO-releasing alginates were highly antibacterial against the four CF-relevant pathogens, achieving a 5-log reduction in biofilm viability after 24-h treatment, with biocidal efficacy dependent on NO-release kinetics. Aerobic biofilms required greater starting NO doses to achieve killing relative to the anaerobic biofilms. Relative to tobramycin (MBEC_24h ≥2000 µg/mL) and vancomycin (MBEC_24h ≥1000 µg/mL), the NO-releasing alginates proved to be more effective (NO dose ≤520 µg/mL) regardless of growth conditions.

Graphical Abstract

Cystic fibrosis (CF) is an autosomal disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. These mutations cause a deficiency in chloride secretion from the epithelial cells in the airways and ultimately interfere with the mucociliary clearance of inhaled microorganisms.^1,2 As a result, a thick stagnant mucus layer accumulates, obstructing airways in the CF lung. Nutrients in the mucus foster a suitable environment for the colonization of bacterial pathogens. As the mucus layer thickens, it protects the bacteria from antibiotic treatments.^3,4 Bacterial biofilms are cooperative communities of bacteria encapsulated by a self-secreted exopolysaccharide matrix that provides additional protection from the host immune response and antibiotics. These combined protective mechanisms further complicate efforts to eradicate pathogens, even in the age of antibiotics.^3,4 Additionally, the efficacy of antibiotics are minimized by the altered microenvironments in biofilms (e.g., lower oxygen concentrations, nutrient depletion) and slower bacterial metabolism.^5–9 Persistent colonization of multiple pathogens, including Pseudomonas aeruginosa, Burkholderia cepacia complex (BCC), Staphylococcus aureus, and methicillin-resistant S. aureus (MRSA), elicits a chronic inflammatory response that ultimately degrade lung function to the point of respiratory failure.^9–12 The need for non-conventional antibacterial agents to eradicate biofilm-based bacteria is great.^13–15

Nitric oxide (NO) is an endogenously produced broad-spectrum antibacterial free radical capable of eradicating both planktonic bacteria and biofilms. Nitric oxide and its reactive byproducts (e.g., peroxynitrite and dinitrogen trioxide) chemically alter bacterial proteins, DNA, and metabolic enzymes, disrupting vital cellular functions and structures to induce killing.^16–19 While early studies demonstrated the efficacy of NO as an antibacterial agent via direct gaseous administration, the therapeutic utility of inhaled gas is limited by NO’s high reactivity and short lifetime in biological media, and the hazards associated with high pressure gas cylinders.¹⁷ The use of NO donors (e.g., N-diazeniumdiolates) for NO storage and triggered release under physiological conditions (e.g., pH 6.5, 37 °C in the CF airways) is an alternative to direct NO inhalation.^16,17 N-diazeniumdiolates undergo proton-initiated decomposition, with NO-release kinetics controlled by factors such as pH, temperature, and the chemical structure of the precursor amine. Unfortunately, the utility of small molecule N-diazeniumdiolate NO donors is limited by the toxicity of the precursor structure when larger NO payloads are required.^16,17,20 This shortcoming has been addressed through the development of macromolecular scaffolds with tunable NO release capabilities. These NO donors facilitate better control over NO-release kinetics and enhanced bacterial killing (i.e., require lower scaffold concentrations).^16,17,20,21

Inhalation of NO-releasing macromolecular biopolymers may represent an attractive therapeutic strategy because of extended NO release durations (i.e., several hours), allowing for expanded NO delivery via intermittent (once or twice daily) treatment. Alginate, a biopolymer composed of 1,4-linked α-l-guluronic acid (G) and β-d-mannuronic acid (M) units,^22–24 holds particular promise as a macromolecular NO donor system for inhalation applications because it is highly water soluble and amenable to nebulization.^22,25–28 Recent studies have demonstrated the ability of low molecular weight alginate oligosaccharides to potentiate the antibacterial efficacy of conventional antibiotics by altering biofilm morphology and mucin assembly, reducing mucus viscoelasticity and enhancing (antibiotic) delivery.^23,29–31

Herein, we report on the antibiofilm action of NO-releasing alginate oligosaccharides against multiple CF-relevant pathogens.^10,32–34 Selection of specific NO donor precursors to modify the alginate allowed for tunable NO-release properties (e.g., total NO payload, NO-release half-lives).^35–37 As pathogens genetically adapt to hypoxic or anaerobic conditions in the CF lung microenvironment and enable resistance to conventional antibiotics, the efficacy of the NO-releasing alginates was tested against biofilms grown under aerobic and anaerobic conditions.⁴ The effect of NO release on antibacterial action was examined as a function of alginate biopolymer chemical modification. Bactericidal activity of the NO-releasing alginates was also compared to tobramycin and vancomycin, two antibiotics commonly used for the treatment of infections in CF, to assess therapeutic potential relative to current standard of care practice.

Results and Discussion

Alginate oligosaccharides (~5 kDa) with different secondary amine functional groups (e.g., DETA, DPTA, PAPA, and SPER) were prepared via EDC/NHS reactions following previously published protocols (Figure 1).³⁷ Alkyl amine modification was confirmed via ¹³C NMR with the observation of an amide bond peak (~160 ppm; Figure S1) and elemental analysis (monitoring the increase in nitrogen content from 0 to 6–11 wt%; Table S1).³⁷ The alkyl amine-modified systems were subsequently functionalized with N-diazeniumdiolates via exposure to high pressures (i.e., 10 bar) of NO in basic aqueous solution.³⁷ Formation of NO donors was confirmed using UV-Vis spectroscopy and the appearance of a characteristic absorbance band at 253 nm (Figure S2).²¹ The resulting NO-releasing alginates possessed a broad range of NO-release kinetics when measured in phosphate buffered saline (PBS) at pH 6.5, with the PAPA- and DETA-modified alginates having the fastest (~0.1 h) and slowest (~2.0 h) NO-release half-lives, respectively (Table S2). In buffer, the biopolymers were characterized as having NO totals comparable to other previously reported NO-releasing biopolymers (0.3–0.6 µmol/mg; Table S2).^37–42

Figure 1.

Alkyl amine-modified alginates.

Nitric oxide release in biological media.

The NO-release properties of the alginate materials were measured in artificial sputum media (ASM, pH 6.5) to determine the total amount of NO available in environments that more closely mimic those in the CF airway.^3,43–45 As expected, the NO payloads were slightly lower (0.2–0.3 µmol/mg) when measured in ASM compared to in PBS due to the rapid scavenging of NO by reactive oxygen species (ROS) and proteins in biological media (Table 1).^46,47 Nevertheless, a range of NO-release kinetics for the alginates was still observed in ASM, with Alg5-DETA/NO maintaining the longest NO-release half-life (~40 min) and Alg5-PAPA/NO the fastest (~4 min), following the same order observed in PBS (Table 1, Figure S3). Compared to the monosubstituted alginates, the hybrid Alg5-PAPA-DPTA/NO biopolymer was characterized as having a similar NO payload to Alg5-PAPA/NO, but NO-release half-life and duration between that of Alg5-PAPA/NO and Alg5-DPTA/NO likely the result of NO donor interactions with components of the ASM media. Indeed, the NO flux of both Alg5-PAPA/NO and Alg5-SPER/NO was decreased in ASM relative to PBS while that of Alg5-DETA/NO and Alg5-DPTA/NO showed opposite behavior.⁴⁶ Prolonged NO release (i.e., lower initial NO flux and longer half-lives) from similar low molecular weight structures (e.g., DETA and DPTA) has previously been attributed to stabilization of the negatively charged N-diazeniumdiolate structure by positively charged terminal primary amines.^48,49 In the case of NO release in ASM, the potential electrostatic interactions between the alginate biopolymers, particularly terminal primary amine groups with components of the media (i.e., mucins and DNA) may destabilize the N-diazeniumdiolate NO donor allowing for greater initial NO release and a shorter overall duration.^3,43,45

Table 1.

Nitric oxide-release properties of NO donor-functionalized alginates in ASM (pH 6.5, 37 °C).^a

Biopolymer	t[NO]b (µmol/mg)	[NO]maxc (ppb/mg)	t1/2d (mins)	tde (h)
Alg5-DETA/NO	0.18 ± 0.04	670 ± 160	43.0 ± 10.9	5.0 ± 0.3
Alg5-DPTA/NO	0.19 ± 0.06	3420 ± 960	14.0 ± 1.2	3.1 ± 0.4
Alg5-SPER/NO	0.27 ± 0.09	2370 ± 1750	5.4 ± 1.6	1.7 ± 0.2
Alg5-PAPA/NO	0.28 ± 0.05	5990 ± 1480	3.8 ± 0.9	1.2 ± 0.8
Alg5-PAPA-DPTA/NO	0.29 ± 0.04	4750 ± 1100	8.4 ± 0.6	3.0 ± 0.6

^aError represent standard deviation for n ≥ 3 separate syntheses.

^bTotal NO released.

^cMaximum flux of NO release.

^dNO-release half-life.

^eDuration of NO release.

To further elucidate how ASM components may influence the NO-release properties of alginates in media, NO-release measurements were collected in mucin and DNA, two of the main components of ASM. Alg5-PAPA/NO and Alg5-DPTA/NO biopolymers were selected for this study as they represent fast and slow NO-releasing alginates. The hybrid biopolymer containing PAPA and DPTA (Alg5-PAPA-DPTA/NO) was also studied as it represents a combination of the two systems. As expected, NO release in each of the three biological media (i.e., ASM, mucin, and DNA) resulted in a decreased NO flux for the fast releasing Alg5-PAPA/NO in the presence of species capable of scavenging NO (Figure 2). However, an increase in NO flux was observed in both ASM and mucin for Alg5-DPTA/NO. The similarity of the NO-release profiles for Alg5-DPTA/NO in the two media clearly reflects destabilization of the N-diazeniumdiolate NO donor by enhanced electrostatic interactions between the protonated amine groups on alginate and the negatively charged moieties on the mucin glycoprotein (i.e., sialic acid residues).^32,48–52 The NO-release profile of the hybrid alginate was unaltered in all media, enabling near constant NO-release kinetics despite a decrease in NO totals in the biological media. The combined, enhanced NO flux for DPTA and decreased flux for PAPA N-diazeniumdiolates accounts for this phenomenon, virtually cancelling the changes related to the biological media altogether. In this regard, the Alg5-PAPA-DPTA/NO can mitigate drastic changes in NO-release kinetics due to the biological media.

Figure 2.

Real-time NO-release profiles for the first hour of release for (A) Alg5-DPTA/NO, (B) Alg5-PAPA/NO, and (C) Alg5-PAPA-DPTA/NO in PBS (black solid), ASM (gray solid), and DNA (black dash), and mucin (gray dash) solutions buffered at pH 6.5.

Antibiofilm efficacy.

Persistent colonization of biofilm-forming pathogens in the airways accounts for ~90% of CF patient mortality.^10,34 The presence of thick mucus in the CF airways, the EPS matrix, and other unique environmental-related factors (e.g., lower oxygen levels) provide additional protection to biofilm-based bacteria by limiting the diffusion and action of traditional antibiotics.^3,4,53 Indeed, slower growth due to viscoelasticity of mucus and the unique biofilm microenvironment (e.g., the presence of an oxygen gradient in the mucus that leads to bacteria adaptation to anaerobic microenvironments) contributes to the demonstrated resistance of bacteria by these biofilms.⁸ It is thus imperative to evaluate the antibiofilm activity of new therapeutics under conditions that mimic CF more accurately (i.e., in ASM under both aerobic and anaerobic conditions).

The antibiofilm activity of control and NO-releasing alginates was evaluated against both aerobic and anaerobic biofilms of four CF-relevant pathogens: P. aeruginosa, BCC, S. aureus, and MRSA.^33,34 The minimum concentration of antibacterial agent required to achieve a 5-logreduction in viability after 24 h (MBEC_24h) was evaluated for each of the alginate compositions. At equivalent concentrations to the MBEC_24h, control (non-NO-releasing) alginates did not alter the bacterial viability of the biofilms (Figure S3). In contrast, treatment with the NO-releasing biopolymers led to a 5-log reduction (≥99.999%) in bacterial viability for each of the NO-releasing alginates regardless of NO-release properties, implicating NO as the required bactericidal agent (Table 2).

Table 2.

Minimum biofilm eradication concentrations (MBEC_24h) of NO-releasing alginates against aerobic biofilms.^a

Sample	MBEC24h (mg/mL)a
	P. aeruginosa	BCC	S. aureus	MRSA
Alg5-DETA/NO	32	32	64	64
Alg5-DPTA/NO	16	16	32	64
Alg5-SPER/NO	16	16	32	64
Alg5-PAPA/NO	16	16	64	32
Alg5-PAPA-DPTA/NO	4	4	16	16
Tobramycin	2	2	16	>64

^aMBEC_24h determined from n ≥ 3 experiments.

Alg5-DPTA/NO, Alg5-SPER/NO, and Alg5-PAPA/NO achieved 5-log biocidal activity at 16 mg/mL against both P. aeruginosa and BCC under aerobic conditions. The Alg5-DETA/NO and Alg5-PAPA-DPTA/NO derivatives deviated from this trend, with the DETA-modified and hybrid alginate systems requiring greater and lower concentrations, respectively. We previously reported that under static (i.e., PBS with 1 vol% broth) conditions, the slower, sustained NO-releasing alginates (e.g., Alg5-DETA/NO) facilitated greater biocidal efficacy against biofilms compared to faster NO-releasing systems (e.g., Alg5-PAPA/NO).³⁷ With NO scavenging (e.g., in ASM), the combination of lower NO totals (~0.2 µmol/mg) and slower release might lead to insufficient delivery to illicit bacterial killing. Indeed, Alg5-DETA/NO proved to be less effective in ASM with greater alginate concentrations required to achieve comparable NO doses and eradication (32 mg/mL for both P. aeruginosa and BCC, and 64 mg/mL for both S. aureus and MRSA). The hybrid Alg5-PAPA-DPTA/NO system proved more lethal as evidenced by a lower MBEC_24h (4 mg/mL for both P. aeruginosa and BCC, and 16 mg/mL for both S. aureus and MRSA) compared to each of the monosubstituted alginates. In agreement with our previous report,³⁷ these results suggest that the kinetics of NO release plays a major role in the biocidal activity of the NO-releasing biopolymer. In particular, the combination of a large initial NO flux with sustained NO release is optimal for potent antibiofilm action, at least for alginates.

Similar to previous reports,^37,54,55 greater concentrations of NO-releasing alginates were needed to achieve biocidal action against S. aureus and MRSA biofilms (Table 2). The thicker peptidoglycan layer, and increased production of polysaccharides and extracellular proteins of Gram-positive species in general likely hinder NO diffusion (into the biofilm), necessitating larger NO fluxes from the biopolymer to achieve killing.⁵⁵ These structural differences are hypothesized to account for the lower susceptibility of both S. aureus and MRSA biofilms.

The efficacy of the alginate biopolymers to eradicate the four pathogens was also compared to tobramycin. Under similar conditions, both P. aeruginosa and BCC were eradicated at lower tobramycin concentrations than those of the NO-releasing alginates (Table 2). However, greater tobramycin concentrations were required to eradicate Gram-positive bacterial biofilms. Against S. aureus, the required tobramycin dose for biocidal action equaled that of Alg5-PAPA-DPTA/NO. Each of the NO-releasing alginates were more efficacious against MRSA, as the Gram-positive bacteria showed resistance to tobramycin up to 64 mg/mL. With NO as the implicated bactericidal agent, tobramycin concentration was also compared to the NO dose delivered during the bacteria killing assay. The NO dose required to achieve antibiofilm activity was derived from both the MBEC_24h of the alginate samples and the measured total NO released in ASM to compare the efficacy of NO to the antibiotic under aerobic conditions (Figure 3A). Against each of the four pathogens studied, lower initial NO doses relative to the antibiotic were required to achieve biocidal activity, demonstrating NO’s potent antimicrobial activity.

Figure 3.

Nitric oxide dose for Alg5-DETA (light gray stripes), Alg5-DPTA/NO (solid light gray), Alg5-SPER/NO (dark gray stripes), Alg5-PAPA/NO (solid dark gray), Alg5-PAPA-DPTA/NO (black stripes) required to treat (A) aerobic and (b) anaerobic biofilms of P. aeruginosa, BCC, S. aureus, and MRSA compared to tobramycin (solid black). Studies consisted of at least three experiments with error bars representing the standard deviation.

Anaerobically grown biofilms required lower alginate and NO concentrations to achieve killing compared to aerobic biofilms (Table 3, Figure 3B). In contrast, greater tobramycin concentrations were necessary to achieve biocidal action under anaerobic versus aerobic conditions, following previous reports describing diminished antibiotic activity in low-oxygen environments.^46,56,57 For S. aureus and MRSA, lower concentrations of the NO-releasing biopolymers were also required compared to tobramycin, demonstrating superior antibiofilm efficacy compared to the aminoglycoside antibiotic. Analogous to the trends observed under aerobic conditions, Alg5-PAPA-DPTA/NO was most effective, achieving a 5-log reduction in biofilm bacterial viability at an alginate concentration of only 2 mg/mL against the four pathogens. Similarly, lower starting NO doses (≤ 140 µg/mL) were needed to treat the anaerobic biofilms (Figure 3B). Reighard et al. reported similar results for NO-releasing chitosan oligosaccharides, attributing the larger macromolecular concentration for aerobic conditions to NO scavenging by oxygen and other reactive oxygen species (ROS).⁴⁶ While NO may react with ROS to form reactive intermediates that lead to potent antibacterial activity, NO’s reaction with oxygen to form nitrate and nitrite is equally likely. Such reactions may lead to lower transient NO concentrations in biological media under aerobic conditions, thereby minimizing the amount of bioavailable NO and increasing the necessary alginate/NO dose for biofilm killing.⁴⁶ This competing mechanism is no longer a condition under anaerobic conditions, in which little to no oxygen is available to react with NO.

Table 3.

Minimum biofilm eradication concentrations (MBEC_24h) of NO-releasing alginates against anaerobic biofilms.^a

Sample	MBEC24h (mg/mL)a
	P. aeruginosa	BCC	S. aureus	MRSA
Alg5-DETA/NO	8	4	16	4
Alg5-DPTA/NO	4	4	8	2
Alg5-SPER/NO	8	4	16	16
Alg5-PAPA/NO	8	4	8	4
Alg5-PAPA-DPTA/NO	2	2	2	2
Tobramycin	4	2	64	>64

^aMBEC_24h determined from n ≥ 3 experiments.

With tobramycin’s poor antibacterial activity against S. aureus and MRSA, the antibiofilm activity of the NO-releasing alginates was also compared to vancomycin, the antibiotic traditionally used to treat infections by Gram-positive bacteria.⁵⁸ Similar to tobramycin, greater concentrations of vancomycin were required to eradicate anaerobic versus aerobic biofilms (Figure 4), indicating a reduced efficacy under low-oxygen conditions. The doses of NO required to eradicate the biofilms were lower for both oxygen levels compared to vancomycin, demonstrating NO’s unique ability to remain active as an antimicrobial regardless of oxygen availability.

Figure 4.

Nitric oxide dose for Alg5-DETA (light gray stripes), Alg5-DPTA/NO (solid light gray), Alg5-SPER/NO (dark gray stripes), Alg5-PAPA/NO (solid dark gray), Alg5-PAPA-DPTA/NO (black stripes) required to treat (A) aerobic and (b) anaerobic biofilms of S. aureus and MRSA. All NO doses were compared to vancomycin (solid black). Studies consisted of at least three experiments with error bars representing the standard deviation.

Effect of NO-release kinetics on antibiofilm efficacy.

Biofilm eradication was also studied as a function of NO-release kinetics. P. aeruginosa biofilms were treated with equal concentrations of either Alg5-DPTA/NO, Alg5-PAPA/NO, or Alg5-PAPA-DPTA/NO at 2 and 4 mg/mL for anaerobic and aerobic growth, respectively (Figure 5). The selected concentrations represent the lowest MBEC_24h for the Alg5 systems (i.e., the MBEC_24h of Alg5-PAPA-DPTA/NO against P. aeruginosa biofilms). Under both aerobic and anaerobic conditions, one-time treatment at 2 h with Alg5-DPTA/ NO and Alg5-PAPA/NO resulted in a 1- and 2-log reduction in biofilm bacterial viability, respectively. Bacteria viability recovered up to ~10⁸ CFU/mL at longer exposure durations for both the PAPA- and DPTA-modified NO-releasing alginates. The greater total NO of Alg5-PAPA/NO at 2 h resulted in a greater viability reduction initially, but the low NO levels released (i.e., below the limit of detection of the instrument) thereafter proved insufficient for maintaining lower viability (i.e., killing). On the other hand, the low NO totals associated with Alg5-DPTA/NO in ASM led to insufficient NO doses to achieve any appreciable bacterial killing, thus requiring a larger concentration (i.e., 16 mg/mL) to eradicate the biofilm. These results suggest the possibility of using multiple treatments at lower alginate concentrations (e.g., 2 or 4 mg/mL) to supplement initial eradication via monosubstituted alginates as an alternative to a significantly larger concentration for one-time treatment. Of course, such a hypothesis must be evaluated pre-clinically and clinically in future work.

Figure 5.

Time-based bactericidal efficacy of Alg5-DPTA/NO (circle), Alg5-PAPA-DPTA/NO (triangle), Alg5-PAPA/NO (square), and tobramycin (cross). Comparison of all NO-releasing alginates at equivalent concentrations of (A) 4 mg/mL under aerobic conditions and (B) 2 mg/mL under anaerobic conditions. The MBEC_24h values under aerobic and anaerobic conditions (2 mg/mL and 4 mg/mL, respectively) were used for tobramycin. Studies consisted of at least three experiments with error bars representing the standard deviation.

In contrast to both Alg5-PAPA/NO and Alg5-DPTA/NO, the hybrid alginate system (Alg5-PAPA-DPTA/NO) elicited an initial 2-log reduction in bacterial viability at the same concentration after 2 h for both aerobic and anaerobic biofilms. The combination of greater NO totals in ASM (~0.3 µmol/mg) coupled with the sustained NO release (~3 h) by using the hybrid biopolymer led to a 5-log reduction after 4 h. Over the same period, tobramycin at its aerobic MBEC_24h (2 mg/mL) only achieved a 2-log reduction (with an overall 3-log reduction over the course of 6 h) suggesting a slower biocidal action mechanism. Similarly, anaerobic biofilms treated with tobramycin at the antibiotic’s anaerobic MBEC_24h achieved a 2-log reduction over the 6-hour treatment period – albeit more slowly. Collectively, the results of this study suggest that the combination of a high initial NO flux and sustained NO release is preferred for biofilm eradication. Moreover, Alg5-PAPA-DPTA/NO exhibited greater potency against the P. aeruginosa biofilms grown in ASM under the two conditions, achieving a 5-log reduction in bacterial viability faster than conventional antibiotics.

Conclusions

The antibiofilm action of NO-releasing alginates were clearly demonstrated against the four tested, CF-relevant pathogens. Of the different alginate systems, Alg5-PAPA-DPTA/NO exhibited the greatest antibacterial action against the test pathogens regardless of biofilm growth conditions (MBEC_24h starting NO dose of ≤520 µg/mL NO). The NO-based hybrid alginate therapeutic also exhibited enhanced activity compared to tobramycin and vancomycin, establishing Alg5-PAPA-DPTA/NO’s prospect as an alternative to conventional antibiotics for treating chronic CF infections. Combined with the ability of alginate oligosaccharides to decrease mucus viscoelasticity,^24,59–61 these results suggest broader utility of NO-releasing alginates as a dual-action CF therapeutic. In particular, NO release may be able to eradicate biofilms embedded deep within the mucus layer where low oxygen concentrations reduce the efficacy of currently employed antibiotics.

Experimental Section

Materials.

Alginic acid sodium salt from brown algae (low viscosity), bis(3-aminopropyl)amine (DPTA), diethylenetriamine (DETA), N-propyl-1,3-propanediamine (PAPA), spermine (SPER), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), N-hydroxysuccinimde (NHS), casamino acids, tobramycin, vancomycin hydrochloride, and pig gastric mucin (PGM) type II were purchased from Sigma-Aldrich (St. Louis, MO, USA). Deoxyribonucleic acid (DNA) sodium salt, egg yolk enrichment, and common laboratory salts and solvents were purchased from Fischer Scientific (Fair Lawn, NJ, USA). Unless otherwise specified, all chemicals were used as received without further purification. Argon (Ar), carbon dioxide (CO₂), nitrogen (N₂), nitric oxide (NO) calibration (25.87 ppm balance N₂), and pure NO (99.5%) gas cylinders were purchased from Airgas National Welders (Raleigh, NC, USA). Distilled water was purified to a resistivity of 18.2 MΩ•cm and a total organic content of ≤6 ppb using a Millipore Milli-Q UV Gradient A10 System (Bedford, MA, USA).

Bacterial Strains and Media.

The laboratory Pseudomonas aeruginosa strain K (PAK) and the Burkholderia cepacia complex (BCC) clinical strain were a gift from Matthew Wolfgang from the Department of Microbiology and Immunology at the University of North Carolina at Chapel Hill (Chapel Hill, NC). Staphylococcus aureus (ATCC #29213) and methicillin-resistant S. aureus (MRSA; ATCC #33591) were obtained from American Type Tissue Culture Collection (Manassas, VA, USA). Luria-Bertani (LB) broth and Tryptic Soy Agar (TSA) plates were obtained from Becton, Dickinson, and Company (Franklin Lakes, NJ, USA). Artificial sputum media (ASM) was prepared following a previously published protocol.^3,62 Briefly, DNA (4 g), pig gastric mucin type II (5 g), casamino acids (5 g), diethylenetriaminepentaacetic acid (DTPA; 5.9 mg), sodium chloride (NaCl; 5 g), and potassium chloride (KCl; 2.2 g) were dissolved in 800 mL of sterile water. A 5-mL aliquot of egg yolk emulsion was added to the resulting solution as a source of lecithin. The pH of the solution was adjusted to 6.5 with 1 M Tris (pH 8.5) and the volume brought up to 1 L with sterile water before being sterilized via filtration using Millipore Steritop filter units (Burlington, MA, USA). The filtered ASM was stored in the dark at 4 °C with each stock solution used within a month. For anaerobic growth, ASM was supplemented with 150 mM potassium nitrate (KNO₃).⁴⁶

Instrumentation.

¹H and ¹³C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker (600 MHz) spectrometer. A PerkinElmer Elemental Analyzer Series 2400 Instrument (Waltham, MA, USA) was used for elemental (carbon, hydrogen, and nitrogen; CHN) analysis. Zeta potential measurements were taken in phosphate buffer (10 mM PB; pH 6.5) using a Zetasizer Nano (Malvern Instruments, UK). Gel permeation chromatography (GPC) measurements were carried out in 0.1 M sodium nitrate (NaNO₃) using an aqueous GPC system equipped with a Waters 2414 refractive index detector (Milford, MA, USA) coupled to a Wyatt miniDawn TREOS multi-angle light scattering detector (Santa Barbara, CA, USA).

Oxidative degradation of alginate.

High molecular weight alginate biopolymers were degraded to lower molecular weight oligosaccharides following previously published protocols.^37,63 Briefly, the biopolymer (2.5 g) was dissolved in 15 wt% hydrogen peroxide (50 mL) and stirred in an oil bath for 1 h at 80 °C. The resulting solution was filtered to remove insoluble material. The alginate oligosaccharides were collected and washed copiously with ethanol before drying in vacuo to yield a white powder. The molecular weight and PDI of the oxidatively degraded biopolymer are provided in Table S1.

Synthesis of polyamine-modified alginate oligosaccharides.

Monofunctional alginate materials used in this study were modified with diethylenetriamine (DETA), bis(3-aminopropyl)amine (DPTA), N-propyl-1,3-propanediamine (PAPA), or spermine (SPER) following a previously published protocol.³⁷ Briefly, alginate (100 mg) was dissolved in 10 mL phosphate buffered saline (PBS; 10 mM, pH 6.5) with a 2:1 molar ratio of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) and a 2:1 molar ratio of N-hydroxysuccinimide (NHS) with respect to the carboxylic acid moeities of alginates. The reaction was left to stir for 1 h. After 1 h, a 4:1 molar ratio of the alkyl amine with respect to the carboxylic acid groups of the alginates was added dropwise to the mixture. The reaction was allowed to proceed for 24 h at room temperature under constant stirring. The amine-modified alginates were precipitated in methanol and collected via centrifugation, washed twice with methanol, and dried in vacuo to yield a white solid for each modification. A hybrid system modified with PAPA and DPTA functional groups was also synthesized following the same procedure, with the PAPA moiety first added dropwise at a 4:1 molar ratio of the alkyl amine with respect to alginate carboxylic acids followed by addition of a 4:1 molar ratio of DPTA with respect to the same groups.

The ¹H and ¹³C NMR peaks of alginate oligosaccharides and the alkyl amine-modified derivatives, nitrogen content, and molecular weight information are provided in the Supporting Information.

Synthesis of N-diazeniumdiolate-modified alginate oligosaccharides.

Nitric oxide donor-modified alginates were synthesized as previously reported.³⁷ Briefly, polyamine-modified alginate (45 mg) was dissolved in 50 mM NaOH solution (3 mL) in a 1-dram glass vial. The (open) vials were placed in a stainless-steel reactor with continuous magnetic stirring. Oxygen was removed from the vessel by purging with argon (10 s, 7 bar) three times. The vessel was then pressurized to 10 bar with NO gas and allowed to react for 3 d. Afterward, the same argon purging protocol was repeated to remove unreacted NO. The NO-releasing alginate oligosaccharides were then precipitated in ethanol, collected by centrifugation, dried overnight in vacuo, and stored at −20 °C as a white powder.

Characterization of nitric oxide release.

Nitric oxide release was evaluated in real-time using a Sievers 280i Chemiluminescence NO analyzer (NOA; Boulder, CO, USA).^47,64 Measurements were taken before each biofilm experiment to ensure sample stability. Prior to analysis, the NOA was calibrated with air passed through a NO zero filter (0 ppm NO) and 25.87 ppm of NO standard gas (balance N₂). In a typical measurement, NO-releasing alginate oligosaccharide (~1 mg) was dissolved in 30 mL of either PBS (10 mM, pH 6.5), mucin (4 wt%, pH 6.5), DNA (4 wt%, pH 6.5), or ASM (pH 6.5). Nitrogen was flowed through the solution at a flow rate of 70 mL/min to carry the liberated NO from the biopolymer to the analyzer. Additional nitrogen flow was supplied to the flask to match the collection rate of the instrument (200 mL/min) to remove water soluble interferents (e.g., nitrite and nitrate).^47,64 Nitric oxide analysis was terminated when NO levels fell below 10 ppb of NO/mg of alginate (the limit of detection of the instrument).

Biofilm eradication assays.

P. aeruginosa, BCC, S. aureus and MRSA bacterial cultures were grown overnight in LB (pH 6.5) at 37 °C and recultured in fresh LB to a concentration of 10⁸ CFU/mL. These solutions were diluted to 10⁶ CFU/mL in sterile media (P. aeruginosa and BCC in ASM; S. aureus and MRSA in ASM with 0.25% glucose) and grown for 48 h at 37 °C with gentle shaking.^3,62,65 Biofilms in ASM were treated with premeasured samples of NO-releasing alginates, control alginates, or antibiotic (i.e., tobramycin, vancomycin) dissolved in ASM (100 µL), with final concentrations ranging from 2 to 64 mg/mL. Treatment was for 24 h at 37 °C in either aerobic (with oxygen) and anaerobic (low oxygen) conditions. All anaerobic biofilm growth and exposures were conducted in a Coy anaerobic chamber equipped with an oxygen and hydrogen monitor (Coy Laboratory Products; Grass Lake, MI, USA). For the anaerobic chamber, oxygen levels were consistently maintained at 0 ppm throughout the experiment. Untreated controls (blanks) were included in each experiment to ensure bacterial viability over the duration of the experiment. The dispersed biofilms were then pipetted out of the wells, serially diluted (10-, 100-, 1000-, and 10,000-fold dilutions), vortexed, plated on TSA plates using an Eddy Jet spiral plater (IUL; Farmingdale, NY, USA), and incubated overnight at 37 °C. Bacterial viability was assessed using a Flash & Go colony counter (IUL; Farmingdale, NY, USA). The minimum biofilm eradication concentration at 24 h (MBEC_24h) was defined as the minimum concentration required to achieve a 5-log reduction in bacterial viability compared to untreated cells (i.e., reduced bacterial counts from 10⁸ to ≤10³ CFU/mL). The limit of detection of this method is 2.5 × 10³ CFU/mL.⁶⁶ The NO dose (µg NO/mL) was derived from the MBEC_24h of the alginate samples (mg/mL) with the NO released measured in ASM (pH 6.5; µmol NO/mg alginate) over the testing period.

Time-based biofilm eradication assay.

P. aeruginosa biofilms were grown aerobically and anaerobically, and treated with either Alg5-PAPA-DPTA/NO, Alg5-PAPA/NO, and Alg5-DPTA/NO (4 and 2 mg/mL, respectively) following the same protocol as for the biofilm eradication assays. Untreated controls were included to ensure bacteria viability over the duration of the experiment. The samples were incubated with gentle shaking at 37 °C for 2, 4, and 6 h before plating on TSA plates using an Eddy Jet spiral plater and incubating overnight at 37 °C. Bacterial viability was assessed using a Flash & Go colony counter.