Sodium nitrite blocks the activity of aminoglycosides against Pseudomonas aeruginosa biofilms

Aminoglycoside uptake, stress, and potentiation in Gram-negative bacteria: new therapies with old molecules

SUMMARYAminoglycosides (AGs) are long-known molecules successfully used against Gram-negative pathogens. While their use declined with the discovery of new antibiotics, they are now classified as critically important molecules because of their effectiveness against multidrug-resistant bacteria. While they can efficiently cross the Gram-negative envelope, the mechanism of AG entry is still incompletely understood, although this comprehension is essential for the development of new therapies in the face of the alarming increase in antibiotic resistance. Increasing antibiotic uptake in bacteria is one strategy to enhance effective treatments. This review aims, first, to consolidate old and recent knowledge about AG uptake; second, to explore the connection between AG-dependent bacterial stress and drug uptake; and finally, to present new strategies of potentiation of AG uptake for more efficient antibiotic therapies. In particular, we emphasize on the connection between sugar transport and AG potentiation.

Complex Interplay of Heme-Copper Oxidases with Nitrite and Nitric Oxide

Nitrite and nitric oxide (NO), two active and critical nitrogen oxides linking nitrate to dinitrogen gas in the broad nitrogen biogeochemical cycle, are capable of interacting with redox-sensitive proteins. The interactions of both with heme-copper oxidases (HCOs) serve as the foundation not only for the enzymatic interconversion of nitrogen oxides but also for the inhibitory activity. From extensive studies, we now know that NO interacts with HCOs in a rapid and reversible manner, either competing with oxygen or not. During interconversion, a partially reduced heme/copper center reduces the nitrite ion, producing NO with the heme serving as the reductant and the cupric ion providing a Lewis acid interaction with nitrite. The interaction may lead to the formation of either a relatively stable nitrosyl-derivative of the enzyme reduced or a more labile nitrite-derivative of the enzyme oxidized through two different pathways, resulting in enzyme inhibition. Although nitrite and NO show similar biochemical properties, a growing body of evidence suggests that they are largely treated as distinct molecules by bacterial cells. NO seemingly interacts with all hemoproteins indiscriminately, whereas nitrite shows high specificity to HCOs. Moreover, as biologically active molecules and signal molecules, nitrite and NO directly affect the activity of different enzymes and are perceived by completely different sensing systems, respectively, through which they are linked to different biological processes. Further attempts to reconcile this apparent contradiction could open up possible avenues for the application of these nitrogen oxides in a variety of fields, the pharmaceutical industry in particular.

Physiological Roles of Nitrite and Nitric Oxide in Bacteria: Similar Consequences from Distinct Cell Targets, Protection, and Sensing Systems

Nitrite and nitric oxide (NO) are two active nitrogen oxides that display similar biochemical properties, especially when interacting with redox-sensitive proteins (i.e., hemoproteins), an observation serving as the foundation of the notion that the antibacterial effect of nitrite is largely attributed to NO formation. However, a growing body of evidence suggests that they are largely treated as distinct molecules by bacterial cells. Although both nitrite and NO are formed and decomposed by enzymes participating in the transformation of these nitrogen species, NO can also be generated via amino acid metabolism by bacterial NO synthetase and scavenged by flavohemoglobin. NO seemingly interacts with all hemoproteins indiscriminately, whereas nitrite shows high specificity to heme-copper oxidases. Consequently, the homeostasis of redox-sensitive proteins may be responsible for the substantial difference in NO-targets identified to date among different bacteria. In addition, most protective systems against NO damage have no significant role in alleviating inhibitory effects of nitrite. Furthermore, when functioning as signal molecules, nitrite and NO are perceived by completely different sensing systems, through which they are linked to different biological processes.

Dispersal of Epithelium-Associated Pseudomonas aeruginosa Biofilms

During chronic lung infections, Pseudomonas aeruginosa grows in highly antibiotic-tolerant communities called biofilms that are difficult for the host to clear. We have developed models for studying P. aeruginosa biofilm dispersal in environments that replicate key features of the airway. We found that mechanisms of biofilm dispersal in these models may employ alternative or additional signaling mechanisms, highlighting the importance of the growth environment in dispersal events. We have adapted the models to accommodate apical fluid flow, bacterial clinical isolates, antibiotics, and primary human airway epithelial cells, all of which are relevant to understanding bacterial behaviors in the context of human disease. We also examined dispersal agents in combination with commonly used antipseudomonal antibiotics and saw improved clearance when nitrite was combined with the antibiotic aztreonam.

Pseudomonas aeruginosa grows in highly antibiotic-tolerant biofilms during chronic airway infections. Dispersal of bacteria from biofilms may restore antibiotic susceptibility or improve host clearance. We describe models to study biofilm dispersal in the nutritionally complex environment of the human airway. P. aeruginosa was cocultured in the apical surface of airway epithelial cells (AECs) in a perfusion chamber. Dispersal, triggered by sodium nitrite, a nitric oxide (NO) donor, was tracked by live cell microscopy. Next, a static model was developed in which biofilms were grown on polarized AECs without flow. We observed that NO-triggered biofilm dispersal was an energy-dependent process. From the existing literature, NO-mediated biofilm dispersal is regulated by DipA, NbdA, RbdA, and MucR. Interestingly, altered signaling pathways appear to be used in this model, as deletion of these genes failed to block NO-induced biofilm dispersal. Similar results were observed using biofilms grown in an abiotic model on glass with iron-supplemented cell culture medium. In cystic fibrosis, airway mucus contributes to the growth environment, and a wide range of bacterial phenotypes are observed; therefore, we tested biofilm dispersal in a panel of late cystic fibrosis clinical isolates cocultured in the mucus overlying primary human AECs. Finally, we examined dispersal in combination with the clinically used antibiotics ciprofloxacin, aztreonam and tobramycin. In summary, we have validated models to study biofilm dispersal in environments that recapitulate key features of the airway and identified combinations of currently used antibiotics that may enhance the therapeutic effect of biofilm dispersal.

IMPORTANCE During chronic lung infections, Pseudomonas aeruginosa grows in highly antibiotic-tolerant communities called biofilms that are difficult for the host to clear. We have developed models for studying P. aeruginosa biofilm dispersal in environments that replicate key features of the airway. We found that mechanisms of biofilm dispersal in these models may employ alternative or additional signaling mechanisms, highlighting the importance of the growth environment in dispersal events. We have adapted the models to accommodate apical fluid flow, bacterial clinical isolates, antibiotics, and primary human airway epithelial cells, all of which are relevant to understanding bacterial behaviors in the context of human disease. We also examined dispersal agents in combination with commonly used antipseudomonal antibiotics and saw improved clearance when nitrite was combined with the antibiotic aztreonam.

Nitrite modulates aminoglycoside tolerance by inhibiting cytochrome heme-copper oxidase in bacteria

As a bacteriostatic agent, nitrite has been used in food preservation for centuries. When used in combination with antibiotics, nitrite is reported to work either cooperatively or antagonistically. However, the mechanism underlying these effects remains largely unknown. Here we show that nitrite mediates tolerance to aminoglycosides in both Gram-negative and Gram-positive bacteria, but has little interaction with other types of antibiotics. Nitrite directly and mainly inhibits cytochrome heme-copper oxidases (HCOs), and by doing so, the membrane potential is compromised, blocking uptake of aminoglycosides. In contrast, reduced respiration (oxygen consumption rate) resulting from nitrite inhibition is not critical for aminoglycoside tolerance. While our data indicate that nitrite is a promising antimicrobial agent targeting HCOs, cautions should be taken when used with other antibiotics, aminoglycosides in particular.

Kunkel Lecture: Fundamental immunodeficiency and its correction

"Fundamental immunodeficiency" is the inability of the encoded immune system to protect an otherwise healthy host from every infection that could threaten its life. In contrast to primary immunodeficiencies, fundamental immunodeficiency is not rare but nearly universal. It results not from variation in a given host gene but from the rate and extent of variation in the genes of other organisms. The remedy for fundamental immunodeficiency is "adopted immunity," not to be confused with adaptive or adoptive immunity. Adopted immunity arises from four critical societal contributions to the survival of the human species: sanitation, nutrition, vaccines, and antimicrobial agents. Immunologists have a great deal to contribute to the development of vaccines and antimicrobial agents, but they have focused chiefly on vaccines, and vaccinology is thriving. In contrast, the effect of antimicrobial agents in adopted immunity, although fundamental, is fragile and failing. Immunologists can aid the development of sorely needed antimicrobial agents, and the study of antimicrobial agents can help immunologists discover targets and mechanisms of host immunity.

Nitrate Sensing and Metabolism Inhibit Biofilm Formation in the Opportunistic Pathogen Burkholderia pseudomallei by Reducing the Intracellular Concentration of c-di-GMP

The opportunistic pathogen Burkholderia pseudomallei is a saprophytic bacterium and the causative agent of melioidosis, an emerging infectious disease associated with high morbidity and mortality. Although melioidosis is most prevalent during the rainy season in endemic areas, domestic gardens and farms can also serve as a reservoir for B. pseudomallei during the dry season, in part due to irrigation and fertilizer use. In the environment, B. pseudomallei forms biofilms and persists in soil near plant root zones. Biofilms are dynamic bacterial communities whose formation is regulated by extracellular cues and corresponding changes in the nearly universal secondary messenger cyclic dimeric GMP. Recent studies suggest B. pseudomallei loads are increased by irrigation and the addition of nitrate-rich fertilizers, whereby such nutrient imbalances may be linked to the transmission epidemiology of this important pathogen. We hypothesized that exogenous nitrate inhibits B. pseudomallei biofilms by reducing the intracellular concentration of c-di-GMP. Bioinformatics analyses revealed B. pseudomallei 1026b has the coding capacity for nitrate sensing, metabolism, and transport distributed on both chromosomes. Using a sequence-defined library of B. pseudomallei 1026b transposon insertion mutants, we characterized the role of denitrification genes in biofilm formation in response to nitrate. Our results indicate that the denitrification pathway is implicated in B. pseudomallei biofilm growth dynamics and biofilm formation is inhibited by exogenous addition of sodium nitrate. Genomics analysis identified transposon insertional mutants in a predicted two-component system (narX/narL), a nitrate reductase (narGH), and a nitrate transporter (narK-1) required to sense nitrate and alter biofilm formation. Additionally, the results presented here show that exogenous nitrate reduces intracellular levels of the bacterial second messenger c-di-GMP. These results implicate the role of nitrate sensing in the regulation of a c-di-GMP phosphodiesterase and the corresponding effects on c-di-GMP levels and biofilm formation in B. pseudomallei 1026b.

Efficient Eradication of Mature Pseudomonas aeruginosa Biofilm via Controlled Delivery of Nitric Oxide Combined with Antimicrobial Peptide and Antibiotics

Fast eradication of mature biofilms is the ‘holy grail’ in the clinical management of device-related infections. Endogenous nitric oxide (NO) produced by macrophages plays an important role in host defense against intracellular pathogens, and NO is a promising agent in preventing biofilms formation in vitro. However, the rate of delivery of NO by various NO donors (e.g., diazeniumdiolates, S-nitrosothiols, etc.) is difficult to control, which hinders fundamental studies aimed at understanding the role of NO in biofilm control. In this study, by using a novel precisely controlled electrochemical NO releasing catheter device, we examine the effect of physiological levels of NO on eradicating mature Pseudomonas aeruginosa biofilm (7 days), as well as the potential application of the combination of NO with antimicrobial agents. It is shown that physiological levels of NO exhibit mixed effects of killing bacteria and dispersing ambient biofilm. The overall biofilm-eradicating effect of NO is quite efficient in a dose-dependent manner over a 3 h period of NO treatment. Moreover, NO also greatly enhances the efficacy of antimicrobial agents, including human beta-defensin 2 (BD-2) and several antibiotics, in eradicating biofilm and its detached cells, which otherwise exhibited high recalcitrance to these antimicrobial agents. The electrochemical NO release technology offers a powerful tool in evaluating the role of NO in biofilm control as well as a promising approach when combined with antimicrobial agents to treat biofilm-associated infections in hospital settings, especially infections resulting from intravascular catheters.

Engineered cationic antimicrobial peptide (eCAP) prevents Pseudomonas aeruginosa biofilm growth on airway epithelial cells

OBJECTIVES

Chronic infections with the opportunistic pathogen Pseudomonas aeruginosa are responsible for the majority of the morbidity and mortality in patients with cystic fibrosis (CF). While P. aeruginosa infections may initially be treated successfully with standard antibiotics, chronic infections typically arise as bacteria transition to a biofilm mode of growth and acquire remarkable antimicrobial resistance. To address the critical need for novel antimicrobial therapeutics that can effectively suppress chronic bacterial infections in challenging physiological environments, such as the CF lung, we have rationally designed a de novo engineered cationic antimicrobial peptide, the 24-residue WLBU2, with broad-spectrum antibacterial activity for pan-drug-resistant P. aeruginosa in liquid culture. In the current study, we tested the hypothesis that WLBU2 also prevents P. aeruginosa biofilm growth.

METHODS

Using abiotic and biotic biofilm assays, co-culturing P. aeruginosa with polarized human airway epithelial cells, we examined the ability of WLBU2 to prevent biofilm biogenesis alone and in combination with currently used antibiotics.

RESULTS

We observed a dose-dependent reduction in biofilm growth on an abiotic surface and in association with CF airway epithelial cells. WLBU2 prevented P. aeruginosa biofilm formation when co-cultured with mucus-producing primary human CF airway epithelial cells and using CF clinical isolates of P. aeruginosa, even at low pH and high salt conditions that mimic the CF airway. When used in combination, WLBU2 significantly increases killing by the commonly used antibiotics tobramycin, ciprofloxacin, ceftazidime and meropenem.

CONCLUSIONS

While other studies have demonstrated the ability of natural and synthetic antimicrobial peptides to prevent abiotic bacterial biofilm formation, the current studies for the first time demonstrate the effective peptide treatment of a biotic bacterial biofilm in a setting similar to the CF airway, and without negative effects on human airway epithelial cells, thus highlighting the unique potential of this engineered cationic antimicrobial peptide for treatment of human respiratory infections.

It is rocket science - why dietary nitrate is hard to 'beet'! Part II: further mechanisms and therapeutic potential of the nitrate-nitrite-NO pathway

Dietary nitrate (found in green leafy vegetables such as rocket and in beetroot) is now recognized to be an important source of nitric oxide, via the nitrate-nitrite-NO pathway. Dietary nitrate confers several cardiovascular beneficial effects on blood pressure, platelets, endothelial function, mitochondrial efficiency and exercise. Having described key twists and turns in the elucidation of the pathway and the underlying mechanisms in Part I, we explore the more recent developments which have served to confirm mechanisms, extend our understanding, and discover new properties and potential therapeutic uses of the pathway in Part II. Even the established dependency on low oxygen states for bioactivation of nitrite has recently been challenged. Dietary nitrate appears to be an important component of 'healthy diets', such as the DASH diet to lower blood pressure and the Mediterranean diet, with its potential to lower cardiovascular risk, possibly through beneficial interactions with a range of other constituents. The World Cancer Research Foundation report strong evidence for vegetables including spinach and lettuce (high nitrate-containing) decreasing cancer risk (mouth, pharynx, larynx, oesophagus and stomach), summarized in a 'Nitrate-Cancer Risk Veg-Table'. The European Space Agency recommends that beetroot, lettuce, spinach and rocket (high-nitrate vegetables) are grown to provide food for long-term space missions. Nitrate, an ancient component of rocket fuel, could support sustainable crops for healthy humans.

A Putative ABC Transporter Permease Is Necessary for Resistance to Acidified Nitrite and EDTA in Pseudomonas aeruginosa under Aerobic and Anaerobic Planktonic and Biofilm Conditions

Pseudomonas aeruginosa (PA) is an important airway pathogen of cystic fibrosis and chronic obstructive disease patients. Multiply drug resistant PA is becoming increasing prevalent and new strategies are needed to combat such insidious organisms. We have previously shown that a mucoid, mucA22 mutant PA is exquisitely sensitive to acidified nitrite (A-NO2−, pH 6.5) at concentrations that are well tolerated in humans. Here, we used a transposon mutagenesis approach to identify PA mutants that are hypersensitive to A-NO2−. Among greater than 10,000 mutants screened, we focused on PA4455, in which the transposon was found to disrupt the production of a putative cytoplasmic membrane-spanning ABC transporter permease. The PA4455 mutant was not only highly sensitive to A-NO2−, but also the membrane perturbing agent, EDTA and the antibiotics doxycycline, tigecycline, colistin, and chloramphenicol, respectively. Treatment of bacteria with A-NO2− plus EDTA, however, had the most dramatic and synergistic effect, with virtually all bacteria killed by 10 mM A-NO2−, and EDTA (1 mM, aerobic, anaerobic). Most importantly, the PA4455 mutant was also sensitive to A-NO2− in biofilms. A-NO2− sensitivity and an anaerobic growth defect was also noted in two mutants (rmlC and wbpM) that are defective in B-band LPS synthesis, potentially indicating a membrane defect in the PA4455 mutant. Finally, this study describes a gene, PA4455, that when mutated, allows for dramatic sensitivity to the potential therapeutic agent, A-NO2− as well as EDTA. Furthermore, the synergy between the two compounds could offer future benefits against antibiotic resistant PA strains.

Increased bactericidal activity of colistin on Pseudomonas aeruginosa biofilms in anaerobic conditions

Tolerance towards antibiotics of Pseudomonas aeruginosa biofilms is recognized as a major cause of therapeutic failure of chronic lung infection in cystic fibrosis (CF) patients. This lung infection is characterized by antibiotic-tolerant biofilms in mucus with zones of O2 depletion mainly due to polymorphonuclear leukocytic activity. In contrast to the main types of bactericidal antibiotics, it has not been possible to establish an association between the bactericidal effects of colistin and the production of detectable levels of OH ˙ on several strains of planktonic P. aeruginosa. Therefore, we propose that production of OH ˙ may not contribute significantly to the bactericidal activity of colistin on P. aeruginosa biofilm. Thus, we investigated the effect of colistin treatment on biofilm of wild-type PAO1, a catalase-deficient mutant (ΔkatA) and a colistin-resistant CF isolate cultured in microtiter plates in normoxic- or anoxic atmosphere with 1 mM nitrate. The killing of bacteria during colistin treatment was measured by CFU counts, and the OH⋅ formation was measured by 3(')-(p-hydroxylphenyl fluorescein) fluorescein (HPF) fluorescence. Validation of the assay was done by hydrogen peroxide treatment. OH⋅ formation was undetectable in aerobic PAO1 biofilms during 3 h of colistin treatment. Interestingly, we demonstrate increased susceptibility of P. aeruginosa biofilms towards colistin during anaerobic conditions. In fact, the maximum enhancement of killing by anaerobic conditions exceeded 2 logs using 4 mg L(-1) of colistin compared to killing at aerobic conditions.

Nitrate, nitrite and nitric oxide reductases: from the last universal common ancestor to modern bacterial pathogens

The electrochemical gradient that ensues from the enzymatic activity of cytochromes such as nitrate reductase, nitric oxide reductase, and quinol oxidase contributes to the bioenergetics of the bacterial cell. Reduction of nitrogen oxides by bacterial pathogens can, however, be uncoupled from proton translocation and biosynthesis of ATP or NH4(+), but still linked to quinol and NADH oxidation. Ancestral nitric oxide reductases, as well as cytochrome c oxidases and quinol bo oxidases evolved from the former, are capable of binding and detoxifying nitric oxide to nitrous oxide. The NO-metabolizing activity associated with these cytochromes can be a sizable source of antinitrosative defense in bacteria during their associations with host cells. Nitrosylation of terminal cytochromes arrests respiration, reprograms bacterial metabolism, stimulates antioxidant defenses and alters antibiotic cytotoxicity. Collectively, the bioenergetics and regulation of redox homeostasis that accompanies the utilization of nitrogen oxides and detoxification of nitric oxide by cytochromes of the electron transport chain increases fitness of many Gram-positive and -negative pathogens during their associations with invertebrate and vertebrate hosts.