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

Anaerobic bacteria in chronic wounds: Roles in disease, infection and treatment failure

Infection is among the most common factors that impede wound healing, yet standard treatments routinely fail to resolve chronic wound infections. The chronic wound environment is largely hypoxic/anoxic, and wounds are predominantly colonised by facultative and obligate anaerobic bacteria. Oxygen (O2) limitation is an underappreciated driver of microbiota composition and behaviour in chronic wounds. In this perspective article, we examine how anaerobic bacteria and their distinct physiologies support persistent, antibiotic-recalcitrant infections. We describe the anaerobic energy metabolisms bacteria rely on for long-term survival in the wound environment, and why many antibiotics become less effective under hypoxic conditions. We also discuss obligate anaerobes, which are among the most prevalent taxa to colonise chronic wounds, yet their potential roles in influencing the microbial community and wound healing have been overlooked. All of the most common obligate anaerobes found in chronic wounds are opportunistic pathogens. We consider how these organisms persist in the wound environment and interface with host physiology to hinder wound healing processes or promote chronic inflammation. Finally, we apply our understanding of anaerobic physiologies to evaluate current treatment practices and to propose new strategies for treating chronic wound infections.

Practical Guidance for Clinical Microbiology Laboratories: Updated guidance for processing respiratory tract samples from people with cystic fibrosis

SUMMARYThis guidance presents recommendations for clinical microbiology laboratories for processing respiratory samples from people with cystic fibrosis (pwCF). Appropriate processing of respiratory samples is crucial to detect bacterial and fungal pathogens, guide treatment, monitor the epidemiology of cystic fibrosis (CF) pathogens, and assess therapeutic interventions. Thanks to CF transmembrane conductance regulator modulator therapy, the health of pwCF has improved, but as a result, fewer pwCF spontaneously expectorate sputum. Thus, the collection of sputum samples has decreased, while the collection of other types of respiratory samples such as oropharyngeal and bronchoalveolar lavage samples has increased. To optimize the detection of microorganisms, including Pseudomonas aeruginosa, Staphylococcus aureus, Haemophilus influenzae, and Burkholderia cepacia complex; other less common non-lactose fermenting Gram-negative bacilli, e.g., Stenotrophomonas maltophilia, Inquilinus, Achromobacter, Ralstonia, and Pandoraea species; and yeasts and filamentous fungi, non-selective and selective culture media are recommended for all types of respiratory samples, including samples obtained from pwCF after lung transplantation. There are no consensus recommendations for laboratory practices to detect, characterize, and report small colony variants (SCVs) of S. aureus, although studies are ongoing to address the potential clinical impact of SCVs. Accurate identification of less common Gram-negative bacilli, e.g., S. maltophilia, Inquilinus, Achromobacter, Ralstonia, and Pandoraea species, as well as yeasts and filamentous fungi, is recommended to understand their epidemiology and clinical importance in pwCF. However, conventional biochemical tests and automated platforms may not accurately identify CF pathogens. MALDI-TOF MS provides excellent genus-level identification, but databases may lack representation of CF pathogens to the species-level. Thus, DNA sequence analysis should be routinely available to laboratories for selected clinical circumstances. Antimicrobial susceptibility testing (AST) is not recommended for every routine surveillance culture obtained from pwCF, although selective AST may be helpful, e.g., for unusual pathogens or exacerbations unresponsive to initial therapy. While this guidance reflects current care paradigms for pwCF, recommendations will continue to evolve as CF research expands the evidence base for laboratory practices.

Mucus polymer concentration and in vivo adaptation converge to define the antibiotic response of Pseudomonas aeruginosa during chronic lung infection

The airway milieu of individuals with muco-obstructive airway diseases (MADs) is defined by the accumulation of dehydrated mucus due to hyperabsorption of airway surface liquid and defective mucociliary clearance. Pathological mucus becomes progressively more viscous with age and disease severity due to the concentration and overproduction of mucin and accumulation of host-derived extracellular DNA (eDNA). Respiratory mucus of MADs provides a niche for recurrent and persistent colonization by respiratory pathogens, including Pseudomonas aeruginosa, which is responsible for the majority of morbidity and mortality in MADs. Despite high concentration inhaled antibiotic therapies and the absence of antibiotic resistance, antipseudomonal treatment failure in MADs remains a significant clinical challenge. Understanding the drivers of antibiotic tolerance is essential for developing more effective treatments that eradicate persistent infections. The complex and dynamic environment of diseased airways makes it difficult to model antibiotic efficacy in vitro. We aimed to understand how mucin and eDNA concentrations, the two dominant polymers in respiratory mucus, alter the antibiotic tolerance of P. aeruginosa. Our results demonstrate that polymer concentration and molecular weight affect P. aeruginosa survival post antibiotic challenge. Polymer-driven antibiotic tolerance was not explicitly associated with reduced antibiotic diffusion. Lastly, we established a robust and standardized in vitro model for recapitulating the ex vivo antibiotic tolerance of P. aeruginosa observed in expectorated sputum across age, underlying MAD etiology, and disease severity, which revealed the inherent variability in intrinsic antibiotic tolerance of host-evolved P. aeruginosa populations.

IMPORTANCE

Antibiotic treatment failure in Pseudomonas aeruginosa chronic lung infections is associated with increased morbidity and mortality, illustrating the clinical challenge of bacterial infection control. Understanding the underlying infection environment, as well as the host and bacterial factors driving antibiotic tolerance and the ability to accurately recapitulate these factors in vitro, is crucial for improving antibiotic treatment outcomes. Here, we demonstrate that increasing concentration and molecular weight of mucin and host eDNA drive increased antibiotic tolerance to tobramycin. Through systematic testing and modeling, we identified a biologically relevant in vitro condition that recapitulates antibiotic tolerance observed in ex vivo treated sputum. Ultimately, this study revealed a dominant effect of in vivo evolved bacterial populations in defining inter-subject ex vivo antibiotic tolerance and establishes a robust and translatable in vitro model for therapeutic development.

The use of combination therapy for the improvement of colistin activity against bacterial biofilm

Colistin is used as a last resort for the management of infections caused by multi-drug resistant (MDR) bacteria. However, the use of this antibiotic could lead to different side effects, such as nephrotoxicity, in most patients, and the high prevalence of colistin-resistant strains restricts the use of colistin in the clinical setting. Additionally, colistin could induce resistance through the increased formation of biofilm; biofilm-embedded cells are highly resistant to antibiotics, and as with other antibiotics, colistin is impaired by bacteria in the biofilm community. In this regard, the researchers used combination therapy for the enhancement of colistin activity against bacterial biofilm, especially MDR bacteria. Different antibacterial agents, such as antimicrobial peptides, bacteriophages, natural compounds, antibiotics from different families, N-acetylcysteine, and quorum-sensing inhibitors, showed promising results when combined with colistin. Additionally, the use of different drug platforms could also boost the efficacy of this antibiotic against biofilm. The mentioned colistin-based combination therapy not only could suppress the formation of biofilm but also could destroy the established biofilm. These kinds of treatments also avoided the emergence of colistin-resistant subpopulations, reduced the required dosage of colistin for inhibition of biofilm, and finally enhanced the dosage of this antibiotic at the site of infection. However, the exact interaction of colistin with other antibacterial agents has not been elucidated yet; therefore, further studies are required to identify the precise mechanism underlying the efficient removal of biofilms by colistin-based combination therapy.

Polymyxin B and ethylenediaminetetraacetic acid act synergistically against Pseudomonas aeruginosa and Staphylococcus aureus

Polymyxin B and ethylenediaminetetraacetic acid are antimicrobials possessing antibiofilm activity. They act by displacement and chelation, respectively, of divalent cations in bacterial membranes and may therefore act synergistically when applied in combination. If so, this combination of agents may be useful for the treatment of diseases like cystic fibrosis (CF), in which biofilms are present on the respiratory epithelium. We used checkerboard assays to investigate the synergy between these agents using reference strains Pseudomonas aeruginosa ATCC 27853 and Staphylococcus aureus ATCC 6538 in planktonic form. We then determined the efficacy of each agent against biofilms of both species grown on 96-pin lids and proceeded to combination testing against the P. aeruginosa reference strain and 10 clinical isolates from patients with CF. Synergism was observed for planktonic forms of both species and for biofilms of P. aeruginosa. The susceptibility of biofilms of P. aeruginosa clinical isolates to these agents was variable compared to the laboratory reference strain. This combination of agents may be useful in the management of biofilm-associated conditions, particularly those amenable to topical therapies. These results provide a basis upon which the antimicrobial and antibiofilm efficacy of preparations containing these agents may be enhanced.IMPORTANCEBacteria living in biofilms produce a protective matrix which makes them difficult to kill. Patients with severe respiratory disease often have biofilms. Polymyxin B is an antibiotic commonly used in topical medications, such as eye drops and nasal sprays. Ethylenediaminetetraacetic acid (EDTA) is used widely as a preservative in medication but also has antimicrobial properties. It has been hypothesized that Polymyxin B and EDTA could have a synergistic relationship: when used in combination their antimicrobial effect is enhanced. Here, we evaluated the levels at which Polymyxin B and EDTA work together to kill common pathogens Pseudomonas aeruginosa and Staphylococcus aureus. We found that Polymyxin B and EDTA were synergistic. This synergy may be useful in the management of planktonic infection with P. aeruginosa and S. aureus, or biofilm infection with P. aeruginosa. This synergy may be beneficial in the treatment of respiratory biofilms, in which P. aeruginosa biofilms are common.

Mechanisms of antibiotic resistance in Pseudomonas aeruginosa biofilms

Pseudomonas aeruginosa is a major cause of life-threatening acute infections and life-long lasting chronic infections. The characteristic biofilm mode of life in P. aeruginosa chronic infections severely limits the efficacy of antimicrobial therapies, as it leads to intrinsic tolerance, involving physical and physiological factors in addition to biofilm-specific genes that can confer a transient protection against antibiotics promoting the development of resistance. Indeed, a striking feature of this pathogen is the extraordinary capacity to develop resistance to nearly all available antibiotics through the selection of chromosomal mutations, evidenced by its outstanding and versatile mutational resistome. This threat is dramatically amplified in chronic infections, driven by the frequent emergence of mutator variants with enhanced spontaneous mutation rates. Thus, this mini review is focused on describing the complex interplay of antibiotic resistance mechanisms in P. aeruginosa biofilms, to provide potentially useful information for the design of effective therapeutic strategies.

Mathematical pharmacodynamic modeling for antimicrobial assessment of ceftazidime/colistin versus gentamicin/meropenem combinations against carbapenem-resistant Pseudomonas aeruginosa biofilm

BACKGROUND

Carbapenem-resistant Pseudomonas aeruginosa (CRPA) represents an escalating healthcare hazard with high mortality worldwide, especially in presence of biofilm. The current study aimed to evaluate the anti-biofilm potentials of ceftazidime, colistin, gentamicin, and meropenem alone and in combinations against biofilm-forming CRPA.

METHODS

Biofilm killing and checkerboard assay were performed to detect the effectiveness of combined antibiotics against biofilms and planktonic cells, respectively. The bacterial bioburden retrieved from the established biofilms following treatment with combined antibiotics was utilized to construct a three-dimensional response surface plot. A sigmoidal maximum effect model was applied to determine the pharmacodynamic parameters (maximal effect, median effective concentration, and Hill factor) of each antibiotic to create a mathematical three-dimensional response surface plot.

RESULTS

Data revealed statistically significant (p < 0.05) superior anti-biofilm potential in the case of colistin followed by a lower effect in the case of gentamicin and meropenem, while ceftazidime exhibited the least anti-biofilm activity. The fractional inhibitory concentration index (FICI ≤ 0.5) indicated synergism following treatment with the combined antibiotics. An elevated anti-biofilm activity was recorded in the case of gentamicin/meropenem compared to ceftazidime/colistin. Synergistic anti-biofilm potentials were also detected via the simulated pharmacodynamic modeling, with higher anti-biofilm activity in the case of the in vitro observation compared to the simulated anti-biofilm profile.

CONCLUSIONS

The present study highlighted the synergistic potentials of the tested antibiotic combinations against P. aeruginosa biofilms and the importance of the mathematical pharmacodynamic modeling in investigating the efficacy of antibiotics in combination as an effective strategy for successful antibiotic therapy to tackle the extensively growing resistance to the currently available antibiotics.

Activity of N-Acetylcysteine Alone and in Combination with Colistin against Pseudomonas aeruginosa Biofilms and Transcriptomic Response to N-Acetylcysteine Exposure

Chronic colonization by Pseudomonas aeruginosa is critical in cystic fibrosis (CF) and other chronic lung diseases, contributing to disease progression. Biofilm growth and a propensity to evolve multidrug resistance phenotypes drastically limit the available therapeutic options. In this perspective, there has been growing interest in evaluating combination therapies, especially for drugs that can be administered by nebulization, which allows high drug concentrations to be reached at the site of infections while limiting systemic toxicity. Here, we investigated the potential antibiofilm activity of N-acetylcysteine (NAC) alone and in combination with colistin against a panel of P. aeruginosa strains (most of which are from CF patients) and the transcriptomic response of a P. aeruginosa CF strain to NAC exposure. NAC alone (8,000 mg/L) showed a limited and strain-dependent antibiofilm activity. Nonetheless, a relevant antibiofilm synergism of NAC-colistin combinations (NAC at 8,000 mg/L plus colistin at 2 to 32 mg/L) was observed with all strains. Synergism was also confirmed with the artificial sputum medium model. RNA sequencing of NAC-exposed planktonic cultures revealed that NAC (8,000 mg/L) mainly induced (i) a Zn2+ starvation response (known to induce attenuation of P. aeruginosa virulence), (ii) downregulation of genes of the denitrification apparatus, and (iii) downregulation of flagellar biosynthesis pathway. NAC-mediated inhibition of P. aeruginosa denitrification pathway and flagellum-mediated motility were confirmed experimentally. These findings suggested that NAC-colistin combinations might contribute to the management of biofilm-associated P. aeruginosa lung infections. NAC might also have a role in reducing P. aeruginosa virulence, which could be relevant in the very early stages of lung colonization. IMPORTANCE Pseudomonas aeruginosa biofilm-related chronic lung colonization contributes to cystic fibrosis (CF) disease progression. Colistin is often a last-resort antibiotic for the treatment of such P. aeruginosa infections, and it has been increasingly used in CF, especially by nebulization. N-acetylcysteine (NAC) is a mucolytic agent with antioxidant activity, commonly administered with antibiotics for the treatment of lower respiratory tract infections. Here, we show that NAC potentiated colistin activity against in vitro biofilms models of P. aeruginosa strains, with both drugs tested at the high concentrations achievable after nebulization. In addition, we report the first transcriptomic data on the P. aeruginosa response to NAC exposure.

Topical Antibiofilm Agents With Potential Utility in the Treatment of Chronic Rhinosinusitis: A Narrative Review

The role of bacterial biofilms in chronic and recalcitrant diseases is widely appreciated, and the treatment of biofilm infection is an increasingly important area of research. Chronic rhinosinusitis (CRS) is a complex disease associated with sinonasal dysbiosis and the presence of bacterial biofilms. While most biofilm-related diseases are associated with highly persistent but relatively less severe inflammation, the presence of biofilms in CRS is associated with greater severity of inflammation and recalcitrance despite appropriate treatment. Oral antibiotics are commonly used to treat CRS but they are often ineffective, due to poor penetration of the sinonasal mucosa and the inherently antibiotic resistant nature of bacteria in biofilms. Topical non-antibiotic antibiofilm agents may prove more effective, but few such agents are available for sinonasal application. We review compounds with antibiofilm activity that may be useful for treating biofilm-associated CRS, including halogen-based compounds, quaternary ammonium compounds and derivatives, biguanides, antimicrobial peptides, chelating agents and natural products. These include preparations that are currently available and those still in development. For each compound, antibiofilm efficacy, mechanism of action, and toxicity as it relates to sinonasal application are summarised. We highlight the antibiofilm agents that we believe hold the greatest promise for the treatment of biofilm-associated CRS in order to inform future research on the management of this difficult condition.

Pseudomonas aeruginosa Is More Tolerant Under Biofilm Than Under Planktonic Growth Conditions: A Multi-Isolate Survey

Biofilm-associated bacteria exhibit profound changes in bacterial physiology. They thrive in the environment but also in the human host in protected sessile communities. Antimicrobial therapy usually fails, despite the absence of genotypic resistance, and it is commonly accepted that biofilm-grown bacteria are up to 1,000-fold more resistant than planktonic cells. We are only at the beginning to understand the reasons for biofilm recalcitrance, and systematic approaches to describe biofilm-induced tolerance phenotypes are lacking. In this study, we investigated a large and highly diverse collection of 352 clinical Pseudomonas aeruginosa isolates for their antimicrobial susceptibility profiles under biofilm growth conditions towards the antibiotics ciprofloxacin, tobramycin, and colistin. We discovered characteristic patterns of drug-specific killing activity and detected conditional tolerance levels far lower (in the range of the minimal inhibitory concentration (MIC)), but also far higher (up to 16,000-fold increase compared to planktonic cells) than generally believed. This extremely broad distribution of biofilm-induced tolerance phenotypes across the clinical isolates was greatly influenced by the choice of the antibiotic. We furthermore describe cross-tolerance against ciprofloxacin and tobramycin, but not colistin, and observed an additive activity between biofilm-induced tolerance and genetically determined resistance. This became less evident when the biofilm-grown cells were exposed to very high antibiotic concentrations. Although much more remains to be learned on the molecular mechanisms underlying biofilm-induced tolerance, our data on intra-species variations in tolerance profiles provide valuable new insights. Furthermore, our observation that colistin appears to act independently of the tolerance mechanisms of individual clinical strains could make colistin a valuable therapeutic option in chronic biofilm-associated infections characterized by the presence of particularly tolerant strains.

The cystic fibrosis lung microenvironment alters antibiotic activity: causes and effects

Chronic airway colonisation by Pseudomonas aeruginosa, a hallmark of cystic fibrosis (CF) lung disease, is associated with increased morbidity and mortality and despite aggressive antibiotic treatment, P. aeruginosa is able to persist in CF airways. In vitro antibiotic susceptibility assays are poor predictors of antibiotic efficacy to treat respiratory tract infections in the CF patient population and the selection of the antibiotic(s) is often made on an empirical base. In the current review, we discuss the factors that are responsible for the discrepancies between antibiotic activity in vitro and clinical efficacy in vivo We describe how the CF lung microenvironment, shaped by host factors (such as iron, mucus, immune mediators and oxygen availability) and the microbiota, influences antibiotic activity and varies widely between patients. A better understanding of the CF microenvironment and population diversity may thus help improve in vitro antibiotic susceptibility testing and clinical decision making, in turn increasing the success rate of antibiotic treatment.

Gallium Porphyrin and Gallium Nitrate Reduce the High Vancomycin Tolerance of MRSA Biofilms by Promoting Extracellular DNA-Dependent Biofilm Dispersion

Biofilms, structured communities of bacterial cells embedded in a self-produced extracellular matrix (ECM) which consists of proteins, polysaccharide intercellular adhesins (PIAs), and extracellular DNA (eDNA), play a key role in clinical infections and are associated with an increased morbidity and mortality by protecting the embedded bacteria against drug and immune response. The high levels of antibiotic tolerance render classical antibiotic therapies impractical for biofilm-related infections. Thus, novel drugs and strategies are required to reduce biofilm tolerance and eliminate biofilm-protected bacteria. Here, we showed that gallium, an iron mimetic metal, can lead to nutritional iron starvation and act as dispersal agent triggering the reconstruction and dispersion of mature methicillin-resistant Staphylococcus aureus (MRSA) biofilms in an eDNA-dependent manner. The extracellular matrix, along with the integral bacteria themselves, establishes the integrated three-dimensional structure of the mature biofilm. The structures and compositions of gallium-treated mature biofilms differed from those of natural or antibiotic-survived mature biofilms but were similar to those of immature biofilms. Similar to immature biofilms, gallium-treated biofilms had lower levels of antibiotic tolerance, and our in vitro tests showed that treatment with gallium agents reduced the antibiotic tolerance of mature MRSA biofilms. Thus, the sequential administration of gallium agents (gallium porphyrin and gallium nitrate) and relatively low concentrations of vancomycin (16 mg/L) effectively eliminated mature MRSA biofilms and eradicated biofilm-enclosed bacteria within 1 week. Our results suggested that gallium agents may represent a potential treatment for refractory biofilm-related infections, such as prosthetic joint infections (PJI) and osteomyelitis, and provide a novel basis for future biofilm treatments based on the disruption of normal biofilm-development processes.

A link between pH homeostasis and colistin resistance in bacteria

Colistin resistance is complex and multifactorial. DbcA is an inner membrane protein belonging to the DedA superfamily required for maintaining extreme colistin resistance of Burkholderia thailandensis. The molecular mechanisms behind this remain unclear. Here, we report that ∆dbcA displays alkaline pH/bicarbonate sensitivity and propose a role of DbcA in extreme colistin resistance of B. thailandensis by maintaining cytoplasmic pH homeostasis. We found that alkaline pH or presence of sodium bicarbonate displays a synergistic effect with colistin against not only extremely colistin resistant species like B. thailandensis and Serratia marcescens, but also a majority of Gram-negative and Gram-positive bacteria tested, suggesting a link between cytoplasmic pH homeostasis and colistin resistance across species. We found that lowering the level of oxygen in the growth media or supplementation of fermentable sugars such as glucose not only alleviated alkaline pH stress, but also increased colistin resistance in most bacteria tested, likely by avoiding cytoplasmic alkalinization. Our observations suggest a previously unreported link between pH, oxygen, and colistin resistance. We propose that maintaining optimal cytoplasmic pH is required for colistin resistance in a majority of bacterial species, consistent with the emerging link between cytoplasmic pH homeostasis and antibiotic resistance.

Catalase Protects Biofilm of Staphylococcus aureus against Daptomycin Activity

Daptomycin is recommended for the treatment of Staphylococcus aureus infections due to its bactericidal activity. However, its mechanism of action is poorly understood. The involvement of reactive oxygen species (ROS) in the bactericidal activity of daptomycin has been proved against planktonic S. aureus, but not against the biofilm of S. aureus. Therefore, we evaluated if ROS contributes to the effect of daptomycin against biofilm of S. aureus. Biofilms of wild type, catalase deficient and daptomycin-resistant S. aureus strains were grown in microtiter-plates. After three days, the biofilms were exposed to daptomycin with or without thiourea in the presence of a ROS indicator. After overnight incubation, the amount of ROS and the percentage of surviving bacteria were determined. The bacterial survival was higher and the amount of ROS was lower in the wild type than in the catalase deficient biofilm, demonstrating a protective effect of catalase against daptomycin. The induction of cytotoxic ROS formation by daptomycin was verified by the addition of thiourea, which reduced the amount of ROS and protected the wild type biofilm against high concentrations of daptomycin. Accordingly, only the highest concentration of daptomycin reduced the bacterial survival and increased the ROS formation in the resistant biofilm. In conclusion, daptomycin induced the production of cytotoxic levels of endogenous ROS in S. aureus biofilm and the presence of catalase protected the biofilm against the lethality of the induced ROS.

Small Molecule Inhibitors of the Bacterioferritin (BfrB)-Ferredoxin (Bfd) Complex Kill Biofilm-Embedded Pseudomonas aeruginosa Cells

Bacteria depend on a well-regulated iron homeostasis to survive adverse environments. A key component of the iron homeostasis machinery is the compartmentalization of Fe3+ in bacterioferritin and its subsequent mobilization as Fe2+ to satisfy metabolic requirements. In Pseudomonas aeruginosa Fe3+ is compartmentalized in bacterioferritin (BfrB), and its mobilization to the cytosol requires binding of a ferredoxin (Bfd) to reduce the stored Fe3+ and release the soluble Fe2+. Blocking the BfrB-Bfd complex in P. aeruginosa by deletion of the bfd gene triggers an irreversible accumulation of Fe3+ in BfrB, concomitant cytosolic iron deficiency and significant impairment of biofilm development. Herein we report that small molecules developed to bind BfrB at the Bfd binding site block the BfrB-Bfd complex, inhibit the mobilization of iron from BfrB in P. aeruginosa cells, elicit a bacteriostatic effect on planktonic cells, and are bactericidal to cells embedded in mature biofilms.

In vitro pharmacokinetics/pharmacodynamics of continuous ceftazidime infusion alone and in combination with colistin against Pseudomonas aeruginosa biofilm

OBJECTIVES

The pharmacokinetics/pharmacodynamics of continuous infusion (CI) beta-lactams for Pseudomonas aeruginosa biofilm infections has not been defined. This study evaluated the efficacy of several dosage regimens of CI ceftazidime, with or without colistin, an antibiotic with a potential antibiofilm effect, against biofilm-embedded P. aeruginosa.

METHODS

Mature biofilms of the reference strain PAO1 and the clinical isolate HUB8 (both ceftazidime- and colistin-susceptible) were investigated over 54h using a dynamic CDC biofilm reactor. CI dosage regimens were ceftazidime monotherapy (4, 10, 20 and 40 mg/L), colistin monotherapy (3.50 mg/L), and combinations of colistin and ceftazidime (4 or 40 mg/L). Efficacy was evaluated by changes in log₁₀colony-forming units (cfu)/mL and confocal microscopy.

RESULTS

At 54 h, the antibiofilm activity of ceftazidime monotherapies was slightly higher for ceftazidime 20 mg/L (-2.84 log₁₀cfu/mL) and 40 mg/L (-3.05) against PAO1, but no differences were seen against HUB8. Ceftazidime-resistant colonies emerged with 4 mg/L regimens in both strains and with other regimens in PAO1. Colistin monotherapy had significant antibiofilm activity against HUB8 (-3.07), but lower activity against PAO1 (-1.12), and colistin-resistant strains emerged. Combinations of ceftazidime and colistin had higher antibiofilm activity at 54 h compared with each monotherapy, and prevented the emergence of resistance to both antibiotics; higher antibiofilm activity was observed with ceftazidime 40 mg/L plus colistin compared with ceftazidime 4 mg/L plus colistin (-4.19 vs. -3.10 PAO1; -4.71 vs. -3.44 HUB8).

CONCLUSIONS

This study demonstrated that, with %T>MIC=100%, CI ceftazidime displayed concentration-dependent antibiofilm activity against P. aeruginosa biofilm, particularly in combination with colistin. These results support the use of high-dosage regimens of CI ceftazidime with colistin against biofilm-associated infections with ceftazidime-susceptible P. aeruginosa.

Recent Advances in Surface Nanoengineering for Biofilm Prevention and Control. Part I: Molecular Basis of Biofilm Recalcitrance. Passive Anti-Biofouling Nanocoatings

Medical device-associated infections are becoming a leading cause of morbidity and mortality worldwide, prompting researchers to find new, more effective ways to control the bacterial colonisation of surfaces and biofilm development. Bacteria in biofilms exhibit a set of "emergent properties", meaning those properties that are not predictable from the study of free-living bacterial cells. The social coordinated behaviour in the biofilm lifestyle involves intricate signaling pathways and molecular mechanisms underlying the gain in resistance and tolerance (recalcitrance) towards antimicrobial agents as compared to free-floating bacteria. Nanotechnology provides powerful tools to disrupt the processes responsible for recalcitrance development in all stages of the biofilm life cycle. The present paper is a state-of-the-art review of the surface nanoengineering strategies currently used to design antibiofilm coatings. The review is structurally organised in two parts according to the targeted biofilm life cycle stages and molecular mechanisms intervening in recalcitrance development. Therefore, in the present first part, we begin with a presentation of the current knowledge of the molecular mechanisms responsible for increased recalcitrance that have to be disrupted. Further, we deal with passive surface nanoengineering strategies that aim to prevent bacterial cells from settling onto a biotic or abiotic surface. Both "fouling-resistant" and "fouling release" strategies are addressed as well as their synergic combination in a single unique nanoplatform.

The Iron-chelator, N,N'-bis (2-hydroxybenzyl) Ethylenediamine-N,N'-Diacetic acid is an Effective Colistin Adjunct against Clinical Strains of Biofilm-Dwelling Pseudomonas aeruginosa

Targeting the iron requirement of Pseudomonas aeruginosa may be an effective adjunctive for conventional antibiotic treatment against biofilm-dwelling P. aeruginosa. We, therefore, assessed the anti-biofilm activity of N,N’-bis (2-hydroxybenzyl) ethylenediamine-N,N’-diacetic acid (HBED), which is a synthetic hexadentate iron chelator. The effect of HBED was studied using short-term (microtitre plate) and longer-term (flow-cell) biofilm models, under aerobic, anaerobic, and microaerobic (flow-cell) conditions and in combination with the polymyxin antibiotic colistimethate sodium (colistin). HBED was assessed against strains of P. aeruginosa from patients with cystic fibrosis and the reference strain PAO1. HBED inhibited growth and biofilm formation of all clinical strains under aerobic and anaerobic conditions, but inhibitory effects against PAO1 were predominantly exerted under anaerobic conditions. PA605, which is a clinical strain with a robust biofilm-forming phenotype, was selected for flow-cell studies. HBED significantly reduced biomass and surface coverage of PA605, and, combined with colistin, HBED significantly enhanced the microcolony killing effects of colistin to result in almost complete removal of the biofilm. HBED combined with colistin is highly effective in vitro against biofilms formed by clinical strains of P. aeruginosa.

Comparative Antibiofilm Efficacy of Meropenem Alone and in Combination with Colistin in an In Vitro Pharmacodynamic Model by Extended-Spectrum-β-Lactamase-Producing Klebsiella pneumoniae

We compared the efficacies of meropenem alone and in combination with colistin against two strains of extended-spectrum-β-lactamase-producing Klebsiella pneumoniae, using an in vitro pharmacodynamic model that mimicked two different biofilm conditions. Meropenem monotherapy achieved remarkable efficacy (even a bactericidal effect) under all conditions, whereas colistin was almost inactive and resistance emerged. The addition of colistin to meropenem produced no relevant benefits, in contrast to experiences with other microorganisms.

Antimicrobial Tolerance and Metabolic Adaptations in Microbial Biofilms

Active bacterial metabolism is a prerequisite for optimal activity of many classes of antibiotics. Hence, bacteria have developed strategies to reduce or modulate metabolic pathways to become tolerant. This review describes the tight relationship between metabolism and tolerance in bacterial biofilms, and how physicochemical properties of the microenvironment at the host-pathogen interface (such as oxygen and nutritional content) are key to this relationship. Understanding how metabolic adaptations lead to tolerance brings us to novel approaches to tackle antibiotic-tolerant biofilms. We describe the use of hyperbaric oxygen therapy, metabolism-stimulating metabolites, and alternative strategies to redirect bacterial metabolism towards an antibiotic-susceptible phenotype.

Evaluation of the interaction between polymyxin B and Pseudomonas aeruginosa biofilm and planktonic cells: reactive oxygen species induction and zeta potential

BACKGROUND

Although the most widely accepted mechanism of action for polymyxins is related to bacterial lysis via disruption, we hypothesized that this antimicrobial drug class could have other effects on Pseudomonas aeruginosa planktonic and sessile cells. Little is known regarding oxidative burst and zeta potential (ZP) data associated with the interaction between polymyxin B and P. aeruginosa cells. The present study evaluated endogenous reactive oxygen species (ROS) production and changes in the net charges of biofilm and planktonic cells in response to polymyxin B.

RESULTS

Polymyxin B induced concentration-dependent killing at all concentrations tested in planktonic and sessile cells from P. aeruginosa strains. Sublethal concentrations of polymyxin B induced oxidative burst. ROS production was higher in resistant planktonic cells than in biofilm cells but this was not observed for susceptible cells. Moreover, no net surface charge alterations were observed in planktonic cells from a susceptible strain treated with polymyxin B, but a significant increase of ZP was noted in planktonic cells from a resistant strain.

CONCLUSION

Oxidative burst generated by planktonic and sessile cells from P. aeruginosa strains against polymyxin B indicates that ROS may have an important role in the mechanism of action of this drug. ZP data revealed that electrostatic interactions of the cationic peptide with the anionic surface of the cells are strain-dependent. Therefore, we suggested that the intracellular effects of polymyxin B should be further investigated to understand polymyxin B-induced stress in P. aeruginosa.

Tolerance and Resistance of Pseudomonas aeruginosa Biofilms to Antimicrobial Agents-How P. aeruginosa Can Escape Antibiotics

Pseudomonas aeruginosa is one of the six bacterial pathogens, Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp., which are commonly associated with antimicrobial resistance, and denoted by their acronym ESKAPE. P. aeruginosa is also recognized as an important cause of chronic infections due to its ability to form biofilms, where the bacteria are present in aggregates encased in a self-produced extracellular matrix and are difficult or impossible to eradicate with antibiotic treatment. P. aeruginosa causes chronic infections in the lungs of patients with cystic fibrosis and chronic obstructive lung disease, as well as chronic urinary tract infections in patients with permanent bladder catheter, and ventilator-associated pneumonia in intubated patients, and is also an important pathogen in chronic wounds. Antibiotic treatment cannot eradicate these biofilm infections due to their intrinsic antibiotic tolerance and the development of mutational antibiotic resistance. The tolerance of biofilms to antibiotics is multifactorial involving physical, physiological, and genetic determinants, whereas the antibiotic resistance of bacteria in biofilms is caused by mutations and driven by the repeated exposure of the bacteria to high levels of antibiotics. In this review, both the antimicrobial tolerance and the development of resistance to antibiotics in P. aeruginosa biofilms are discussed. Possible therapeutic approaches based on the understanding of the mechanisms involved in the tolerance and resistances of biofilms to antibiotics are also addressed.

Efficacy of ceftolozane/tazobactam, alone and in combination with colistin, against multidrug-resistant Pseudomonas aeruginosa in an in vitro biofilm pharmacodynamic model

OBJECTIVES

Ceftolozane/tazobactam is a potential tool for infections caused by multidrug-resistant (MDR) Pseudomonas aeruginosa (P. aeruginosa), but its efficacy against some difficult-to-treat infections has not been well defined.

METHODS

Using an in vitro pharmacodynamic biofilm model, this study evaluated the comparative efficacy of ceftolozane/tazobactam against MDR/extensively drug-resistant (XDR) P. aeruginosa strains, alone and in combination with colistin. Simulated regimens of ceftolozane/tazobactam (2 g/1 g every 8 h), meropenem (2 g every 8 h) and ceftazidime (2 g every 8 h), alone and in combination with colistin (continuous infusion) were evaluated against three colistin-susceptible and ceftazidime-resistant strains: MDR-HUB1, ceftolozane/tazobactam-susceptible and meropenem-susceptible; XDR-HUB2, ceftolozane/tazobactam-susceptible and meropenem-resistant; MDR-HUB3, ceftolozane/tazobactam-resistant and meropenem-susceptible. Antibiotic efficacy was evaluated by decreases in bacterial counts (Δlog CFU/mL) from biofilm-embedded bacteria over 54 h. Resistance emergence was screened.

RESULTS

Among monotherapies, ceftolozane/tazobactam had low killing but no resistance appeared, ceftazidime was ineffective, colistin was initially effective but regrowth and resistance occurred, and meropenem was bactericidal against carbapenem-susceptible strains. Ceftolozane/tazobactam plus colistin was the most effective combination against the meropenem-resistant XDR-HUB2 strain (Δlog CFU/mL 54-0 h = -4.42 vs. -3.54 for meropenem-colistin; P = 0.002), whereas this combination against MDR-HUB1 (-4.36) was less effective than meropenem-colistin (-6.25; P < 0.001). Ceftolozane/tazobactam plus colistin was ineffective against the ceftolozane/tazobactam-resistant strain; meropenem plus colistin was the most bactericidal therapy (-6.37; P < 0.001 vs. others). Combinations of active beta-lactams plus colistin prevented the emergence of colistin-resistant strains.

CONCLUSIONS

Combinations of colistin plus ceftolozane/tazobactam and meropenem were the most appropriate treatments for biofilm-related infections caused by XDR and MDR P. aeruginosa strains, respectively. These combinations could be considered as potential treatment options for these difficult to treat infections.

Biofilm formation - what we can learn from recent developments

Although biofilms have been observed early in the history of microbial research, their impact has only recently been fully recognized. Biofilm infections, which contribute to up to 80% of human microbial infections, are associated with common human disorders, such as diabetes mellitus and poor dental hygiene, but also with medical implants. The associated chronic infections such as wound infections, dental caries and periodontitis significantly enhance morbidity, affect quality of life and can aid development of follow-up diseases such as cancer. Biofilm infections remain challenging to treat and antibiotic monotherapy is often insufficient, although some rediscovered traditional compounds have shown surprising efficiency. Innovative anti-biofilm strategies include application of anti-biofilm small molecules, intrinsic or external stimulation of production of reactive molecules, utilization of materials with antimicrobial properties and dispersion of biofilms by digestion of the extracellular matrix, also in combination with physical biofilm breakdown. Although basic principles of biofilm formation have been deciphered, the molecular understanding of the formation and structural organization of various types of biofilms has just begun to emerge. Basic studies of biofilm physiology have also resulted in an unexpected discovery of cyclic dinucleotide second messengers that are involved in interkingdom crosstalk via specific mammalian receptors. These findings even open up new venues for exploring novel anti-biofilm strategies.

Cysteamine, an Endogenous Aminothiol, and Cystamine, the Disulfide Product of Oxidation, Increase Pseudomonas aeruginosa Sensitivity to Reactive Oxygen and Nitrogen Species and Potentiate Therapeutic Antibiotics against Bacterial Infection

Cysteamine is an endogenous aminothiol produced in mammalian cells as a consequence of coenzyme A metabolism through the activity of the vanin family of pantetheinase ectoenzymes. It is known to have a biological role in oxidative stress, inflammation, and cell migration. There have been several reports demonstrating anti-infective properties targeting viruses, bacteria, and even the malarial parasite. We and others have previously described broad-spectrum antimicrobial and antibiofilm activities of cysteamine. Here, we go further to demonstrate redox-dependent mechanisms of action for the compound and how its antimicrobial effects are, at least in part, due to undermining bacterial defenses against oxidative and nitrosative challenges. We demonstrate the therapeutic potentiation of antibiotic therapy against Pseudomonas aeruginosa in mouse models of infection. We also demonstrate potentiation of many different classes of antibiotics against a selection of priority antibiotic-resistant pathogens, including colistin (often considered an antibiotic of last resort), and we discuss how this endogenous antimicrobial component of innate immunity has a role in infectious disease that is beginning to be explored and is not yet fully understood.

The Consequences of Being in an Infectious Biofilm: Microenvironmental Conditions Governing Antibiotic Tolerance

The main driver behind biofilm research is the desire to understand the mechanisms governing the antibiotic tolerance of biofilm-growing bacteria found in chronic bacterial infections. Rather than genetic traits, several physical and chemical traits of the biofilm have been shown to be attributable to antibiotic tolerance. During infection, bacteria in biofilms exhibit slow growth and a low metabolic state due to O₂ limitation imposed by intense O₂ consumption of polymorphonuclear leukocytes or metabolically active bacteria in the biofilm periphery. Due to variable O₂ availability throughout the infection, pathogen growth can involve aerobic, microaerobic and anaerobic metabolism. This has serious implications for the antibiotic treatment of infections (e.g., in chronic wounds or in the chronic lung infection of cystic fibrosis patients), as antibiotics are usually optimized for aerobic, fast-growing bacteria. This review summarizes knowledge about the links between the microenvironment of biofilms in chronic infections and their tolerance against antibiotics.

Towards individualized diagnostics of biofilm-associated infections: a case study

Organized within biofilm communities, bacteria exhibit resistance towards a broad spectrum of antibiotics. Thus, one might argue that bacteria isolated from biofilm-associated chronic infections should be subjected to resistance profiling under biofilm growth conditions. Various test systems have been developed to determine the biofilm-associated resistance; however, it is not clear to what extent the in vitro results reflect the situation in vivo, and whether the biofilm-resistance profile should guide clinicians in their treatment choice. To address this issue, we used confocal microscopy in combination with live/dead staining, and profiled biofilm-associated resistance of a large number (>130) of clinical Pseudomonas aeruginosa isolates from overall 15 cystic fibrosis patients. Our results demonstrate that in addition to a general non-responsiveness of bacteria when grown under biofilm conditions, there is an isolate-specific and antibiotic-specific biofilm-resistance profile. This individual resistance profile is independent on the structural properties of the biofilms. Furthermore, biofilm resistance is not linked to the resistance profile under planktonic growth conditions, or a mucoid, or small colony morphology of the tested isolates. Instead, it seems that individual biofilm structures evolve during biofilm-associated growth and are shaped by environment-specific cues. In conclusion, our results demonstrate that biofilm resistance profiles are isolate specific and cannot be deduced from commonly studied phenotypes. Further clinical studies will have to show the added value of biofilm-resistance profiling. Individualized diagnosis of biofilm resistance might lead to more rational recommendations for antimicrobial therapy and, thus, increased effectiveness of the treatment of chronically infected patients.

Antibiotic treatment of biofilm infections

Bacterial biofilms are associated with a wide range of infections, from those related to exogenous devices, such as catheters or prosthetic joints, to chronic tissue infections such as those occurring in the lungs of cystic fibrosis patients. Biofilms are recalcitrant to antibiotic treatment due to multiple tolerance mechanisms (phenotypic resistance). This causes persistence of biofilm infections in spite of antibiotic exposure which predisposes to antibiotic resistance development (genetic resistance). Understanding the interplay between phenotypic and genetic resistance mechanisms acting on biofilms, as well as appreciating the diversity of environmental conditions of biofilm infections which influence the effect of antibiotics are required in order to optimize the antibiotic treatment of biofilm infections. Here, we review the current knowledge on phenotypic and genetic resistance in biofilms and describe the potential strategies for the antibiotic treatment of biofilm infections. Of note is the optimization of PK/PD parameters in biofilms, high-dose topical treatments, combined and sequential/alternate therapies or the use antibiotic adjuvants.

Microenvironmental characteristics and physiology of biofilms in chronic infections of CF patients are strongly affected by the host immune response

In vitro studies of Pseudomonas aeruginosa and other pathogenic bacteria in biofilm aggregates have yielded detailed insight into their potential growth modes and metabolic flexibility under exposure to gradients of substrate and electron acceptor. However, the growth pattern of P. aeruginosa in chronic lung infections of cystic fibrosis (CF) patients is very different from what is observed in vitro, for example, in biofilms grown in flow chambers. Dense in vitro biofilms of P. aeruginosa exhibit rapid O₂ depletion within <50-100 μm due to their own aerobic metabolism. In contrast, in vivo investigations show that P. aeruginosa persists in the chronically infected CF lung as relatively small cell aggregates that are surrounded by numerous PMNs, where the activity of PMNs is the major cause of O₂ depletion rendering the P. aeruginosa aggregates anoxic. High levels of nitrate and nitrite enable P. aeruginosa to persist fueled by denitrification in the PMN-surrounded biofilm aggregates. This configuration creates a potentially long-term stable ecological niche for P. aeruginosa in the CF lung, which is largely governed by slow growth and anaerobic metabolism and enables persistence and resilience of this pathogen even under the recurring aggressive antimicrobial treatments of CF patients. As similar slow growth of other CF pathogens has recently been observed in endobronchial secretions, there is now a clear need for better in vitro models that simulate such in vivo growth patterns and anoxic microenvironments in order to help unravel the efficiency of existing or new antimicrobials targeting anaerobic metabolism in P. aeruginosa and other CF pathogens. We also advocate that host immune responses such as PMN-driven O₂ depletion play a central role in the formation of anoxic microniches governing bacterial persistence in other chronic infections such as chronic wounds.

Probiotic Lactobacillus reuteri has antifungal effects on oral Candida species in vitro

Background: An alternative approach for managing Candida infections in the oral cavity by modulating the oral microbiota with probiotic bacteria has been proposed.

Objective: The aim was to investigate the antifungal potential of the probiotic bacterium Lactobacillus reuteri (DSM 17938 and ATCC PTA 5289) against six oral Candida species (C. albicans, C. glabrata, C. krusei, C. tropicalis, C. dubliniensis, and C. parapsilosis).

Design: The lactobacilli were tested for their ability to co-aggregate with and inhibit the growth of the yeasts assessed by spectrophotometry and the agar overlay inhibition assay. Additionally, the pH was evaluated with microsensors, and the production of hydrogen peroxide (H2O2) by the lactobacilli was verified.

Results: Both L. reuteri strains showed co-aggregation abilities with the yeasts. The lactobacilli almost completely inhibited the growth of C. albicans and C. parapsilosis, but did not affect C. krusei. Statistically significant differences in co-aggregation and growth inhibition capacities between the two L. reuteri strains were observed (p<0.001). The pH measurements suggested that C. krusei can resist the acids produced by the lactobacilli.

Conclusions:L. reuteri exhibited antifungal properties against five of the six most common oral Candida species. Further, the results reconfirms that the probiotic capacity of L. reuteri is strain specific.