Pseudomonas aeruginosa adapts to octenidine via a combination of efflux and membrane remodelling

Membrane Modification and Adaptation of Metabolism by Acinetobacter baumannii Prompts Resistance to Antimicrobial Activity of Outer Membrane Perturbing Peptide L8

Multidrug resistant (MDR) Acinetobacter baumannii has emerged as one of the most concerning nosocomial pathogens worldwide. One approach to target MDR A. baumannii is treatment with synergistic combinations of outer membrane-permeabilizing antimicrobial peptides (AMP) and antibiotics that otherwise only act against Gram-positive bacteria. Resistance against AMPs is rarely observed, especially when administered in combination with other drugs. Recently, we described the synergistic antimicrobial activity of AMPs L8 and L8S1 with rifampicin against a clinical isolate of A. baumannii. In the current work we explore the mechanisms of action of these peptides. We demonstrate that L8 and L8S1 perturb the cell envelope of A. baumannii. Moreover, we show that resistance against peptide L8 could be acquired in vitro either by increasing the amount of PE lipid on the surface or by increasing biofilm formation. Interestingly, the resistance to the antimicrobial activity of the peptides did not affect membrane perturbation or synergistic activity of the peptides with rifampicin, suggesting a dual mechanism of action for these peptides.

In vitro long-term exposure to chlorhexidine or triclosan induces cross-resistance against azoles in Nakaseomyces glabratus

BACKGROUND

Topical antiseptics are crucial for preventing infections and reducing transmission of pathogens. However, commonly used antiseptic agents have been reported to cause cross-resistance to other antimicrobials in bacteria, which has not yet been described in yeasts. This study aims to assess the in vitro efficacy of antiseptics against clinical and reference isolates of Candida albicans and Nakaseomyces glabratus, and whether prolonged exposure to antiseptics promotes the development of antifungal (cross)resistance.

METHODS

A high-throughput approach for in vitro resistance development was established to simultaneously expose 96 C. albicans and N. glabratus isolates to increasing concentrations of a given antiseptic - chlorhexidine, triclosan or octenidine. Susceptibility testing and whole genome sequencing of yeast isolates pre- and post-exposure were performed.

RESULTS

Long-term exposure to antiseptics does not result in the development of stable resistance to the antiseptics themselves. However, 50 N. glabratus isolates acquired resistance to azole antifungals after long-term exposure to triclosan or chlorhexidine, revealing newly acquired mutations in the PDR1 and PMA1 genes.

CONCLUSIONS

Chlorhexidine as well as triclosan, but not octenidine, were able to introduce selective pressure promoting resistance to azole antifungals. Although we assessed this phenomenon only in vitro, these findings warrant critical monitoring in clinical settings.

Activity of Antiseptics Against Pseudomonas aeruginosa and Its Adaptation Potential

BACKGROUND/OBJECTIVES

Pseudomonas aeruginosa rapidly acquires antibiotic resistance and demonstrates increasing tolerance to antiseptics. This study evaluated the activity of eight antiseptics against P. aeruginosa, assessed its ability to develop adaptation to these antiseptics, and, for the first time, determined the Karpinski Adaptation Index (KAI) for this bacterium.

METHODS

The minimal inhibitory concentration (MIC), susceptibility to antibiotics, bactericidal time according to EN 1040:2005, adaptation potential, and KAI of P. aeruginosa strains were evaluated.

RESULTS

The most effective antiseptics against P. aeruginosa, based on MIC activity, were octenidine dihydrochloride (OCT; mean MIC 11.3 ± 4.5 µg/mL), polyhexamethylene biguanide (PHMB; MIC 22.6 ± 8.0 µg/mL), and chlorhexidine digluconate (CHX; MIC 26.6 ± 14.4 µg/mL). Sodium hypochlorite (NaOCl) and ethacridine lactate (ET) showed moderate activity, while boric acid (BA), povidone-iodine (PVI), and potassium permanganate (KMnO4) exhibited the weakest MIC activity. MIC values for NaOCl (95 ± 15.4 µg/mL) and KMnO4 (>10 mg/mL) were close to or exceeded the clinical concentrations used in commercial products. OCT, CHX, and PVI exhibited the fastest bactericidal effect within 1 min. Bactericidal times were up to 15 min for PHMB, up to 60 min for ET, and more than 60 min for BA, NaOCl, and KMnO4. The lowest KAI values, indicating a low resistance risk, were observed for OCT (0.12), PHMB (0.19), and BA (0.19). Moderate resistance risk was noted for PVI (0.21), CHX (0.29), and ET (0.47). The highest KAI values, signifying a very high resistance risk, were found for NaOCl (1.0) and KMnO4 (≥1.0).

CONCLUSIONS

Antiseptics like OCT, CHX, and partially PVI can be critical in quick antibacterial action on infected wounds, while agents such as PHMB might be reserved for cases where prolonged contact times are possible. Given the rapid adaptation of P. aeruginosa to the clinical concentrations of NaOCl and KMnO4 currently in use, reconsideration of their effectiveness in treating skin and mucous membrane infections is recommended.

Adaptation to Sodium Hypochlorite and Potassium Permanganate May Lead to Their Ineffectiveness Against Candida albicans

Background/Objectives: Adaptation can reduce or completely eliminate the effectiveness of antibiotics and antiseptics at clinical concentrations. To our knowledge, no studies have examined fungal adaptation to antiseptics. This study aimed to preliminarily investigate the potential for Candida albicans adaptation to eight antiseptics. Methods: The minimal inhibitory concentration (MIC), drug susceptibility, adaptation to antiseptics, and Karpinski Adaptation Index (KAI) of C. albicans strains were assessed. Results: The antiseptics with the most effective MICs activity against C. albicans were octenidine dihydrochloride (OCT), chlorhexidine digluconate (CHX), and polyhexamethylene biguanide (polyhexanide, PHMB). Sodium hypochlorite (NaOCl) and ethacridine lactate (ET) demonstrated moderate activity, while boric acid (BA), povidone-iodine (PVI), and potassium permanganate (KMnO4) showed the weakest activity. The MIC values for NaOCl and KMnO4 were close to or equal to the clinical concentrations used in commercial products. The studied strains were susceptible to econazole, miconazole, and voriconazole. Resistance to other drugs occurred in 10-30% of the strains. Antifungal resistance remained unchanged after antiseptic adaptation testing. The lowest KAI values, indicating very low resistance risk, were observed for CHX, OCT, and PHMB. PVI and BA presented a low risk, ET a moderate risk. KMnO4 and NaOCl had the highest KAI values, indicating high and very high resistance risk in Candida yeasts. Conclusions:C. albicans strains can adapt to antiseptics to varying extents. For most antiseptics, adaptation does not significantly affect their clinical efficacy. However, due to adaptation, NaOCl and KMnO4 may become ineffective against C. albicans strains even at clinical concentrations.

Highly Effective Biocides against Pseudomonas aeruginosa Reveal New Mechanistic Insights Across Gram-Negative Bacteria

Pseudomonas aeruginosa is a major nosocomial pathogen that persists in healthcare settings despite rigorous disinfection protocols due to intrinsic mechanisms conferring resistance. We sought to systematically assess cationic biocide efficacy against this pathogen using a panel of multidrug-resistant P. aeruginosa clinical isolates. Our studies revealed widespread resistance to commercial cationic disinfectants that are the current standard of care, raising concerns about their efficacy. To address this shortcoming, we highlight a new class of quaternary phosphonium compounds that are highly effective against all members of the panel. To understand the difference in efficacy, mechanism of action studies were carried out, which identified a discrete inner-membrane selective target. Resistance selection studies implicated the SmvRA efflux system (a transcriptionally regulated, inner membrane-associated efflux system) as a major determinant of resistance. This system is also implicated in resistance to two commercial bolaamphiphile antiseptics, octenidine and chlorhexidine, which was further validated herein. In sum, this work highlights, for the first time, a discrete inner-membrane specific mechanism for the bolaamphiphile class of disinfectants that contrasts with the prevailing model of indiscriminate membrane interactions of commercial amphiphiles paving the way for future innovations in disinfectant research.

Antiseptic management of critical wounds: differential bacterial response upon exposure to antiseptics and first insights into antiseptic/phage interactions

BACKGROUND

With the antibiotic crisis, the topical antibacterial control including chronic wounds gains increasing importance. However, little is known regarding tolerance development when bacteria face repetitive exposure to the identical antiseptics as commonly found in clinical practice.

MATERIALS AND METHODS

Clinical isolates foremost of chronic wounds were exposed in vitro to dilutions of two antiseptics used for wound therapy: polyhexanide or octenidine. Adaptive response was determined by growth/kill curves, minimal inhibitory concentration (MIC), and whole genome sequencing. Antiseptic/bacteriophage combinations were studied by liquid-infection assays and bacterial plating.

RESULTS

Polyhexanide acted stronger against Escherichia coli and Proteus mirabilis while octenidine was more potent against Staphylococcus aureus . Otherwise, the antiseptic efficacy varied across isolates of Klebsiella pneumoniae , Pseudomonas aeruginosa , and Acinetobacter baumannii . Upon repetitive exposure with constant antiseptic concentrations P. aeruginosa and P. mirabilis adaptation was evident by a reduced lag-phase and a twofold increased MIC. Under increasing octenidine concentrations, P. aeruginosa adapted to an eightfold higher dosage with mutations in smvA , opgH , and kinB affecting an efflux pump, alginate and biofilm formation, respectively. S. aureus adapted to a fourfold increase of polyhexanide with a mutation in the multiple peptide resistance factor MprF, also conferring cross-resistance to daptomycin. Antiseptic/bacteriophage combinations enhanced bacterial inhibition and delayed adaptation.

CONCLUSION

Different bacterial species/strains respond unequally to low-level antiseptic concentrations. Bacterial adaptation potential at phenotypic and genotypic levels may indicate the necessity for a more nuanced selection of antiseptics. Bacteriophages represent a promising yet underexplored strategy for supporting antiseptic treatment, which may be particularly beneficial for the management of critical wounds.

Evaluating the Expression of Efflux Pumps in Pseudomonas aeruginosa in Exposure to Sodium Dodecyl Sulfate, Didecyldimethylammonium Chloride, and Octenidine Dihydrochloride

Emerging resistance of Gram-negative bacteria, including Pseudomonas aeruginosa, to commonly used detergents and disinfectant is encountering us with hazard. Inappropriate use of disinfectants has forced bacteria to gain resistance. The ability of bacteria to extrude substrates from the cellular interior to the external environment has enabled them to persist in exposure to toxic compounds, which is due to existence of transport proteins. Efflux pumps, in Gram-negative bacteria, are proteins responsible for exporting molecules outside of the cell, by crossing the two membranes. In this study, 40 P. aeruginosa strains from hospitals, clinics, and burn center laundries and 40 P. aeruginosa strains from urban laundries were collected. This study evaluated the minimum inhibitory concentration (MIC) level of sodium dodecyl sulfate (SDS), didecyldimethylammonium chloride (DDAC), and octenidine dihydrochloride (Od) in P. aeruginosa strains. The real-time PCR was carried out to evaluate the expression of MexAB-OprM, MexCD-OprJ, and MexXY-OprM efflux system. The obtained results indicated a higher MIC level for SDS, DDAC, and Od in medical laundries. The sub-MIC level of DDAC and Od increased the expression level of MexAB-OprM, MexCD-OprJ, and MexXY-OprM in P. aeruginosa strains, suggesting that efflux pumps contribute to disinfectant resistance in P. aeruginosa.

Identification of mechanisms modulating chlorhexidine and octenidine susceptibility in Proteus mirabilis

AIMS

We aimed to identify mechanisms underlying the tolerance of Proteus mirabilis-a common cause of catheter associated urinary tract infection-to the clinically used biocides chlorhexidine (CHD) and octenidine (OCT).

METHODS AND RESULTS

We adapted three clinical isolates to grow at concentrations of 512 µg ml-1 CHD and 128 µg ml-1 OCT. Genetic characterization and complementation studies revealed mutations inactivating the smvR repressor and increasing smvA efflux expression were associated with adaptation to both biocides. Mutations in mipA (encoding the MltA interacting protein) were less prevalent than smvR mutations and only identified in CHD adapted populations. Mutations in the rppA response regulator were exclusive to one adapted isolate and were linked with reduced polymyxin B susceptibility and a predicted gain of function after biocide adaptation. Biocide adaptation had no impact on crystalline biofilm formation.

CONCLUSIONS

SmvR inactivation is a key mechanism in both CHD and OCT tolerance. MipA inactivation alone confers moderate protection against CHD, and rppA showed no direct role in either CHD or OCT susceptibility.

Glyoxylate Shunt and Pyruvate-to-Acetoin Shift Are Specific Stress Responses Induced by Colistin and Ceragenin CSA-13 in Enterobacter hormaechei ST89

Ceragenins, including CSA-13, are cationic antimicrobials that target the bacterial cell envelope differently than colistin. However, the molecular basis of their action is not fully understood. Here, we examined the genomic and transcriptome responses by Enterobacter hormaechei after prolonged exposure to either CSA-13 or colistin. Resistance of the E. hormaechei 4236 strain (sequence type 89 [ST89]) to colistin and CSA-13 was induced in vitro during serial passages with sublethal doses of tested agents. The genomic and metabolic profiles of the tested isolates were characterized using a combination of whole-genome sequencing (WGS) and transcriptome sequencing (RNA-seq), followed by metabolic mapping of differentially expressed genes using Pathway Tools software. The exposure of E. hormaechei to colistin resulted in the deletion of the mgrB gene, whereas CSA-13 disrupted the genes encoding an outer membrane protein C and transcriptional regulator SmvR. Both compounds upregulated several colistin-resistant genes, such as the arnABCDEF operon and pagE, including genes coding for DedA proteins. The latter proteins, along with beta-barrel protein YfaZ and VirK/YbjX family proteins, were the top overexpressed cell envelope proteins. Furthermore, the l-arginine biosynthesis pathway and putrescine-ornithine antiporter PotE were downregulated in both transcriptomes. In contrast, the expression of two pyruvate transporters (YhjX and YjiY) and genes involved in pyruvate metabolism, as well as genes involved in generating proton motive force (PMF), was antimicrobial specific. Despite the similarity of the cell envelope transcriptomes, distinctly remodeled carbon metabolism (i.e., toward fermentation of pyruvate to acetoin [colistin] and to the glyoxylate pathway [CSA-13]) distinguished both antimicrobials, which possibly reflects the intensity of the stress exerted by both agents. IMPORTANCE Colistin and ceragenins, like CSA-13, are cationic antimicrobials that disrupt the bacterial cell envelope through different mechanisms. Here, we examined the genomic and transcriptome changes in Enterobacter hormaechei ST89, an emerging hospital pathogen, after prolonged exposure to these agents to identify potential resistance mechanisms. Interestingly, we observed downregulation of genes associated with acid stress response as well as distinct dysregulation of genes involved in carbon metabolism, resulting in a switch from pyruvate fermentation to acetoin (colistin) and the glyoxylate pathway (CSA-13). Therefore, we hypothesize that repression of the acid stress response, which alkalinizes cytoplasmic pH and, in turn, suppresses resistance to cationic antimicrobials, could be interpreted as an adaptation that prevents alkalinization of cytoplasmic pH in emergencies induced by colistin and CSA-13. Consequently, this alteration critical for cell physiology must be compensated via remodeling carbon and/or amino acid metabolism to limit acidic by-product production.

The Art of War with Pseudomonas aeruginosa: Targeting Mex Efflux Pumps Directly to Strategically Enhance Antipseudomonal Drug Efficacy

Pseudomonas aeruginosa (P. aeruginosa) poses a grave clinical challenge due to its multidrug resistance (MDR) phenotype, leading to severe and life-threatening infections. This bacterium exhibits both intrinsic resistance to various antipseudomonal agents and acquired resistance against nearly all available antibiotics, contributing to its MDR phenotype. Multiple mechanisms, including enzyme production, loss of outer membrane proteins, target mutations, and multidrug efflux systems, contribute to its antimicrobial resistance. The clinical importance of addressing MDR in P. aeruginosa is paramount, and one pivotal determinant is the resistance-nodulation-division (RND) family of drug/proton antiporters, notably the Mex efflux pumps. These pumps function as crucial defenders, reinforcing the emergence of extensively drug-resistant (XDR) and pandrug-resistant (PDR) strains, which underscores the urgency of the situation. Overcoming this challenge necessitates the exploration and development of potent efflux pump inhibitors (EPIs) to restore the efficacy of existing antipseudomonal drugs. By effectively countering or bypassing efflux activities, EPIs hold tremendous potential for restoring the antibacterial activity against P. aeruginosa and other Gram-negative pathogens. This review focuses on concurrent MDR, highlighting the clinical significance of efflux pumps, particularly the Mex efflux pumps, in driving MDR. It explores promising EPIs and delves into the structural characteristics of the MexB subunit and its substrate binding sites.

The In Vitro Impact of Isoxazole Derivatives on Pathogenic Biofilm and Cytotoxicity of Fibroblast Cell Line

The microbial, biofilm-based infections of chronic wounds are one of the major challenges of contemporary medicine. The use of topically administered antiseptic agents is essential to treat wound-infecting microorganisms. Due to observed microbial tolerance/resistance against specific clinically-used antiseptics, the search for new, efficient agents is of pivotal meaning. Therefore, in this work, 15 isoxazole derivatives were scrutinized against leading biofilm wound pathogens Staphylococcus aureus and Pseudomonas aeruginosa, and against Candida albicans fungus. For this purpose, the minimal inhibitory concentration, biofilm reduction in microtitrate plates, modified disk diffusion methods and antibiofilm dressing activity measurement methods were applied. Moreover, the cytotoxicity and cytocompatibility of derivatives was tested toward wound bed-forming cells, referred to as fibroblasts, using normative methods. Obtained results revealed that all isoxazole derivatives displayed antimicrobial activity and low cytotoxic effect, but antimicrobial activity of two derivatives, 2-(cyclohexylamino)-1-(5-nitrothiophen-2-yl)-2-oxoethyl 5-amino-3-methyl-1,2-oxazole-4-carboxylate (PUB9) and 2-(benzylamino)-1-(5-nitrothiophen-2-yl)-2-oxoethyl 5-amino-3-methyl-1,2-oxazole-4-carboxylate (PUB10), was noticeably higher compared to the other compounds analyzed, especially PUB9 with regard to Staphylococcus aureus, with a minimal inhibitory concentration more than x1000 lower compared to the remaining derivatives. The PUB9 and PUB10 derivatives were able to reduce more than 90% of biofilm-forming cells, regardless of the species, displaying at the same time none (PUB9) or moderate (PUB10) cytotoxicity against fibroblasts and high (PUB9) or moderate (PUB10) cytocompatibility against these wound cells. Therefore, taking into consideration the clinical demand for new antiseptic agents for non-healing wound treatment, PUB9 seems to be a promising candidate to be further tested in advanced animal models and later, if satisfactory results are obtained, in the clinical setting.

Role of Efflux Pumps on Antimicrobial Resistance in Pseudomonas aeruginosa

Antimicrobial resistance is an old and silent pandemic. Resistant organisms emerge in parallel with new antibiotics, leading to a major global public health crisis over time. Antibiotic resistance may be due to different mechanisms and against different classes of drugs. These mechanisms are usually found in the same organism, giving rise to multidrug-resistant (MDR) and extensively drug-resistant (XDR) bacteria. One resistance mechanism that is closely associated with the emergence of MDR and XDR bacteria is the efflux of drugs since the same pump can transport different classes of drugs. In Gram-negative bacteria, efflux pumps are present in two configurations: a transmembrane protein anchored in the inner membrane and a complex formed by three proteins. The tripartite complex has a transmembrane protein present in the inner membrane, a periplasmic protein, and a porin associated with the outer membrane. In Pseudomonas aeruginosa, one of the main pathogens associated with respiratory tract infections, four main sets of efflux pumps have been associated with antibiotic resistance: MexAB-OprM, MexXY, MexCD-OprJ, and MexEF-OprN. In this review, the function, structure, and regulation of these efflux pumps in P. aeruginosa and their actions as resistance mechanisms are discussed. Finally, a brief discussion on the potential of efflux pumps in P. aeruginosa as a target for new drugs is presented.

Octenidine's Efficacy: A Matter of Interpretation or the Influence of Experimental Setups?

With its broad antimicrobial spectrum and non-specific mode of action via membrane disruption, any resistance to octenidine (OCT) seems unlikely and has not been observed in clinical settings so far. In this study, we aimed to investigate the efficacy of OCT against Escherichia coli and mutants lacking specific lipid head groups which, due to altered membrane properties, might be the root cause for resistance development of membrane-active compounds. Furthermore, we aimed to test its efficacy under different experimental conditions including different solvents for OCT, bacterial concentration and methods for analysis. Our primary goal was to estimate how many OCT molecules are needed to kill one bacterium. We performed susceptibility assays by observing bacterial growth behavior, using a Bioscreen in an analogous manner for every condition. The growth curves were recorded for 20 h at 420-580 nm in presence of different OCT concentrations and were used to assess the inhibitory concentrations (IC_100%) for OCT. Bacterial concentrations given in cell numbers were determined, followed by Bioscreen measurement by manual colony counting on agar plates and QUANTOM^TM cell staining. This indicated a significant variance between both methods, which influenced IC_100% of OCT, especially when used at low doses. The binding capacity of OCT to E. coli was investigated by measuring UV-absorbance of OCT exposed to bacteria and a common thermodynamic framework based on Bioscreen measurements. Results showed that OCT's antimicrobial activity in E. coli is not affected by changes at the membrane level but strongly dependent on experimental settings in respect to solvents and applied bacterial counts. More OCT was required when the active was dissolved in phosphate or Hepes buffers instead of water and when higher bacterial concentration was used. Furthermore, binding studies revealed that 10⁷-10⁸ OCT molecules bind to bacteria, which is necessary for the saturation of the bacterial surface to initiate the killing cascade. Our results clearly demonstrate that in vitro data, depending on the applied materials and the methods for determination of IC_100%, can easily be misinterpreted as reduced bacterial susceptibility towards OCT.

Efflux-mediated tolerance to cationic biocides, a cause for concern?

AbstractWith an increase in the number of isolates resistant to multiple antibiotics, infection control has become increasingly important to help combat the spread of multi-drug-resistant pathogens. An important component of this is through the use of disinfectants and antiseptics (biocides). Antibiotic resistance has been well studied in bacteria, but little is known about potential biocide resistance genes and there have been few reported outbreaks in hospitals resulting from a breakdown in biocide effectiveness. Development of increased tolerance to biocides has been thought to be more difficult due to the mode of action of biocides which affect multiple cellular targets compared with antibiotics. Very few genes which contribute towards increased biocide tolerance have been identified. However, the majority of those that have are components or regulators of different efflux pumps or genes which modulate membrane function/modification. This review will examine the role of efflux in increased tolerance towards biocides, focusing on cationic biocides and heavy metals against Gram-negative bacteria. As many efflux pumps which are upregulated by biocide presence also contribute towards an antimicrobial resistance phenotype, the role of these efflux pumps in cross-resistance to both other biocides and antibiotics will be explored.