Factors Determining Staphylococcus aureus Susceptibility to Photoantimicrobial Chemotherapy: RsbU Activity, Staphyloxanthin Level, and Membrane Fluidity

Antivirulence activities of Rutin-loaded chitosan nanoparticles against pathogenic Staphylococcus aureus

BACKGROUND

Staphylococcus aureus is an infectious bacterium that is frequently found in healthcare settings and the community. This study aimed to prepare rutin-loaded chitosan nanoparticles (Rut-CS NPs) and assess their antibacterial activity against pathogenic strains of S. aureus.

RESULTS

The synthesized Rut-CS NPs exhibited an amorphous morphology with a size ranging from 160 to 240 nm and a zeta potential of 37.3 mV. Rut-CS NPs demonstrated significant antibacterial activity against S. aureus strains. Following exposure to Rut-CS NPs, the production of staphyloxanthin pigment decreased by 43.31-89.63%, leading to increased susceptibility of S. aureus to hydrogen peroxide. Additionally, visual inspection of cell morphology indicated changes in membrane integrity and permeability upon Rut-CS NPs exposure, leading to a substantial increase (107.07-191.08%) in cytoplasmic DNA leakage in the strains. Furthermore, ½ MIC of Rut-CS NPs effectively inhibited the biofilm formation (22.5-37.5%) and hemolytic activity (69-82.59%) in the S. aureus strains.

CONCLUSIONS

Our study showcases that Rut-CS NPs can serve as a novel treatment agent to combat S. aureus infections by altering cell morphology and inhibiting virulence factors of S. aureus.

Decrease of ESKAPE virulence with a cationic heme-mimetic gallium porphyrin photosensitizer: The Trojan horse strategy that could help address antimicrobial resistance

INTRODUCTION

Emerging antibiotic resistance among bacterial pathogens has forced an urgent need for alternative non-antibiotic strategies development that could combat drug resistant-associated infections. Suppression of virulence of ESKAPE pathogens' by targeting multiple virulence traits provides a promising approach.

OBJECTIVES

Here we propose an iron-blocking antibacterial therapy based on a cationic heme-mimetic gallium porphyrin (GaCHP), which antibacterial efficacy could be further enhanced by photodynamic inactivation.

METHODS

We used gallium heme mimetic porphyrin (GaCHP) excited with light to significantly reduce microbial viability and suppress both the expression and biological activity of several virulence traits of both Gram-positive and Gram-negative ESKAPE representatives, i.e., S. aureus and P. aeruginosa. Moreover, further improvement of the proposed strategy by combining it with routinely used antimicrobials to resensitize the microbes to antibiotics and provide enhanced bactericidal efficacy was investigated.

RESULTS

The proposed strategy led to substantial inactivation of critical priority pathogens and has been evidenced to suppress the expression and biological activity of multiple virulence factors in S. aureus and P. aeruginosa. Finally, the combination of GaCHP phototreatment and antibiotics resulted in promising strategy to overcome antibiotic resistance of the studied microbes and to enhance disinfection of drug resistant pathogens.

CONCLUSION

Lastly, considering high safety aspects of the proposed treatment toward host cells, i.e., lack of mutagenicity, no dark toxicity and mild phototoxicity, we describe an efficient alternative that simultaneously suppresses the functionality of multiple virulence factors in ESKAPE pathogens.

Anti-virulence potential of patuletin, a natural flavone, against Staphylococcus aureus: In vitro and In silico investigations

Staphylococcus aureus is a highly prevalent and aggressive human pathogen causing a wide range of infections. This study aimed to explore the potential of Patuletin, a rare natural flavone, as an anti-virulence agent against S. aureus. At a sub-inhibitory concentration (1/4 MIC), Patuletin notably reduced biofilm formation by 27 % and 23 %, and decreased staphyloxanthin production by 53 % and 46 % in Staphylococcus aureus isolate SA25923 and clinical isolate SA1, respectively. In order to gain a more comprehensive understanding of the in vitro findings, several in silico analyses were conducted. Initially, a 3D-flexible alignment study demonstrated a favorable structural similarity between Patuletin and B70, the co-crystallized ligand of CrtM, an enzyme that plays a pivotal role in the biosynthesis of staphyloxanthin. Molecular docking highlighted the strong binding of Patuletin to the active site of CrtM, with a high affinity of -20.95 kcal/mol. Subsequent 200 ns molecular dynamics simulations, along with MM-GBSA, ProLIF, PLIP, and PCAT analyses, affirmed the stability of the Patuletin-CrtM complex, revealing no significant changes in CrtM's structure upon binding. Key amino acids crucial for binding were also identified. Collectively, this study showcased the effective inhibition of CrtM activity by Patuletin in silico and its attenuation of key virulence factors in vitro, including biofilm formation and staphyloxanthin production. These findings hint at Patuletin's potential as a valuable therapeutic agent, especially in combination with antibiotics, to counter antibiotic-resistant Staphylococcus aureus infections.

Staphylococcus aureus Modulates Carotenoid and Phospholipid Content in Response to Oxygen-Restricted Growth Conditions, Triggering Changes in Membrane Biophysical Properties

Staphylococcus aureus membranes contain carotenoids formed during the biosynthesis of staphyloxanthin. These carotenoids are considered virulence factors due to their activity as scavengers of reactive oxygen species and as inhibitors of antimicrobial peptides. Here, we show that the growth of S. aureus under oxygen-restricting conditions downregulates carotenoid biosynthesis and modifies phospholipid content in biofilms and planktonic cells analyzed using LC-MS. At oxygen-restrictive levels, the staphyloxanthin precursor 4,4-diapophytofluene accumulates, indicating that the dehydrogenation reaction catalyzed by 4,4'-diapophytoene desaturases (CrtN) is inhibited. An increase in lysyl-phosphatidylglycerol is observed under oxygen-restrictive conditions in planktonic cells, and high levels of cardiolipin are detected in biofilms compared to planktonic cells. Under oxygen-restriction conditions, the biophysical parameters of S. aureus membranes show an increase in lipid headgroup spacing, as measured with Laurdan GP, and decreased bilayer core order, as measured with DPH anisotropy. An increase in the liquid-crystalline to gel phase melting temperature, as measured with FTIR, is also observed. S. aureus membranes are therefore less condensed under oxygen-restriction conditions at 37 °C. However, the lack of carotenoids leads to a highly ordered gel phase at low temperatures, around 15 °C. Carotenoids are therefore likely to be low in S. aureus found in tissues with low oxygen levels, such as abscesses, leading to altered membrane biophysical properties.

Photoactivated Gallium Porphyrin Reduces Staphylococcus aureus Colonization on the Skin and Suppresses Its Ability to Produce Enterotoxin C and TSST-1

Staphylococcus aureus is a key pathogen in atopic dermatitis (AD) pathogenicity. Over half of AD patients are carriers of S. aureus. Clinical isolates derived from AD patients produce various staphylococcal enterotoxins, such as staphylococcal enterotoxin C or toxic shock syndrome toxin. The production of these virulence factors is correlated with more severe AD. In this study, we propose cationic heme-mimetic gallium porphyrin (Ga3+CHP), a novel gallium metalloporphyrin, as an anti-staphylococcal agent that functions through dual mechanisms: a light-dependent mechanism (antimicrobial photodynamic inactivation, aPDI) and a light-independent mechanism (suppressing iron metabolism). Ga3+CHP has two additive quaternary ammonium groups that increase its water solubility. Furthermore, Ga3+CHP is an efficient generator of singlet oxygen and can be recognized by heme-target systems such as Isd, which improves the intracellular accumulation of this compound. Ga3+CHP activated with green light effectively reduced the survival of clinical S. aureus isolates derived from AD patients (>5 log10 CFU/mL) and affected their enterotoxin gene expression. Additionally, there was a decrease in the biological functionality of studied toxins regarding their superantigenicity. In aPDI conditions, there was no pronounced toxicity in HaCaT keratinocytes with both normal and suppressed filaggrin gene expression, which occurs in ∼50% of AD patients. Additionally, no mutagenic activity was observed. Green light-activated gallium metalloporphyrins may be a promising chemotherapeutic to reduce S. aureus colonization on the skin of AD patients.

Alleviating the virulence of Pseudomonas aeruginosa and Staphylococcus aureus by ascorbic acid nanoemulsion

The high incidence of persistent multidrug resistant bacterial infections is a worldwide public health burden. Alternative strategies are required to deal with such issue including the use of drugs with anti-virulence activity. The application of nanotechnology to develop advanced Nano-materials that target quorum sensing regulated virulence factors is an attractive approach. Synthesis of ascorbic acid Nano-emulsion (ASC-NEs) and assessment of its activity in vitro against the virulence factors and its protective ability against pathogenesis as well as the effect against expression of quorum sensing genes of Pseudomonas aeruginosa and Staphylococcus aureus isolates. Ascorbic acid Nano-emulsion was characterized by DLS Zetasizer Technique, Zeta potential; Transmission Electron Microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR). The antibacterial activity of ASC-NEs was tested by the broth microdilution method and the activity of their sub-MIC against the expression of quorum sensing controlled virulence was investigated using phenotypic experiments and RT-PCR. The protective activity of ASC-NEs against P. aeruginosa as well as S. aureus pathogenesis was tested in vivo. Phenotypically, ASC-NEs had strong virulence inhibitory activity against the tested bacteria. The RT-PCR experiment showed that it exhibited significant QS inhibitory activity. The in vivo results showed that ASC-NEs protected against staphylococcal infection, however, it failed to protect mice against Pseudomonal infection. These results suggest the promising use of nanoformulations against virulence factors in multidrug resistant P. aeruginosa and S. aureus. However, further studies are required concerning the potential toxicity, clearance and phamacokinetics of the nanoformulations.

Staphylococcus aureus Carotenoids Modulate the Thermotropic Phase Behavior of Model Systems That Mimic Its Membrane Composition

Staphylococcus aureus (S. aureus) is a pathogenic gram-positive bacterium that normally resides in the skin and nose of the human body. It is subject to fluctuations in environmental conditions that may affect the integrity of the membrane. S. aureus produces carotenoids, which act as antioxidants. However, these carotenoids have also been implicated in modulating the biophysical properties of the membrane. Here, we investigate how carotenoids modulate the thermotropic phase behavior of model systems that mimic the phospholipid composition of S. aureus. We found that carotenoids depress the main phase transition of DMPG and CL, indicating that they strongly affect cooperativity of membrane lipids in their gel phase. In addition, carotenoids modulate the phase behavior of mixtures of DMPG and CL, indicating that they may play a role in modulation of lipid domain formation in S. aureus membranes.

Novel silver metformin nano-structure to impede virulence of Staphylococcus aureus

Staphylococcus aureus is a prevalent etiological agent of health care associated and community acquired infections. Antibiotic abuse resulted in developing multidrug resistance in S. aureus that complicates treatment of infections. Targeting bacterial virulence using FDA approved medication offers an alternative to the antibiotics with no stress on bacterial viability. Using nanomaterials as anti-virulence agent against S. aureus virulence factors is a valuable approach. This study aims to investigate the impact of metformin (MET), metformin nano (MET-Nano), silver metformin nano structure (Ag-MET-Ns) and silver nanoparticles (AgNPs) on S. aureus virulence and pathogenicity. The in vitro results showed a higher inhibitory activity against S. aureus virulence factors with both MET-Nano and Ag-MET-Ns treatment. However, genotypically, it was found that except for agrA and icaR genes that are upregulated, the tested agents significantly downregulated the expression of crtM, sigB, sarA and fnbA genes, with Ag-MET-Ns being the most efficient one. MET-Nano exhibited the highest protection against S. aureus infection in mice. These data indicate the promising anti-virulence activity of nanoformulations especially Ag-MET-Ns against multidrug resistant S. aureus by inhibiting quorum sensing signaling system.

A new formation of silver metformin nanostructure.

The in vitro inhibition of S. aureus virulence factors.

Nano structure form improves the activity of anti-virulence agents.

Antibiofilm and staphyloxanthin inhibitory potential of terbinafine against Staphylococcus aureus: in vitro and in vivo studies

Background

Antimicrobial resistance is growing substantially, which necessitates the search for novel therapeutic options. Terbinafine, an allylamine antifungal agent that exhibits a broad spectrum of activity and is used in the treatment of dermatophytosis, could be a possible option to disarm S. aureus virulence.

Methods

Terbinafine inhibitory effect on staphyloxanthin was characterized by quantitative measurement of staphyloxanthin intermediates and molecular docking. The effect of terbinafine on S. aureus stress survival was characterized by viable counting. The anti-biofilm activity of terbinafine on S. aureus was assessed by the crystal violet assay and microscopy. Changes in S. aureus membrane following treatment with terbinafine were determined using Fourier transform infrared (FTIR) analysis. The synergistic action of terbinafine in combination with conventional antibiotics was characterized using the checkerboard assay. qRT-PCR was used to evaluate the impact of terbinafine on S. aureus gene expression. The influence of terbinafine on S. aureus pathogenesis was investigated in mice infection model.

Results

Terbinafine inhibits staphyloxanthin biosynthesis through targeting dehydrosqualene desaturase (CrtN). Docking analysis of terbinafine against the predicted active site of CrtN reveals a binding energy of − 9.579 kcal/mol exemplified by the formation of H-bonds, H-arene bonds, and hydrophobic/hydrophilic interactions with the conserved amino acids of the receptor pocket. Terbinafine treated S. aureus was more susceptible to both oxidative and acid stress as well as human blood killing as compared to untreated cells. Targeting staphyloxanthin by terbinafine rendered S. aureus more sensitive to membrane acting antibiotics. Terbinafine interfered with S. aureus biofilm formation through targeting cell autoaggregation, hydrophobicity, and exopolysaccharide production. Moreover, terbinafine demonstrated a synergistic interaction against S. aureus when combined with conventional antibiotics. Importantly, terbinafine attenuated S. aureus pathogenesis using mice infection model. qRT-PCR revealed that terbinafine repressed expression of the transcriptional regulators sigB, sarA, and msaB, as well as icaA in S. aureus.

Conclusions

Present findings strongly suggest that terbinafine could be used safely and efficiently as an anti-virulent agent to combat S. aureus infections.

Celastrol mitigates staphyloxanthin biosynthesis and biofilm formation in Staphylococcus aureus via targeting key regulators of virulence; in vitro and in vivo approach

Background

Staphylococcus aureus is a leading cause of human infections. The spread of antibiotic-resistant staphylococci has driven the search for novel strategies to supersede antibiotics use. Thus, targeting bacterial virulence rather than viability could be a possible alternative.

Results

The influence of celastrol on staphyloxanthin (STX) biosynthesis, biofilm formation, antibiotic susceptibility and host pathogenesis in S. aureus has been investigated. Celastrol efficiently reduced STX biosynthesis in S. aureus. Liquid chromatography-mass spectrometry (LC–MS) and molecular docking revealed that celastrol inhibits STX biosynthesis through its effect on CrtM. Quantitative measurement of STX intermediates showed a significant pigment inhibition via interference of celastrol with CrtM and accumulation of its substrate, farnesyl diphosphate. Importantly, celastrol-treated S. aureus was more sensitive to environmental stresses and human blood killing than untreated bacteria. Similarly, inhibition of STX upon celastrol treatment rendered S. aureus more susceptible to membrane targeting antibiotics. In addition to its anti-pigment capability, celastrol exhibits significant anti-biofilm activity against S. aureus as indicated by crystal violet assay and microscopy. Celastrol-treated cells showed deficient exopolysaccharide production and cell hydrophobicity. Moreover, celastrol markedly synergized the action of conventional antibiotics against S. aureus and reduced bacterial pathogenesis in vivo using mice infection model. These findings were further validated using qRT-PCR, demonstrating that celastrol could alter the expression of STX biosynthesis genes as well as biofilm formation related genes and bacterial virulence.

Conclusions

Celastrol is a novel anti-virulent agent against S. aureus suggesting, a prospective therapeutic role for celastrol as a multi-targeted anti-pathogenic agent.

Staphyloxanthin inhibitory potential of thymol impairs antioxidant fitness, enhances neutrophil mediated killing and alters membrane fluidity of methicillin resistant Staphylococcus aureus

Staphylococcus aureus is a leading pathogen responsible for mild to severe invasive infections in humans. Especially, methicillin resistant Staphylococcus aureus (MRSA) is prevalent in hospital and community associated infections. Staphyloxanthin is a golden yellow color eponymous pigment produced by S. aureus and provides resistance to reactive oxygen species (ROS) and host neutrophil-based killing. In addition, this membrane pigment contributes to membrane rigidity and helps MRSA to survive under stress conditions. Targeting virulence of pathogen without exerting selection pressure is the recent approach to fight bacterial infections without developing drug resistance. The present study for the first time evaluated the staphyloxanthin inhibitory potential of thymol against MRSA. Qualitative and quantitative analyses demonstrated 90% of staphyloxanthin inhibition at 100 µg/mL concentration of thymol without alteration in growth. Molecular docking analysis and in vitro measurement of metabolic intermediates of staphyloxanthin revealed that thymol could possibly interact with CrtM to inhibit staphyloxanthin. Absorbance and infra red spectra further validated the inhibition of staphyloxanthin by thymol. In addition, thymol treatment significantly reduced the resistance of MRSA to ROS and neutrophil-based killing as exhibited by oxidant susceptibility assays and ex vivo innate immune clearance assay using human whole blood and neutrophils. Further, reduction in staphyloxanthin by thymol treatment increased the membrane fluidity and made MRSA cells more susceptible to membrane targeting antibiotic polymyxin B. Especially, thymol was found to be non-cytotoxic to human peripheral blood mononuclear cells. Our study validated the antivirulence potential of thymol against MRSA by inhibiting staphyloxanthin and suggests the prospective therapeutic role of thymol to combat MRSA infections.

Factors Determining the Susceptibility of Bacteria to Antibacterial Photodynamic Inactivation

Photodynamic inactivation of microorganisms (aPDI) is an excellent method to destroy antibiotic-resistant microbial isolates. The use of an exogenous photosensitizer or irradiation of microbial cells already equipped with endogenous photosensitizers makes aPDI a convenient tool for treating the infections whenever technical light delivery is possible. Currently, aPDI research carried out on a vast repertoire of depending on the photosensitizer used, the target microorganism, and the light delivery system shows efficacy mostly on in vitro models. The search for mechanisms underlying different responses to photodynamic inactivation of microorganisms is an essential issue in aPDI because one niche (e.g., infection site in a human body) may have bacterial subpopulations that will exhibit different susceptibility. Rapidly growing bacteria are probably more susceptible to aPDI than persister cells. Some subpopulations can produce more antioxidant enzymes or have better performance due to efficient efflux pumps. The ultimate goal was and still is to identify and characterize molecular features that drive the efficacy of antimicrobial photodynamic inactivation. To this end, we examined several genetic and biochemical characteristics, including the presence of individual genetic elements, protein activity, cell membrane content and its physical properties, the localization of the photosensitizer, with the result that some of them are important and others do not appear to play a crucial role in the process of aPDI. In the review, we would like to provide an overview of the factors studied so far in our group and others that contributed to the aPDI process at the cellular level. We want to challenge the question, is there a general pattern of molecular characterization of aPDI effectiveness? Or is it more likely that a photosensitizer-specific pattern of molecular characteristics of aPDI efficacy will occur?

Potentiated antimicrobial blue light killing of methicillin resistant Staphylococcus aureus by pyocyanin

As antimicrobial resistance continues to threaten the efficacy of conventional antibiotic therapy, it is paramount that we investigate innovative approaches to treat infectious diseases. In this study, we investigated the antimicrobial capabilities of the innovative combination of antimicrobial blue light (aBL; 405 nm wavelength) with the Pseudomonas aeruginosa pigment pyocyanin against methicillin resistant Staphylococcus aureus (MSRA. We explored the effects of different radiant exposures of aBL and increasing concentrations of pyocyanin against planktonic cells and those within biofilms. In addition, we investigated the effect of the aBL/pyocyanin on the endogenous staphyloxanthin pigment, as well as the role of hydrogen peroxide and singlet oxygen scavenging in the efficacy of this combination. Lastly, we investigated the potential for the aBL/pyocyanin to reduce the MRSA burden within a proof-of-principle mouse abrasion infection model. We found pyocyanin to be a powerful potentiator of aBL activity under all in vitro conditions tested. In addition, we serendipitously discovered the capability of the aBL/pyocyanin combination to bleach staphyloxanthin within colonies of MRSA. Furthermore, we established that singlet oxygen is an important mediator during combined aBL/pyocyanin exposure. Moreover, we found that the combination of aBL and pyocyanin could significantly reduce the viability of MRSA within a proof-of-principle early onset MRSA skin abrasion infection. Exposure to the treatment did not have deleterious effects on skin tissue. In conclusion, the combination of aBL and pyocyanin represents a potentially powerful therapeutic modality for the treatment of infections caused by MRSA.

Increased photoinactivation stress tolerance of Streptococcus agalactiae upon consecutive sublethal phototreatments

Streptococcus agalactiae (Group B Streptococcus, GBS) is a common commensal bacterium in adults but remains a leading source of invasive infections in newborns, pregnant women, and the elderly, and more recently, causes an increased incidence of invasive disease in nonpregnant adults. Reduced penicillin susceptibility and emerging resistance to non-β-lactams pose challenges for the development and implementation of novel, nonantimicrobial strategies to reduce the burden of GBS infections. Antimicrobial photodynamic inactivation (aPDI) via the production of singlet oxygen or other reactive oxygen species leads to the successful eradication of pathogenic bacteria, affecting numerous cellular targets of microbial pathogens and indicating a low risk of resistance development. Nevertheless, we have previously reported possible aPDI tolerance development upon repeated sublethal aPDI applications; thus, the current work was aimed at investigating whether aPDI tolerance could be observed for GBS and what mechanisms could cause it. To address this problem, 10 cycles of sublethal aPDI treatments employing rose bengal as a photosensitizer, were applied to the S. agalactiae ATCC 27956 reference strain and two clinical isolates (2306/02 and 2974/07, serotypes III and V, respectively). We demonstrated aPDI tolerance development and stability after 5 cycles of subculturing with no aPDI exposure. Though the treatment resulted in a stable phenotype, no increases in mutation rate or accumulated genetic alterations were observed (employing a RIF-, CIP-, STR-resistant mutant selection assay and cyl sequencing, respectively). qRT-PCR analysis demonstrated that 10 sublethal aPDI exposures led to increased expression of all tested major oxidative stress response elements; changes in sodA, ahpC, npx, cylE, tpx and recA expression indicate possible mechanisms of developed tolerance. Increased expression upon sublethal aPDI treatment was reported for all but two genes, namely, ahpC and cylE. aPDI targeting cylE was further supported by colony morphology changes induced with 10 cycles of aPDI (increased SCV population, increased hemolysis, increased numbers of dark- and unpigmented colonies). In oxidant killing assays, aPDI-tolerant strains demonstrated no increased tolerance to hypochlorite, superoxide (paraquat), singlet oxygen (new methylene blue) or oxidative stress induced by aPDI employing a structurally different photosensitizer, i.e., zinc phthalocyanine, indicating a lack of cross resistance. The results indicate that S. agalactiae may develop stable aPDI tolerance but not resistance when subjected to multiple sublethal phototreatments, and this risk should be considered significant when defining efficient anti-S. agalactiae aPDI protocols.

Validation of stable reference genes in Staphylococcus aureus to study gene expression under photodynamic treatment: a case study of SEB virulence factor analysis

Staphylococcal enterotoxin B (SEB), encoded by the seb gene, is a virulence factor produced by Staphylococcus aureus that is involved mainly in food poisoning and is known to act as an aggravating factor in patients with atopic dermatitis. Research results in animal infection models support the concept that superantigens, including SEB contribute to sepsis and skin and soft tissue infections. In contrast to antibiotics, antimicrobial photodynamic inactivation (aPDI) is a promising method to combat both bacterial cells and virulence factors. The main aims of this research were to (1) select the most stable reference genes under sublethal aPDI treatments and (2) evaluate the impact of aPDI on seb. Two aPDI combinations were applied under sublethal conditions: rose bengal (RB) and green light (λ_max = 515 nm) and new methylene blue (NMB) and red light (λ_max = 632 nm). The stability of ten candidate reference genes (16S rRNA, fabD, ftsZ, gmk, gyrB, proC, pyk, rho, rpoB and tpiA) was evaluated upon aPDI using four software packages—BestKeeper, geNorm, NormFinder and RefFinder. Statistical analyses ranked ftsZ and gmk (RB + green light) and ftsZ, proC, and fabD (NMB + red light) as the most stable reference genes upon photodynamic treatment. Our studies showed downregulation of seb under both aPDI conditions, suggesting that aPDI could decrease the level of virulence factors.

Farnesol potentiates photodynamic inactivation of Staphylococcus aureus with the use of red light-activated porphyrin TMPyP

Photodynamic inactivation (PDI) or antibacterial photodynamic therapy (aPDT) is a method based on the use of a photosensitizer, light of a proper wavelength and oxygen, which combined together leads to an oxidative stress and killing of target cells. PDI can be applied towards various pathogenic bacteria independently on their antibiotic resistance profile. Optimization of photodynamic treatment to eradicate the widest range of human pathogens remains challenging despite the availability of numerous photosensitizing compounds. Therefore, a search for molecules that could act as adjuvants potentiating antibacterial photoinactivation is of high scientific and clinical importance. Here we propose farnesol (FRN), a well described sesquiterpene, as a potent adjuvant of PDI, which specifically sensitizes Staphylococcus aureus to 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin tetratosylate (TMPyP) upon red light irradiation. Interestingly, the observed potentiation strongly depends on the presence of light. Analysis of this combined action of FRN and TMPyP, however, showed no influence of farnesol on TMPyP photochemical properties, i.e. the amount of reactive oxygen species that were produced by TMPyP in the presence of FRN. The accumulation rate of TMPyP in Staphylococcus aureus cells did not change, as well as the influence of staphyloxanthin inhibition. The precise mechanism of observed sensitization is unclear and probably involves specific molecular targets.

Carotenoids are used as regulators for membrane fluidity by Staphylococcus xylosus

Carotenoids are associated with several important biological functions as antenna pigments in photosynthesis or protectives against oxidative stress. Occasionally they were also discussed as part of the cold adaptation mechanism of bacteria. For two Staphylococcus xylosus strains we demonstrated an increased content of staphyloxanthin and other carotenoids after growth at 10 °C but no detectable carotenoids after grow at 30 °C. By in vivo measurements of generalized polarization and anisotropy with two different probes Laurdan and TMA-DPH we detected a strong increase in membrane order with a simultaneous increase in membrane fluidity at low temperatures accompanied by a broadening of the phase transition. Increased carotenoid concentration was also correlated with an increased resistance of the cells against freeze-thaw stress. In addition, the fatty acid profile showed a moderate adaptation to low temperature by increasing the portion of anteiso-branched fatty acids. The suppression of carotenoid synthesis abolished the effects observed and thus confirmed the causative function of the carotenoids in the modulation of membrane parameters. A differential transcriptome analysis demonstrated the upregulation of genes involved in carotenoid syntheses under low temperature growth conditions. The presented data suggests that upregulated synthesis of carotenoids is a constitutive component in the cold adaptation strategy of Staphylococcus xylosus and combined with modifications of the fatty acid profile constitute the adaptation to grow under low temperature conditions.

Within-host evolution of bovine Staphylococcus aureus selects for a SigB-deficient pathotype characterized by reduced virulence but enhanced proteolytic activity and biofilm formation

Staphylococcus aureus is a major cause of bovine mastitis, commonly leading to long-lasting, persistent and recurrent infections. Thereby, S. aureus constantly refines and permanently adapts to the bovine udder environment. In this work, we followed S. aureus within-host adaptation over the course of three months in a naturally infected dairy cattle with chronic, subclinical mastitis. Whole genome sequence analysis revealed a complete replacement of the initial predominant variant by another isogenic variant. We report for the first time within-host evolution towards a sigma factor SigB-deficient pathotype in S. aureus bovine mastitis, associated with a single nucleotide polymorphism in rsbU (G368A → G122D), a contributor to SigB-functionality. The emerged SigB-deficient pathotype exhibits a substantial shift to new phenotypic traits comprising strong proteolytic activity and poly-N-acetylglucosamine (PNAG)-based biofilm production. This possibly unlocks new nutritional resources and promotes immune evasion, presumably facilitating extracellular persistence within the host. Moreover, we observed an adaptation towards attenuated virulence using a mouse infection model. This study extends the role of sigma factor SigB in S. aureus pathogenesis, so far described to be required for intracellular persistence during chronic infections. Our findings suggest that S. aureus SigB-deficiency is an alternative mechanism for persistence and underpin the clinical relevance of staphylococcal SigB-deficient variants which are consistently isolated during human chronic infections.

Development of Staphylococcus aureus tolerance to antimicrobial photodynamic inactivation and antimicrobial blue light upon sub-lethal treatment

Antimicrobial photodynamic inactivation (aPDI) and antimicrobial blue light (aBL) are considered low-risk treatments for the development of bacterial resistance and/or tolerance due to their multitargeted modes of action. In this study, we assessed the development of Staphylococcus aureus tolerance to these phototreatments. Reference S. aureus USA300 JE2 was subjected to 15 cycles of both sub-lethal aPDI (employing an exogenously administered photosensitizer (PS), i.e., rose Bengal (RB)) and sub-lethal aBL (employing endogenously produced photosensitizing compounds, i.e., porphyrins). We demonstrate substantial aPDI/aBL tolerance development and tolerance stability after 5 cycles of subculturing without aPDI/aBL exposure (the development of aPDI/aBL tolerance was also confirmed with the employment of clinical MRSA and MSSA strain as well as other representatives of Gram-positive microbes, i.e. Enterococcus faecium and Streptococcus agalactiae). In addition, a rifampicin-resistant (RIFR) mutant selection assay showed an increased mutation rate in S. aureus upon sub-lethal phototreatments, indicating that the increased aPDI/aBL tolerance may result from accumulated mutations. Moreover, qRT-PCR analysis following sub-lethal phototreatments demonstrated increased expression of umuC, which encodes stress-responsive error-prone DNA polymerase V, an enzyme that increases the rate of mutation. Employment of recA and umuC transposon S. aureus mutants confirmed SOS-induction dependence of the tolerance development. Interestingly, aPDI/aBL-tolerant S. aureus exhibited increased susceptibility to gentamicin (GEN) and doxycycline (DOX), supporting the hypothesis of genetic alterations induced by sub-lethal phototreatments. The obtained results indicate that S. aureus may develop stable tolerance to studied phototreatments upon sub-lethal aPDI/aBL exposure; thus, the risk of tolerance development should be considered significant when designing aPDI/aBL protocols for infection treatments in vitro and in clinical settings.

Photoinactivation of ESKAPE pathogens: overview of novel therapeutic strategy

The emergence of antimicrobial drug resistance requires development of alternative therapeutic options. Multidrug-resistant strains of Enterococcus spp., Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumanii, Pseudomonas aeruginosa and Enterobacter spp. are still the most commonly identified antimicrobial-resistant pathogens. These microorganisms are part of the so-called 'ESKAPE' pathogens to emphasize that they currently cause the majority of hospital acquired infections and effectively 'escape' the effects of antibacterial drugs. Thus, alternative, safer and more efficient antimicrobial strategies are urgently needed, especially against 'ESKAPE' superbugs. Antimicrobial photodynamic inactivation is a therapeutic option used in the treatment of infectious diseases. It is based on a combination of a photosensitizer, light and oxygen to remove highly metabolically active cells.

Antimicrobials Are a Photodynamic Inactivation Adjuvant for the Eradication of Extensively Drug-Resistant Acinetobacter baumannii

The worldwide emergence of extensively drug resistant (XDR) Acinetobacter baumannii has reduced the number of antimicrobials that exert high bactericidal activity against this pathogen. This is the reason why many scientists are focusing on investigations concerning novel non-antibiotic strategies such as antimicrobial photodynamic inactivation (aPDI) or the use of antimicrobial blue light (aBL). Therefore, the aim of the current study was to screen for antimicrobial synergies of routinely used antibiotics and phototherapies, including both aPDI involving exogenously administered photosensitizing molecules, namely, rose bengal, and aBL, involving excitation of endogenously produced photoactive compounds. The synergy testing was performed in accordance with antimicrobial susceptibility testing (AST) standards, including various methodological approaches, i.e., antibiotic diffusion tests, checkerboard assays, CFU counting and the evaluation of postantibiotic effects (PAEs). We report that combining antimicrobials and aPDI/aBL treatment led to a new strategy that overcomes drug resistance in XDR A. baumannii, rendering this pathogen susceptible to various categories of antibiotics. Sublethal aPDI/aBL treatment in the presence of sub-MIC levels of antimicrobials effectively killed A. baumannii expressing drug resistance to studied antibiotics when treated with only antibiotic therapy. The susceptibility of XDR A. baumannii to a range of antibiotics was enhanced following sublethal aPDI/aBL. Furthermore, 3′-(p-aminophenyl) fluorescein (APF) testing indicated that significantly increased reactive oxygen species production upon combined treatment could explain the observed synergistic activity. This result represents a conclusive example of the synergistic activity between photodynamic inactivation and clinically used antimicrobials leading to effective eradication of XDR A. baumannii isolates and indicates a potent novel therapeutic approach.

Impact of Deficiencies in Branched-Chain Fatty Acids and Staphyloxanthin in Staphylococcus aureus

Staphylococcus aureus is a well-known human pathogen with the ability to cause mild superficial skin infections to serious deep-tissue infections, such as osteomyelitis, pneumonia, and infective endocarditis. A key to S. aureus infections and its pathogenicity is its ability to survive in adverse environments, especially at lower temperatures, by regulation of its cell membrane. Branched-chain fatty acids (BCFAs) and staphyloxanthin have been shown to regulate membrane fluidity and staphylococcal virulence. This study was conducted with the hypothesis that the simultaneous lack of BCFAs and staphyloxanthin will have a far greater implication on environmental survival and virulence of S. aureus. Lack of a functional branched-chain α-keto acid dehydrogenase (BKD) enzyme because of a mutation in the lpdA gene led to a decrease in the production of BCFAs, membrane fluidity, slower growth, and poor in vivo survival of S. aureus. A mutation in the crtM gene eliminated the production of staphyloxanthin but it did not affect membrane BCFA levels, fluidity, growth, or in vivo survival. A crtM:lpdA double mutant showed much slower growth and attenuation compared to individual mutants. The results of this study suggest that simultaneous targeting of the BCFA and staphyloxanthin biosynthetic pathways can be a strategy to control S. aureus infections.

Antimicrobial blue light photoinactivation of Pseudomonas aeruginosa: Quorum sensing signaling molecules, biofilm formation and pathogenicity

Pseudomonas aeruginosa is a common causative bacterium of acute and chronic infections that have been responsible for high mortality over the past decade. P. aeruginosa produces many virulence factors such as toxins, enzymes and dyes that are strongly dependent on quorum sensing (QS) signaling systems. P. aeruginosa has three major QS systems (las, rhl and Pseudomonas quinolone signal) that regulate the expression of genes encoding virulence factors as well as biofilm production and maturation. Antimicrobial blue light (aBL) is considered a therapeutic option for bacterial infections and has other benefits, such as reducing bacterial virulence. Therefore, this study investigated the efficacy of aBL to reduce P. aeruginosa pathogenicity. aBL treatment resulted in the reduced activity of certain QS signaling molecules in P. aeruginosa and inhibited biofilm formation. in vivo tests using a Caenorhabditis elegans infection model indicated that sublethal aBL decreased the pathogenicity of P. aeruginosa. aBL may be a new virulence-targeting therapeutic approach.

Role of SigB and Staphyloxanthin in Radiation Survival of Staphylococcus aureus

Staphylococcus aureus is a potent human pathogen. The virulence of this bacterium depends on a multitude of factors that it produces. One such virulence factor is the golden pigment, staphyloxanthin, which has been shown to protect the bacterium from oxidative stress. Expression of the staphyloxanthin biosynthetic pathway is dependent on SigB, a global stress response regulator in S. aureus. This study investigated the role of staphyloxanthin and SigB in protection of S. aureus from radiation damage. Using stationary-phase bacterial cells, it was determined that the staphyloxanthin-deficient (crt mutant) strain was significantly sensitive to UV radiation (~ threefold), but not sensitive to X-radiation. However, a SigB-deficient S. aureus that also lacks staphyloxanthin, was significantly sensitive to both UV- and X-radiation. To confirm that protection from X-radiation was due to hydroxyl radicals, effect of 3 M glycerol, a known hydroxyl scavenger, was also investigated. Glycerol increased the survival of the S. aureus sigB mutant to the wild-type level suggesting that the X-radiation sensitivity of these mutants was due to deficiency in scavenging hydroxyl radicals. In summary, SigB is critical for protection of S. aureus cells from radiation damage.

Interrelationships between Fatty Acid Composition, Staphyloxanthin Content, Fluidity, and Carbon Flow in the Staphylococcus aureus Membrane

Fatty acids play a major role in determining membrane biophysical properties. Staphylococcus aureus produces branched-chain fatty acids (BCFAs) and straight-chain saturated fatty acids (SCSFAs), and can directly incorporate exogenous SCSFAs and straight-chain unsaturated fatty acids (SCUFAs). Many S. aureus strains produce the triterpenoid pigment staphyloxanthin, and the balance of BCFAs, SCSFAs and staphyloxanthin determines membrane fluidity. Here, we investigated the relationship of fatty acid and carotenoid production in S. aureus using a pigmented strain (Pig1), its carotenoid-deficient mutant (Pig1ΔcrtM) and the naturally non-pigmented Staphylococcus argenteus that lacks carotenoid biosynthesis genes and is closely related to S. aureus. Fatty acid compositions in all strains were similar under a given culture condition indicating that staphyloxanthin does not influence fatty acid composition. Strain Pig1 had decreased membrane fluidity as measured by fluorescence anisotropy compared to the other strains under all conditions indicating that staphyloxanthin helps maintain membrane rigidity. We could find no evidence for correlation of expression of crtM and fatty acid biosynthesis genes. Supplementation of medium with glucose increased SCSFA production and decreased BCFA and staphyloxanthin production, whereas acetate-supplementation also decreased BCFAs but increased staphyloxanthin production. We believe that staphyloxanthin levels are influenced more through metabolic regulation than responding to fatty acids incorporated into the membrane.