Cold atmospheric pressure plasma-antibiotic synergy in Pseudomonas aeruginosa biofilms is mediated via oxidative stress response

Cold atmospheric-pressure plasma (CAP) has emerged as a potential alternative or adjuvant to conventional antibiotics for the treatment of bacterial infections, including those caused by antibiotic-resistant pathogens. The potential of sub-lethal CAP exposures to synergise conventional antimicrobials for the eradication of Pseudomonas aeruginosa biofilms is investigated in this study. The efficacy of antimicrobials following or in the absence of sub-lethal CAP pre-treatment in P. aeruginosa biofilms was assessed. CAP pre-treatment resulted in an increase in both planktonic and biofilm antimicrobial sensitivity for all three strains tested (PAO1, PA14, and PA10548), with both minimum inhibitory concentrations (MICs) and minimum biofilm eradication concentrations (MBECs) of individual antimicrobials, being significantly reduced following CAP pre-treatment of the biofilm (512-fold reduction with ciprofloxacin/gentamicin; and a 256-fold reduction with tobramycin). At all concentrations of antimicrobial used, the combination of sub-lethal CAP exposure and antimicrobials was effective at increasing time-to-peak metabolism, as measured by isothermal microcalorimetry, again indicating enhanced susceptibility. CAP is known to damage bacterial cell membranes and DNA by causing oxidative stress through the in situ generation of reactive oxygen and nitrogen species (RONS). While the exact mechanism is not clear, oxidative stress on outer membrane proteins is thought to damage/perturb cell membranes, confirmed by ATP and LDH leakage, allowing antimicrobials to penetrate the bacterial cell more effectively, thus increasing bacterial susceptibility. Transcriptomic analysis, reveals that cold-plasma mediated oxidative stress caused upregulation of P. aeruginosa superoxide dismutase, cbb3 oxidases, catalases, and peroxidases, and upregulation in denitrification genes, suggesting that P. aeruginosa uses these enzymes to degrade RONS and mitigate the effects of cold plasma mediated oxidative stress. CAP treatment also led to an increased production of the signalling molecule ppGpp in P. aeruginosa, indicative of a stringent response being established. Although we did not directly measure persister cell formation, this stringent response may potentially be associated with the formation of persister cells in biofilm cultures. The production of ppGpp and polyphosphate may be associated with protein synthesis inhibition and increase efflux pump activity, factors which can result in antimicrobial tolerance. The transcriptomic analysis also showed that by 6 h post-treatment, there was downregulation in ribosome modulation factor, which is involved in the formation of persister cells, suggesting that the cells had begun to resuscitate/recover. In addition, CAP treatment at 4 h post-exposure caused downregulation of the virulence factors pyoverdine and pyocyanin; by 6 h post-exposure, virulence factor production was increasing. Transcriptomic analysis provides valuable insights into the mechanisms by which P. aeruginosa biofilms exhibits enhanced susceptibility to antimicrobials. Overall, these findings suggest, for the first time, that short CAP sub-lethal pre-treatment can be an effective strategy for enhancing the susceptibility of P. aeruginosa biofilms to antimicrobials and provides important mechanistic insights into cold plasma-antimicrobial synergy. Transcriptomic analysis of the response to, and recovery from, sub-lethal cold plasma exposures in P. aeruginosa biofilms improves our current understanding of cold plasma biofilm interactions.

Synergistic inactivation of Escherichia coli O157:H7 and Staphylococcus aureus by gallic acid and thymol and its potential application on fresh-cut tomatoes

Antibacterial activity against Escherichia coli O157:H7 and Staphylococcus aureus of five typical plant-derived compounds [gallic acid (G.A), citral (Cit), thymol (Thy), salicylic acid (S.A), lauric acid (L.A)] were investigated by determining the minimum inhibitory concentration (MIC) and the fractional inhibitory concentration index (FICI). The results showed that only a combination of Thy and G.A (TGA), with a concentration of 0.1 and 1.25 mg/mL, respectively, had a synergistic effect (FICI = 0.5) on both E. coli O157:H7 and S. aureus. The amount of Thy and G.A in mixture were four-fold lower than the MICs of the individuals shown to cause the equivalent antimicrobial activity in trypticase soy broth (TSB). The microbial reduction obtained in TSB with addition of TGA were significantly higher (P < 0.05) than the reduction shown for the broth supplemented with the separated phenolics. TGA caused the changes of morphology and membrane integrity of bacteria. Additionally, the application of TGA on fresh-cut tomatoes are investigated. Fresh-cut tomatoes inoculated with E. coli O157:H7and S. aureus were washed for 2min, 5min, 10min at 4 °C, 25 °C, 40 °C in 0.3% NaOCl, or water containing TGA at various concentrations. Overall, the reduction of TGA achieved against S. aureus is higher than E. coli O157:H7. Same concentrations of combined antimicrobials at a temperature of 40 °C further increased the degree of microbial inactivation, with an additional 0.89-1.51 log CFU/g reduction compared to that at 25 °C. Moreover, 1/2MIC_Thy+1/2MIC_G.A at 25 °C for 10min or 40 °C for 5min were generally acceptable with sensorial scores higher than 7. Our results showed that TGA could work synergistically on the inactivation of the tested bacteria and may be used as an alternative disinfectant of fresh produce.

Physical Mechanisms of Bacterial Killing by Histones

Antibiotic resistance is a global epidemic, becoming increasingly pressing due to its rapid spread. There is thus a critical need to develop new therapeutic approaches. In addition to searching for new antibiotics, looking into existing mechanisms of natural host defense may enable researchers to improve existing defense mechanisms, and to develop effective, synthetic drugs guided by natural principles. Histones, primarily known for their role in condensing mammalian DNA, are antimicrobial and share biochemical similarities with antimicrobial peptides (AMPs); however, the mechanism by which histones kill bacteria is largely unknown. Both AMPs and histones are similar in size, cationic, contain a high proportion of hydrophobic amino acids, and possess the ability to form alpha helices. AMPs, which mostly kill bacteria through permeabilization or disruption of the biological membrane, have recently garnered significant attention for playing a key role in host defenses. This chapter outlines the structure and function of histone proteins as they compare to AMPs and provides an overview of their role in innate immune responses, especially regarding the action of specific histones against microorganisms and their potential mechanism of action against microbial pathogens.

What Is the Future of Clinical Microbiology?

Clinical microbiology has advanced tremendously in the past 10 years. In this comic, the role of technology, the need for skilled microbiologists, and the meaning of progress in clinical microbiology are considered.

When One Drug Is Not Enough: Context, Methodology, and Future Prospects in Antibacterial Synergy Testing

Antibacterial combinations have long been used to accomplish a variety of therapeutic goals, including prevention of resistance and enhanced antimicrobial activity. In vitro synergy testing methods, including the checkerboard array, the time-kill study, diffusion assays, and pharmacokinetic/pharmacodynamic models, are used commonly in the research setting, but are not routinely performed in the clinical microbiology laboratory because of test complexity and uncertainty about their predictive value for patient outcomes. Optimized synergy testing techniques and better data on the relationship between in vitro results and clinical outcomes are needed to guide the rational use of antimicrobial combinations in the multidrug resistance era.

Composite particle formulations of colistin and meropenem with improved in-vitro bacterial killing and aerosolization for inhalation

Antibiotic combination therapy is promising for the treatment of lower respiratory tract infections caused by multi-drug resistant Gram-negative pathogens. Inhaled antibiotic therapy offers the advantage of direct delivery of the drugs to the site of infection, as compared to the parenteral administrations. In this study, we developed composite particle formulations of colistin and meropenem. The formulations were characterized for particle size, morphology, specific surface area, surface chemical composition, in-vitro aerosolization performance and in-vitro antibacterial activity. The combinations demonstrated enhanced antibacterial activity against clinical isolates of Acinetobacter baumannii N16870 and Pseudomonas aeruginosa 19147, when compared with antibiotic monotherapy. Spray-dried meropenem alone showed a poor aerosolization performance as indicated by a low fine particle fraction (FPF) of 32.5 ± 3.3%. Co-spraying with colistin improved the aerosolization of meropenem with up to a two-fold increase in the FPF. Such improvements in aerosolization can be attributed to the enrichment of colistin on the surface of composite particles as indicated by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), and the increases in particle porosity. Intermolecular interactions between colistin and meropenem were observed for the combination formulations as measured by FT-IR. In conclusion, our results show that co-spray drying with colistin improves the antibacterial activity and aerosol performance of meropenem and produces a formulation with synergistic bacterial killing.

Rifaximin combined with polymyxins: A potential regimen for selective decontamination of multidrug-resistant bacteria in the digestive tract?

Selective decontamination of the digestive tract (SDD) using combinations of oral non-absorbable antibiotics has been proposed as a means of preventing multidrug-resistant (MDR) infections. The minimum inhibitory concentrations (MICs) of rifaximin (RIFAX) were determined against 262 Gram-negative and Gram-positive bacterial isolates by broth microtitre assay. Rifampicin (RIF) was used as a comparator in the analysis. Synergistic interactions between RIFAX and polymyxin B (PMB) were assessed by using the chequerboard method and calculating the fractional inhibitory concentration index (FICI). The antimicrobial activities of both RIFAX and RIF were similar with little variation in the overall MIC distributions for Gram-negative non-fermenters and Gram-positive bacteria. However, against Enterobacteriaceae higher MICs (>16mg/L) were observed for RIFAX than for RIF (50% vs 27%). Amongst the 262 isolates tested, 100 could be considered resistant to RIFAX. Overall, the combination of RIFAX and PMB was more active against all of the isolates tested compared with either drug alone, with reductions of 2-11 doubling dilutions in individual MICs. Potent synergy was observed with the RIFAX+PMB combination using FICI criteria (FICI range 0.02-0.5). The data presented here suggest that combination therapy may be significantly more effective against isolates with RIFAX and/or PMB resistance and could be considered as part of a SDD regimen aimed at reducing enteric carriage of MDR pathogens in colonised and infected patients.

In vivo activity of daptomycin/colistin combination therapy in a Galleria mellonella model of Acinetobacter baumannii infection

Antimicrobial treatment of multidrug-resistant Acinetobacter baumannii (MDR-AB) infections continues to pose significant challenges. With limited options, clinicians have been pushed towards using unorthodox combinations of licensed antibiotics. Although daptomycin/colistin combination appears to be a promising treatment option based on in vitro data, further preclinical work is needed. In this study, the A. baumannii-Galleria mellonella system was employed to study the in vivo efficacy of this combination in order to determine whether it should be explored further for the treatment of MDR-AB infections. The antimicrobial activity of colistin alone and in combination with daptomycin was assessed versus an A. baumannii type strain (ATCC 19606) and a MDR-AB clinical strain (GN2231) isolated in Anhui, China. Synergy studies were performed using the microtitre plate chequerboard assay and time-kill methodology. The in vivo activity of daptomycin/colistin combination was assessed using a G. mellonella larvae model. The combination of daptomycin and colistin was bactericidal against both strains tested. In chequerboard assays, daptomycin was highly active against A. baumannii when combined with colistin [fractional inhibitory concentration index (FICI) of <0.5]. Treatment of G. mellonella larvae infected with lethal doses of A. baumannii resulted in significantly enhanced survival rates when daptomycin was given with colistin compared with colistin treatment alone (P<0.05). This work suggests that daptomycin/colistin combination is highly active against A. baumannii both in vitro and in a simple invertebrate model of infection.

New non-alcoholic formulation for hand disinfection

Hand washing is considered as the single most important strategy to prevent infections. World health organization (WHO) defines hand hygiene as a primary issue of personal care with particular reference to hospital personnel and health facility workers. In this work, we investigated a new combination for hand disinfection as an alternative to alcohol-based and chlorhexidine products. The new combination of 5-pyrrolidone-2-carboxylic acid (PCA) and copper sulphate pentahydrate (CS) was tested upon different bacterial species that normally colonize hands, including Staphylococcus aureus, methicillin resistant S. aureus (MR S. aureus), Staphylococcus epidermidis, multidrug resistant S. epidermidis (MDR S. epidermidis), Streptococcus pyogenes, Streptococcus agalactiae, Escherichia coli, Candida albicans and three clinical isolates: MR S. aureus, MDR S. epidermidis, and an E. coli strain. Minimal inhibitory concentrations (MICs), Minimal bactericidal concentrations (MBCs), fractional inhibitory concentration (FIC) indices, and fractional bactericidal concentration (FBC) indices were evaluated. Ethanol 70% V/V, isopropanol 60% V/V, and 4% w/V chlorhexidine solution were used as reference hand disinfectants. Copper sulphate pentahydrate was very effective against all tested microorganisms: The MIC and MBC for CS ranged from 781 mg/l against S. pyogenes to 12500 mg/l against E. coli strains and C. albicans. In addition, PCA exhibited a good antimicrobial activity, in particular, against S. pyogenes and S. agalactiae. The combination of CS and PCA showed a strong synergistic effect and all FIC indices were ≤0·500. The combination of CS and PCA were more effective than ethanol 70% V/V and isopropanol 60% V/V. In addition to antimicrobial activity, the new formulation possesses peculiar features such as residual activity and moisturizing effect. This work identifies a new strategy for hand disinfection.