Activity of meropenem with and without ciprofloxacin and colistin against Pseudomonas aeruginosa and Acinetobacter baumannii

Increased tolerance to commonly used antibiotics in a Pseudomonas aeruginosa ex vivo porcine keratitis model

Introduction. Bacterial keratitis, particularly caused by Pseudomonas aeruginosa, is challenging to treat because of multi-drug tolerance, often associated with the formation of biofilms. Antibiotics in development are typically evaluated against planktonic bacteria in a culture medium, which may not accurately represent the complexity of infections in vivo.Hypothesis/Gap Statement. Developing a reliable, economic ex vivo keratitis model that replicates some complexity of tissue infections could facilitate a deeper understanding of antibiotic efficacy, thus aiding in the optimization of treatment strategies for bacterial keratitis.Methodology. Here we investigated the efficacy of three commonly used antibiotics (gentamicin, ciprofloxacin and meropenem) against Pseudomonas aeruginosa cytotoxic strain PA14 and invasive strain PA01 using an ex vivo porcine keratitis model.Results. Both strains of P. aeruginosa were susceptible to the MIC of the three tested antibiotics. However, significantly higher concentrations were necessary to inhibit bacterial growth in the minimum biofilm eradication concentration (MBEC) assay, with both strains tolerating concentrations greater than 512 mg l-1 of meropenem. When MIC and higher concentrations than MBEC (1024 mg l-1) of antibiotics were applied, ciprofloxacin exhibited the highest potency against both P. aeruginosa strains, followed by meropenem, while gentamicin showed the least potency. Despite this, none of the antibiotic concentrations used effectively cleared the infection, even after 18 h of continuous exposure.Conclusions. Further exploration of antibiotic concentrations and aligning dosing with clinical studies to validate the model is needed. Nonetheless, our ex vivo porcine keratitis model could be a valuable tool for assessing antibiotic efficacy.

Clinical Efficacy and Safety of Colistin Sulfate in the Treatment of Carbapenem-Resistant Organism Infections in Patients with Hematological Diseases

INTRODUCTION

Carbapenem-resistant organisms (CRO) have emerged as a significant worldwide issue. However, the availability of efficacious antibiotics for treating CRO infections remains limited. Polymyxins, including colistin sulfate, represent the last-line therapeutic option against CRO infections. This study aims to retrospectively evaluate the clinical effectiveness and safety of colistin sulfate in managing CRO infections among patients with hematological diseases.

METHODS

Between April 2022 and January 2023, a total of 118 hematological patients diagnosed with CRO infection were treated with colistin sulfate at Suzhou Hongci Hospital of Hematology. The assessment encompassed the clinical efficacy, bacterial clearance rate, adverse reactions, and 30-day all-cause mortality.

RESULTS

The study found that the total effective rate of colistin sulfate in the treatment of CRO infection was 74.6%, with a bacterial clearance rate of 72.6%. Throughout the treatment, nephrotoxicity occurred in 7.6% of cases, neurotoxicity in 2.5% of cases, and the 30-day all-cause mortality rate was 22.9%. Multivariate logistic analysis revealed that the treatment course and combination medication with other antimicrobials were independent factors affecting the clinical efficacy of colistin sulfate.

CONCLUSION

Our study demonstrates that the treatment of colistin sulfate can achieve high clinical efficacy and microbial responses, with a low risk of nephrotoxicity. This study provides evidence of the positive clinical efficacy and safety of colistin sulfate treatment in these patients. High-quality randomized controlled trials are still needed to further confirm the beneficial role of colistin sulfate.

Antibacterial and Anti-Efflux Activities of Cinnamon Essential Oil against Pan and Extensive Drug-Resistant Pseudomonas aeruginosa Isolated from Human and Animal Sources

Pseudomonas aeruginosa is notorious for its ability to develop a high level of resistance to antimicrobial agents. Resistance-nodulation-division (RND) efflux pumps could mediate drug resistance in P. aeruginosa. The present study aimed to evaluate the antibacterial and anti-efflux activities of cinnamon essential oil either alone or combined with ciprofloxacin against drug resistant P. aeruginosa originated from human and animal sources. The results revealed that 73.91% of the examined samples were positive for P. aeruginosa; among them, 77.78% were of human source and 72.73% were recovered from animal samples. According to the antimicrobial resistance profile, 48.73% of the isolates were multidrug-resistant (MDR), 9.2% were extensive drug-resistant (XDR), and 0.84% were pan drug-resistant (PDR). The antimicrobial potential of cinnamon oil against eleven XDR and one PDR P. aeruginosa isolates was assessed by the agar well diffusion assay and broth microdilution technique. The results showed strong antibacterial activity of cinnamon oil against all tested P. aeruginosa isolates with inhibition zones' diameters ranging from 34 to 50 mm. Moreover, the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values of cinnamon oil against P. aeruginosa isolates ranged from 0.0562-0.225 µg/mL and 0.1125-0.225 µg/mL, respectively. The cinnamon oil was further used to evaluate its anti-efflux activity against drug-resistant P. aeruginosa by phenotypic and genotypic assays. The cartwheel test revealed diminished efflux pump activity post cinnamon oil exposure by two-fold indicating its reasonable impact. Moreover, the real-time quantitative polymerase chain reaction (RT-qPCR) results demonstrated a significant (p < 0.05) decrease in the expression levels of MexA and MexB genes of P. aeruginosa isolates treated with cinnamon oil when compared to the non-treated ones (fold changes values ranged from 0.4204-0.7474 for MexA and 0.2793-0.4118 for MexB). In conclusion, we suggested the therapeutic use of cinnamon oil as a promising antibacterial and anti-efflux agent against drug-resistant P. aeruginosa.

The resistance mechanisms of bacteria against ciprofloxacin and new approaches for enhancing the efficacy of this antibiotic

For around three decades, the fluoroquinolone (FQ) antibiotic ciprofloxacin has been used to treat a range of diseases, including chronic otorrhea, endocarditis, lower respiratory tract, gastrointestinal, skin and soft tissue, and urinary tract infections. Ciprofloxacin's main mode of action is to stop DNA replication by blocking the A subunit of DNA gyrase and having an extra impact on the substances in cell walls. Available in intravenous and oral formulations, ciprofloxacin reaches therapeutic concentrations in the majority of tissues and bodily fluids with a low possibility for side effects. Despite the outstanding qualities of this antibiotic, Salmonella typhi, Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa have all shown an increase in ciprofloxacin resistance over time. The rise of infections that are resistant to ciprofloxacin shows that new pharmacological synergisms and derivatives are required. To this end, ciprofloxacin may be more effective against the biofilm community of microorganisms and multi-drug resistant isolates when combined with a variety of antibacterial agents, such as antibiotics from various classes, nanoparticles, natural products, bacteriophages, and photodynamic therapy. This review focuses on the resistance mechanisms of bacteria against ciprofloxacin and new approaches for enhancing its efficacy.

The Potential of Antibiotics and Nanomaterial Combinations as Therapeutic Strategies in the Management of Multidrug-Resistant Infections: A Review

Antibiotic resistance has become a major public health concern around the world. This is exacerbated by the non-discovery of novel drugs, the development of resistance mechanisms in most of the clinical isolates of bacteria, as well as recurring infections, hindering disease treatment efficacy. In vitro data has shown that antibiotic combinations can be effective when microorganisms are resistant to individual drugs. Recently, advances in the direction of combination therapy for the treatment of multidrug-resistant (MDR) bacterial infections have embraced antibiotic combinations and the use of nanoparticles conjugated with antibiotics. Nanoparticles (NPs) can penetrate the cellular membrane of disease-causing organisms and obstruct essential molecular pathways, showing unique antibacterial mechanisms. Combined with the optimal drugs, NPs have established synergy and may assist in regulating the general threat of emergent bacterial resistance. This review comprises a general overview of antibiotic combinations strategies for the treatment of microbial infections. The potential of antibiotic combinations with NPs as new entrants in the antimicrobial therapy domain is discussed.

Preferential Selection of Low-Frequency, Lipopolysaccharide-Modified, Colistin-Resistant Mutants with a Combination of Antimicrobials in Acinetobacter baumannii

Colistin, which targets lipopolysaccharide (LPS), is used as a last-resort drug against severe infections caused by drug-resistant Acinetobacter baumannii. However, A. baumannii possesses two colistin-resistance mechanisms. LPS modification caused by mutations in pmrAB genes is often observed in clinical isolates of multidrug-resistant Gram-negative pathogens. In addition to LPS modification, A. baumannii has a unique colistin resistance mechanism, a complete loss of LPS due to mutations in the lpxACD genes, which are involved in LPS biosynthesis. This study aimed to elucidate the detailed mechanism of the emergence of colistin-resistant A. baumannii using strains with the same genetic background. Various colistin-resistant strains were generated experimentally using colistin alone and in combination with other antimicrobials, such as meropenem and ciprofloxacin, and the mutation spectrum was analyzed. In vitro selection of A. baumannii in the presence of colistin led to the emergence of strains harboring mutations in lpxACD genes, resulting in LPS-deficient colistin-resistant strains. However, combination of colistin with other antimicrobials led to the selection of pmrAB mutant strains, resulting in strains with modified LPS (LPS-modified strains). Further, the LPS-deficient strains showed decreased fitness and increased susceptibility to many antibiotics and disinfectants. As LPS-deficient strains have a higher biological cost than LPS-modified strains, our findings suggested that pmrAB mutants are more likely to be isolated in clinical settings. We provide novel insights into the mechanisms of resistance to colistin and provide substantial solutions along with precautions for facilitating current research and treatment of colistin-resistant A. baumannii infections. IMPORTANCE Acinetobacter baumannii has developed resistance to various antimicrobial drugs, and its drug-resistant strains cause nosocomial infections. Controlling these infections has become a global clinical challenge. Carbapenem antibiotics are the frontline treatment drugs for infectious diseases caused by A. baumannii. For patients with infections caused by carbapenem-resistant A. baumannii, colistin-based therapy is often the only treatment option. However, A. baumannii readily acquires resistance to colistin. Many patients infected with colistin-resistant A. baumannii undergo colistin treatment before isolation of the colistin-resistant strain, and it is hypothesized that colistin resistance predominantly emerges under selective pressure during colistin therapy. Although the concomitant use of colistin and carbapenems has been reported to have a synergistic effect in vitro against carbapenem-resistant A. baumannii strains, our observations strongly suggest the need for attention to the emergence of strains with a modified lipopolysaccharide during treatment.

New Variants of Pseudomonas aeruginosa High-Risk Clone ST233 Associated with an Outbreak in a Mexican Paediatric Hospital

Recent multidrug resistance in Pseudomonas aeruginosa has favoured the adaptation and dissemination of worldwide high-risk strains. In June 2018, 15 P. aeruginosa strains isolated from patients and a contaminated multi-dose meropenem vial were characterized to assess their association to an outbreak in a Mexican paediatric hospital. The strains were characterized by antibiotic susceptibility profiling, virulence factors’ production, and biofilm formation. The clonal relationship among isolates was determined with pulse-field gel electrophoresis (PFGE) and multi-locus sequence typing (MLST) sequencing. Repressor genes for the MexAB-OprM efflux pump were sequenced for haplotype identification. Of the strains, 60% were profiled as extensively drug-resistant (XDR), 33% as multidrug-resistant (MDR), and 6.6% were classified as sensitive (S). All strains presented intermediate resistance to colistin, and 80% were sensitive to aztreonam. Pyoverdine was the most produced virulence factor. The PFGE technique was performed for the identification of the outbreak, revealing eight strains with the same electrophoretic pattern. ST235 and ten new sequence types (STs) were identified, all closely related to ST233. ST3241 predominated in 26.66% of the strains. Twenty-five synonymous and seventeen nonsynonymous substitutions were identified in the regulatory genes of the MexAB-OprM efflux pump, and nalC was the most variable gene. Six different haplotypes were identified. Strains from the outbreak were metallo-β-lactamases and phylogenetically related to the high-risk clone ST233.

In vitro activities of antibiotic combinations against mature biofilms of ventilator-associated pneumonia isolates

Background: The authors aimed to determine the efficacy of frequently used antibiotics, alone or in combination, against biofilms of ventilator-associated pneumonia isolates. Materials & methods: The authors determined the MICs, minimum biofilm inhibitory concentrations and minimum biofilm eradication concentrations of meropenem, ciprofloxacin and colistin as well as their combinations against planktonic forms and biofilms of Pseudomonas aeruginosa, Klebsiella pneumoniae and Acinetobacter baumannii clinical isolates. Results: Generally, the minimum biofilm inhibitory concentrations and minimum biofilm eradication concentrations of the antibiotics were 1000-fold higher than their MICs, and synergy was provided by different concentrations of meropenem-colistin and meropenem-ciprofloxacin combinations with checkerboard and time-kill curve methods. Conclusion: The combination of meropenem and ciprofloxacin seems to be a good candidate for the treatment of biofilm-associated infections; none of the concentrations obtained as a result of the synergy test were clinically significant.

Use of the Hollow-Fiber Infection Model to Measure the Effect of Combination Therapy of Septic Shock Exposures of Meropenem and Ciprofloxacin against Intermediate and Resistant Pseudomonas aeruginosa Clinical Isolates

Meropenem-ciprofloxacin combination therapy was compared to the respective monotherapy in a Hollow-Fiber Infection Model against two Pseudomonas aeruginosa isolates. Following initial kill of ∼ 5-logs by each monotherapy, rapid regrowth occurred within 24 h, reaching 108 - 1010 CFU/mL at 120 h. In contrast, combination therapy achieved > 5-log kill within 6 h and suppressed bacterial regrowth throughout. The results suggest that meropenem-ciprofloxacin combination may provide significantly enhanced bacterial killing and resistance suppression against P. aeruginosa.

In vitro synergistic antimicrobial activity of a combination of meropenem, colistin, tigecycline, rifampin, and ceftolozane/tazobactam against carbapenem-resistant Acinetobacter baumannii

We investigated the in vitro activity of various antimicrobial combinations against carbapenem-resistant Acinetobacter baumannii (CRAB) isolates. The in vitro activity of six two-drug combinations against CRAB isolates collected from the blood samples of patients with bloodstream infection was evaluated using the checkerboard method and time-kill assay [0.5 ×, 1 ×, and 2 × minimum inhibitory concentration (MIC)] to identify potential synergistic and bactericidal two-drug combinations against CRAB isolates. The effects of meropenem, colistin, tigecycline, rifampin, and ceftolozane/tazobactam combinations were investigated. All 10 CRAB isolates in our study produced the OXA-58-type and OXA-23-type carbapenem-hydrolyzing oxacillinases. The colistin-ceftolozane/tazobactam combination showed synergistic effects in both the time-kill assay (using an antibiotic concentration of 1 × MIC) and the checkerboard method. It also showed bactericidal effects in the time-kill assay. For all 10 CRAB isolates, time-kill curves showed synergistic bactericidal activity of the colistin-ceftolozane/tazobactam combination at 0.5 × MIC. Overall, there was substantial discordance of synergistic activity between the checkerboard microdilution and time-kill assays (with a concordance of 31.7%). Our study demonstrated that two-drug combinations of colistin and ceftolozane/tazobactam could be useful treatment alternatives for CRAB infections. The effects of these antibiotic combinations should be evaluated using in vivo experimental models.

Novel Combination Therapy for Extensively Drug-Resistant Acinetobacter baumannii Necrotizing Pneumonia Complicated by Empyema: A Case Report

We report our clinical and laboratory experience treating a 50-year-old patient who was critically ill with extensively drug-resistant Acinetobacter baumannii necrotizing pneumonia complicated by empyema in Detroit, Michigan. A precision medicine approach using whole-genome sequencing, susceptibility testing, and synergy analysis guided the selection of rational combination antimicrobial therapy.

Potential Synergistic Antibiotic Combinations against Fluoroquinolone-Resistant Pseudomonas aeruginosa

The rise in multiple-drug-resistant (MDR) phenotypes in Gram-negative pathogens is a major public health crisis. Pseudomonas aeruginosa is one of the leading causes of nosocomial infections in clinics. Treatment options for P. aeruginosa have become increasingly difficult due tdo its remarkable capacity to resist multiple antibiotics. The presence of intrinsic resistance factors and the ability to quickly adapt to antibiotic monotherapy warrant us to look for alternative strategies like combinatorial antibiotic therapy. Here, we report the frequency of P. aeruginosa multidrug-resistant and extensively drug-resistance (XDR) phenotypes in a super-specialty tertiary care hospital in north India. Approximately 60 percent of all isolated P. aeruginosa strains displayed the MDR phenotype. We found highest antibiotic resistance frequency in the emergency department (EMR), as 20 percent of isolates were resistant to 15 antipseudomonal antibiotics. Presence of plasmids with quinolone-resistance determinants were major drivers for resistance against fluoroquinolone. Additionally, we explored the possible combinatorial therapeutic options with four antipseudomonal antibiotics—colistin, ciprofloxacin, tobramycin, and meropenem. We uncovered an association between different antibiotic interactions. Our data show that the combination of colistin and ciprofloxacin could be an effective combinatorial regimen to treat infections caused by MDR and XDR P. aeruginosa.

Boosting antibiotics performance by new formulations with deep eutectic solvents

The critical scenario of antimicrobial resistance to antibiotics highlights the need for improved therapeutics and/or formulations. Herein, we demonstrate that deep eutectic solvents (DES) formulations are very promising to remarkably improve the solubility, stability and therapeutic efficacy of antibiotics, such as ciprofloxacin. DES aqueous solutions enhance the solubility of ciprofloxacin up to 430-fold while extending the antibiotic stability. The developed formulations can improve, by 2 to 4-fold, the susceptibility of Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus) bacteria to the antibiotic. They also improve the therapeutic efficacy at concentrations where bacteria present resistance, without promoting tolerance development to ciprofloxacin. Furthermore, the incorporation of DES decreases the toxicity of ciprofloxacin towards immortalized human epidermal keratinocytes (HaCat cells). The results herein reveal the pioneering use of DES in fluoroquinolone-based formulations and their impact on the antibiotic's characteristics and on its therapeutic action.

Nanotargeting of Resistant Infections with a Special Emphasis on the Biofilm Landscape

Bacterial resistance to antimicrobial compounds is a growing concern in medical and public health circles. Overcoming the adaptable and duplicative resistance mechanisms of bacteria requires chemistry-based approaches. Engineered nanoparticles (NPs) now offer unique advantages toward this effort. However, most in situ infections (in humans) occur as attached biofilms enveloped in a protective surrounding matrix of extracellular polymers, where survival of microbial cells is enhanced. This presents special considerations in the design and deployment of antimicrobials. Here, we review recent efforts to combat resistant bacterial strains using NPs and, then, explore how NP surfaces may be specifically engineered to enhance the potency and delivery of antimicrobial compounds. Special NP-engineering challenges in the design of NPs must be overcome to penetrate the inherent protective barriers of the biofilm and to successfully deliver antimicrobials to bacterial cells. Future challenges are discussed in the development of new antibiotics and their mechanisms of action and targeted delivery via NPs.

Systematic review and meta-analysis of in vitro efficacy of antibiotic combination therapy against carbapenem-resistant Gram-negative bacilli

The superiority of combination therapy for carbapenem-resistant Gram-negative bacilli (CR-GNB) infections remains controversial. In vitro models may predict the efficacy of antibiotic regimens against CR-GNB. A systematic review and meta-analysis was performed including pharmacokinetic/pharmacodynamic (PK/PD) and time-kill (TK) studies examining the in vitro efficacy of antibiotic combinations against CR-GNB [PROSPERO registration no. CRD42019128104]. The primary outcome was in vitro synergy based on the effect size (ES): high, ES ≥ 0.75, moderate, 0.35 < ES < 0.75; low, ES ≤ 0.35; and absent, ES = 0). A network meta-analysis assessed the bactericidal effect and re-growth rate (secondary outcomes). An adapted version of the ToxRTool was used for risk-of-bias assessment. Over 180 combination regimens from 136 studies were included. The most frequently analysed classes were polymyxins and carbapenems. Limited data were available for ceftazidime/avibactam, ceftolozane/tazobactam and imipenem/relebactam. High or moderate synergism was shown for polymyxin/rifampicin against Acinetobacter baumannii [ES = 0.91, 95% confidence interval (CI) 0.44-1.00], polymyxin/fosfomycin against Klebsiella pneumoniae (ES = 1.00, 95% CI 0.66-1.00) and imipenem/amikacin against Pseudomonas aeruginosa (ES = 1.00, 95% CI 0.21-1.00). Compared with monotherapy, increased bactericidal activity and lower re-growth rates were reported for colistin/fosfomycin and polymyxin/rifampicin in K. pneumoniae and for imipenem/amikacin or imipenem/tobramycin against P. aeruginosa. High quality was documented for 65% and 53% of PK/PD and TK studies, respectively. Well-designed in vitro studies should be encouraged to guide the selection of combination therapies in clinical trials and to improve the armamentarium against carbapenem-resistant bacteria.

Clinically Relevant Epithelial Lining Fluid Concentrations of Meropenem with Ciprofloxacin Provide Synergistic Killing and Resistance Suppression of Hypermutable Pseudomonas aeruginosa in a Dynamic Biofilm Model

Treatment of exacerbations of chronic Pseudomonas aeruginosa infections in patients with cystic fibrosis (CF) is highly challenging due to hypermutability, biofilm formation, and an increased risk of resistance emergence. We evaluated the impact of ciprofloxacin and meropenem as monotherapy and in combination in the dynamic in vitro CDC biofilm reactor (CBR). Two hypermutable P. aeruginosa strains, PAOΔmutS (MIC of ciprofloxacin [MIC_{ciprofloxacin}], 0.25 mg/liter; MIC_meropenem, 2 mg/liter) and CW44 (MIC_{ciprofloxacin}, 0.5 mg/liter; MIC_meropenem, 4 mg/liter), were investigated for 120 h. Concentration-time profiles achievable in epithelial lining fluid (ELF) following FDA-approved doses were simulated in the CBR. Treatments were ciprofloxacin at 0.4 g every 8 h as 1-h infusions (80% ELF penetration), meropenem at 6 g/day as a continuous infusion (CI) (30% and 60% ELF penetration), and their combinations. Counts of total and less-susceptible planktonic and biofilm bacteria and MICs were determined. Antibiotic concentrations were quantified by an ultrahigh-performance liquid chromatography photodiode array (UHPLC-PDA) assay. For both strains, all monotherapies failed, with substantial regrowth and resistance of planktonic (≥8 log₁₀ CFU/ml) and biofilm (>8 log₁₀ CFU/cm²) bacteria at 120 h (MIC_{ciprofloxacin}, up to 8 mg/liter; MIC_meropenem, up to 64 mg/liter). Both combination treatments demonstrated synergistic bacterial killing of planktonic and biofilm bacteria of both strains from ∼48 h onwards and suppressed regrowth to ≤4 log₁₀ CFU/ml and ≤6 log₁₀ CFU/cm² at 120 h. Overall, both combination treatments suppressed the amplification of resistance of planktonic bacteria for both strains and of biofilm bacteria for CW44. The combination with meropenem at 60% ELF penetration also suppressed the amplification of resistance of biofilm bacteria for PAOΔmutS Thus, combination treatment demonstrated synergistic bacterial killing and resistance suppression against difficult-to-treat hypermutable P. aeruginosa strains.

Antimicrobial activity of novel chalcones and modulation of virulence factors in hospital strains of Acinetobacter baumannii and Pseudomonas aeruginosa

Acinetobacter baumannii and Pseudomonas aeruginosa are frequent multiresistant nosocomial pathogens that cause wound and pulmonary infections in hospitalized patients. As being increasingly resistant to most clinically available antibiotics, there is a constant need for exploration of new substances that could kill them or inhibit their growth, or alternatively inhibit some of their essential virulence factors. Chalcones are chemical compounds with well-documented antimicrobial potential. The aim of this study was to examine effectiveness of four newly-synthesized chalcones against the multiresistant clinical strains of A. baumannii and P. aeruginosa. Antibacterial activity of chalcones was investigated with broth-microdilution test and time-dependent killing assay. Synergistic effects of tested compounds with antibiotics (meropenem, amikacin and ciprofloxacin) were determined by checkerboard assay. The effects of chalcones on expression of virulence factors in P. aeruginosa (pyocyanin production, swimming and swarming motility) and A. baumannii (twitching and surface-associated motility), along with their biofilm production, were also examined. The obtained results indicate substantial antimicrobial activity of the tested chalcones (MICs = 100-175 μg/mL) and several synergistic interactions with antibiotics, as well as notable reduction in expression of all investigated virulence factors. These promising results may constitute a good basis for further research.

Rational Combinations of Polymyxins with Other Antibiotics

Combinations of antimicrobial agents are often used in the management of infectious diseases. Antimicrobial agents used as part of combination therapy are often selected empirically. As regrowth and the emergence of polymyxin (either colistin or polymyxin B) resistance has been observed with polymyxin monotherapy, polymyxin combination therapy has been suggested as a possible means by which to increase antimicrobial activity and reduce the development of resistance. This chapter provides an overview of preclinical and clinical investigations of CMS/colistin and polymyxin B combination therapy. In vitro data and animal model data suggests a potential clinical benefit with many drug combinations containing clinically achievable concentrations of polymyxins, even when resistance to one or more of the drugs in combination is present and including antibiotics normally inactive against Gram-negative organisms. The growing body of data on the emergence of polymyxin resistance with monotherapy lends theoretical support to a benefit with combination therapy. Benefits include enhanced bacterial killing and a suppression of polymyxin resistant subpopulations. However, the complexity of the critically ill patient population, and high rates of treatment failure and death irrespective of infection-related outcome make demonstrating a potential benefit for polymyxin combinations extremely challenging. Polymyxin combination therapy in the clinic remains a heavily debated and controversial topic. When combinations are selected, optimizing the dosage regimens for the polymyxin and the combinatorial agent is critical to ensure that the benefits outweigh the risk of the development of toxicity. Importantly, patient characteristics, pharmacokinetics, the site of infection, pathogen and resistance mechanism must be taken into account to define optimal and rational polymyxin combination regimens in the clinic.

Colistin Induces S. aureus Susceptibility to Bacitracin

Bacitracin has been used in topical preparations with polymyxin B for bacterial infections. Colistin belongs to the polymyxin group of antibiotics and is effective against most Gram-negative bacilli. This study investigated whether colistin could affect the susceptibility of S. aureus to bacitracin. S. aureus isolates were first incubated with colistin and the susceptibility of S. aureus to bacitracin was increased. The effect of the combination of colistin and bacitracin on S. aureus was then confirmed by the checkerboard assay and the time-kill kinetics. The Triton X-100-induced autolysis was significantly increased after S. aureus was exposed to colistin. Exposure to colistin also led to a less positive charge on the cell surface and a significant leakage of Na+, Mg2, K+, Ca2+, Mn2+, Cu2+, and Zn2+. Finally, disruptions on the cell surface and an irregular morphology were observed when the bacteria were exposed to colistin and bacitracin. Bacitracin had a stronger antibacterial activity against S. aureus in the presence of colistin. This could be due to the fact that colistin damaged the bacterial membrane. This study suggests that combination of colistin with bacitracin has a potential for treating clinical S. aureus infections.

Pseudomonas aeruginosa bacteremia among liver transplant recipients

Pseudomonas aeruginosa bacteremia remains as a life-threatening complication after liver transplantation (LT) and is intractable because of the high rate of drug resistance to commonly used antibiotics. To better understand the characteristics of this postoperative complication, PubMed and Embase searches as well as reference mining was done for relevant literature from the start of the databases through August 2018. Among LT recipients, the incidence of P. aeruginosa bacteremia ranged from 0.5% to 14.4% and mortality rates were up to 40%. Approximately 35% of all episodes of bloodstream infections (BSIs) were P. aeruginosa bacteremia, of which 47% were multidrug resistant and 63% were extensively drug resistant. Several factors are known to affect the mortality of LT recipients with P. aeruginosa bacteremia, including hypotension, mechanical ventilation, and increasing severity of illness. In LT recipients with P. aeruginosa bacteremia, alteration in DNA gyrase A genes and overexpression of proteins involved in efflux systems, namely the expression of KPC-2-type carbapenemase, NDM-1, and VIM-2-type MBL, contribute to the high resistance of P. aeruginosa to a wide variety of antibiotics. Because of complicated mechanisms of drug resistance, P. aeruginosa causes high morbidity and mortality in bacteremic LT patients. Consequently, early detection and treatment with adequate early targeted coverage for P. aeruginosa BSI are of paramount importance in the early posttransplantation period to obtain a better prognosis for LT patients.

Meropenem Combined with Ciprofloxacin Combats Hypermutable Pseudomonas aeruginosa from Respiratory Infections of Cystic Fibrosis Patients

Hypermutable Pseudomonas aeruginosa organisms are prevalent in chronic respiratory infections and have been associated with reduced lung function in cystic fibrosis (CF); these isolates can become resistant to all antibiotics in monotherapy. This study aimed to evaluate the time course of bacterial killing and resistance of meropenem and ciprofloxacin in combination against hypermutable and nonhypermutable P. aeruginosa Static concentration time-kill experiments over 72 h assessed meropenem and ciprofloxacin in mono- and combination therapies against PAO1 (nonhypermutable), PAOΔmutS (hypermutable), and hypermutable isolates CW8, CW35, and CW44 obtained from CF patients with chronic respiratory infections. Meropenem (1 or 2 g every 8 h [q8h] as 3-h infusions and 3 g/day as a continuous infusion) and ciprofloxacin (400 mg q8h as 1-h infusions) in monotherapies and combinations were further evaluated in an 8-day hollow-fiber infection model study (HFIM) against CW44. Concentration-time profiles in lung epithelial lining fluid reflecting the pharmacokinetics in CF patients were simulated and counts of total and resistant bacteria determined. All data were analyzed by mechanism-based modeling (MBM). In the HFIM, all monotherapies resulted in rapid regrowth with resistance at 48 h. The maximum daily doses of 6 g meropenem (T_>MIC of 80% to 88%) and 1.2 g ciprofloxacin (area under the concentration-time curve over 24 h in the steady state divided by the MIC [AUC/MIC], 176), both given intermittently, in monotherapy failed to suppress regrowth and resulted in substantial emergence of resistance (≥7.6 log₁₀ CFU/ml resistant populations). The combination of these regimens achieved synergistic killing and suppressed resistance. MBM with subpopulation and mechanistic synergy yielded unbiased and precise curve fits. Thus, the combination of 6 g/day meropenem plus ciprofloxacin holds promise for future clinical evaluation against infections by susceptible hypermutable P. aeruginosa.

An update on technical, interpretative and clinical relevance of antimicrobial synergy testing methodologies

Testing for antimicrobial interactions has gained popularity in the last decade due to the increasing prevalence of drug-resistant organisms and limited options for the treatment of these infections. In vitro combination testing provides information, on which two or more antimicrobials can be combined for a good clinical outcome. Amongst the various in vitro methods of drug interactions, time-kill assay (TKA), checkerboard (CB) assay and E-test-based methods are most commonly used. Comparative performance of these methods reveals the TKA as the most promising method to detect synergistic combinations followed by CB assay and E-test. Various combinations of antimicrobials have been tested to demonstrate synergistic activity. Promising results were obtained for the combinations of meropenem plus colistin and rifampicin plus colistin against Acinetobacter baumannii, colistin plus carbapenem and carbapenem plus fluoroquinolones against Pseudomonas aeruginosa and colistin/polymyxin B plus rifampicin/meropenem against Klebsiella pneumoniae. Antagonism was detected in only few instances. The presence of synergy or antagonism with a combination seems to correlate with minimum inhibitory concentration of the agent and molecular mechanism involved in the resistance. Further studies need to be conducted to assess the utility of in vitro testing to predict clinical outcome and direct therapy for drug-resistant organisms.

Synergy effect of meropenem-based combinations against Acinetobacter baumannii: a systematic review and meta-analysis

PURPOSE

The main objective of our meta-analysis was to examine the in vitro synergistic effect of meropenem-based combination therapies against Acinetobacter baumannii through a systematic review of the existing literature.

METHODS

An extensive search was performed with no restrictions on date of publication, language, and publication type. Our study evaluated the main conclusions drawn from various studies describing the synergistic activity of combination therapies in vitro.

RESULTS

In this review, 56 published studies were included. Our report included data on 20 types of antibiotics combined with meropenem in 1,228 Acinetobacter baumannii isolates. In time-kill studies, meropenem combined with polymyxin B and rifampicin showed synergy rates of 98.3% (95% CI, 83.7%-100.0%) and 89.4% (95% CI, 57.2%-100.0%), respectively, for Acinetobacter baumannii, modest synergy rates were found for meropenem combined with several antibiotics such as colistin and sulbactam, and no synergy effect was displayed in the combination of meropenem and ciprofloxacin, whereas in checkerboard method, the synergy rates of polymyxin B and rifampicin were 37.0% (95% CI, 0.00%-100.0%) and 56.3% (95% CI, 8.7%-97.8%), respectively.

CONCLUSION

We found that time-kill studies generally identified the greatest synergy, while checkerboard and Etest methods yielded relatively poor synergy rates. Further well-designed in vivo studies should be carried out to confirm these findings.

Synergistic Killing of Polymyxin B in Combination With the Antineoplastic Drug Mitotane Against Polymyxin-Susceptible and -Resistant Acinetobacter baumannii: A Metabolomic Study

Polymyxins are currently used as the last-resort antibiotics against multidrug-resistant Acinetobacter baumannii. As resistance to polymyxins emerges in A. baumannii with monotherapy, combination therapy is often the only remaining treatment option. A novel approach is to employ the combination of polymyxin B with non-antibiotic drugs. In the present study, we employed metabolomics to investigate the synergistic mechanism of polymyxin B in combination with the antineoplastic drug mitotane against polymyxin-susceptible and -resistant A. baumannii. The metabolomes of four A. baumannii strains were analyzed following treatment with polymyxin B, mitotane and the combination. Polymyxin B monotherapy induced significant perturbation in glycerophospholipid (GPL) metabolism and histidine degradation pathways in polymyxin-susceptible strains, and minimal perturbation in polymyxin-resistant strains. Mitotane monotherapy induced minimal perturbation in the polymyxin-susceptible strains, but caused significant perturbation in GPL metabolism, pentose phosphate pathway and histidine degradation in the LPS-deficient polymyxin-resistant strain (FADDI-AB065). The polymyxin B – mitotane combination induced significant perturbation in all strains except the lipid A modified polymyxin-resistant FADDI-AB225 strain. For the polymyxin-susceptible strains, the combination therapy significantly perturbed GPL metabolism, pentose phosphate pathway, citric acid cycle, pyrimidine ribonucleotide biogenesis, guanine ribonucleotide biogenesis, and histidine degradation. Against FADDI-AB065, the combination significantly perturbed GPL metabolism, pentose phosphate pathway, citric acid cycle, and pyrimidine ribonucleotide biogenesis. Overall, these novel findings demonstrate that the disruption of the citric acid cycle and inhibition of nucleotide biogenesis are the key metabolic features associated with synergistic bacterial killing by the combination against polymyxin-susceptible and -resistant A. baumannii.

Pharmacological Inhibition of the Pseudomonas aeruginosa MvfR Quorum-Sensing System Interferes with Biofilm Formation and Potentiates Antibiotic-Mediated Biofilm Disruption

Pseudomonas aeruginosa biofilms contribute to its survival on biotic and abiotic surfaces and represent a major clinical threat due to their high tolerance to antibiotics. Therefore, the discovery of antibiofilm agents may hold great promise. We show that pharmacological inhibition of the P. aeruginosa quorum-sensing regulator MvfR (PqsR) using a benzamide-benzimidazole compound interferes with biofilm formation and potentiates biofilm sensitivity to antibiotics. Such a strategy could have great potential against P. aeruginosa persistence in diverse environments.

The distribution of carbapenem- and colistin-resistance in Gram-negative bacteria from the Tamil Nadu region in India

PURPOSE

The occurrence of carbapenem- and colistin-resistance among Gram-negative bacteria is increasing worldwide. The aim of this study was to understand the distribution of carbapenem- and colistin-resistance in two areas in Tamil Nadu, India.

METHODOLOGY

The clinical isolates (n=89) used in this study were collected from two diagnostic centres in Tamil Nadu, India. The bacterial isolates were screened for meropenem- and colistin-resistance. Further, resistance genes blaNDM-1, blaOXA-48-like, blaIMP, blaVIM, blaKPC, mcr-1 and mcr-2 and integrons were studied. The synergistic effect of meropenem in combination with colistin was assessed.

RESULTS

A total of 89 bacterial isolates were studied which included Escherichia coli (n=43), Klebsiella pneumoniae (n=18), Pseudomonas aeruginosa (n=10), Enterobacter cloacae (n=6), Acinetobacter baumannii (n=5), Klebsiella oxytoca (n=4), Proteus mirabilis (n=2) and Salmonella paratyphi (n=1). MIC testing showed that 58/89 (65 %) and 29/89 (32 %) isolates were resistant to meropenem and colistin, respectively, whereas 27/89 (30 %) isolates were resistant to both antibiotics. Escherichia coli, K. pneumoniae, K. oxytoca, Pseudomonas aeruginosa, and Enterobacter cloacae isolates were blaNDM-1-positive (n=20). Some strains of Escherichia coli, K. pneumoniae and K. oxytoca were blaOXA-181-positive (n=4). Class 1, 2 and 3 integrons were found in 24, 20 and 3 isolates, respectively. Nine NDM-1-positive Escherichia coli strains could transfer carbapenem resistance via plasmids to susceptible Escherichia coli AB1157. Meropenem and colistin showed synergy in 10/20 (50 %) isolates by 24 h time-kill studies.

CONCLUSION

Our results highlight the distribution of carbapenem- and colistin-resistance in Gram-negative bacteria isolated from the Tamil Nadu region in South India.

Biology of Acinetobacter baumannii: Pathogenesis, Antibiotic Resistance Mechanisms, and Prospective Treatment Options

Acinetobacter baumannii is undoubtedly one of the most successful pathogens responsible for hospital-acquired nosocomial infections in the modern healthcare system. Due to the prevalence of infections and outbreaks caused by multi-drug resistant A. baumannii, few antibiotics are effective for treating infections caused by this pathogen. To overcome this problem, knowledge of the pathogenesis and antibiotic resistance mechanisms of A. baumannii is important. In this review, we summarize current studies on the virulence factors that contribute to A. baumannii pathogenesis, including porins, capsular polysaccharides, lipopolysaccharides, phospholipases, outer membrane vesicles, metal acquisition systems, and protein secretion systems. Mechanisms of antibiotic resistance of this organism, including acquirement of β-lactamases, up-regulation of multidrug efflux pumps, modification of aminoglycosides, permeability defects, and alteration of target sites, are also discussed. Lastly, novel prospective treatment options for infections caused by multi-drug resistant A. baumannii are summarized.

Investigating the in vitro synergistic activities of several antibiotic combinationsagainst carbapenem-resistant Acinetobacter baumannii isolates

BACKGROUND/AIM

Acinetobacter baumannii (A. baumannii) is one of the most common healthcare-associated infectious agents worldwide. The aim of this study was to investigate the in vitro synergistic activities of several antibiotic combinations against carbapenem-resistant (CR) A. baumannii isolates.

MATERIALS AND METHODS

Eighteen CR A. baumannii strains were isolated from the patients who were hospitalized in the intensive care unit between June 2012 and August 2012. The in vitro effects of single and binary combinations of meropenem (MEM), colistin (CST), tigecycline (TGC), and sulbactam (SUL) on these isolates were determined using the Epsilometer test (E-test) method.

RESULTS

All 18 isolates were resistant to MEM and SUL and susceptible to CST. TGC was detected as susceptible in two of the isolates and intermediate susceptibility results were observed in the remaining isolates. With MEM-CST and MEM-TGC combinations, synergism was determined against all isolates. The synergistic and/or additive effect ratios were detected in MEM-SUL, CST-SUL, TGC-SUL, and CST-TGC combinations as 16.7%, 38.9%, 16.7%, and 5.6%, respectively.

CONCLUSION

Among the tested antimicrobial combinations, the in vitro combination of MEM with TGC or CST was most effective against the CR A. baumannii strains.

Dynamic interaction of colistin and meropenem on a WT and a resistant strain of Pseudomonas aeruginosa as quantified in a PK/PD model

OBJECTIVES

Combination therapy can be a strategy to ensure effective bacterial killing when treating Pseudomonas aeruginosa, a Gram-negative bacterium with high potential for developing resistance. The aim of this study was to develop a pharmacokinetic/pharmacodynamic (PK/PD) model that describes the in vitro bacterial time-kill curves of colistin and meropenem alone and in combination for one WT and one meropenem-resistant strain of P. aeruginosa.

METHODS

In vitro time-kill curve experiments were conducted with a P. aeruginosa WT (ATCC 27853) (MICs: meropenem 1 mg/L; colistin 1 mg/L) and a meropenem-resistant type (ARU552) (MICs: meropenem 16 mg/L; colistin 1.5 mg/L). PK/PD models characterizing resistance were fitted to the observed bacterial counts in NONMEM. The final model was applied to predict the bacterial killing of ARU552 for different combination dosages of colistin and meropenem.

RESULTS

A model with compartments for growing and resting bacteria, where the bacterial killing by colistin reduced with continued exposure and a small fraction (0.15%) of the start inoculum was resistant to meropenem, characterized the bactericidal effect and resistance development of the two antibiotics. For a typical patient, a loading dose of colistin combined with a high dose of meropenem (2000 mg q8h) was predicted to result in a pronounced kill of the meropenem-resistant strain over 24 h.

CONCLUSIONS

The developed PK/PD model successfully described the time course of bacterial counts following exposures to colistin and meropenem, alone and in combination, for both strains, and identified a dynamic drug interaction. The study illustrates the application of a PK/PD model and supports high-dose combination therapy of colistin and meropenem to overcome meropenem resistance.

Optimizing Polymyxin Combinations Against Resistant Gram-Negative Bacteria

Polymyxin combination therapy is increasingly used clinically. However, systematic investigations of such combinations are a relatively recent phenomenon. The emerging pharmacodynamic (PD) and pharmacokinetic (PK) data on CMS/colistin and polymyxin B suggest that caution is required with monotherapy. Given this situation, polymyxin combination therapy has been suggested as a possible way to increase bacterial killing and reduce the development of resistance. Considerable in vitro data have been generated in support of this view, particularly recent studies utilizing dynamic models. However, most existing animal data are of poor quality with major shortcomings in study design, while clinical data are generally limited to retrospective analysis and small, low-power, prospective studies. This article provides an overview of clinical and preclinical investigations of CMS/colistin and polymyxin B combination therapy.

Polymyxin combinations: pharmacokinetics and pharmacodynamics for rationale use

Since their reintroduction into the clinic in the 1980s, the polymyxin antibiotics colistin-administered intravenously as an inactive prodrug, colistin methanesulfonate (CMS)-and polymyxin B have assumed an important role as salvage therapy for otherwise untreatable gram-negative infections. However, the emerging pharmacodynamic and pharmacokinetic data on CMS/colistin and polymyxin B indicate that polymyxin monotherapy is unlikely to generate plasma concentrations that are reliably efficacious. Additionally, regrowth and the emergence of resistance with monotherapy are commonly reported even when concentrations exceed those achieved clinically. Given this situation, polymyxin combination therapy, which is increasingly being used clinically, has been suggested as a possible means of increasing antimicrobial activity and reducing the development of resistance. Although considerable in vitro data support this view, investigations of polymyxin combination therapy in patients have only recently commenced. The currently available clinical data for polymyxin combinations are generally limited to retrospective analyses and small, low-powered, prospective studies using traditional dosage regimens that achieve low plasma concentrations. Considering the potential for rapid development of resistance to polymyxins, well-designed clinical trials that include higher-dose polymyxin regimens are urgently required to provide a more definitive answer regarding the role of polymyxin combination therapy compared with monotherapy. In this article, we provide an overview of key in vitro and clinical investigations examining CMS/colistin and polymyxin B combination therapy.

A meta-analysis of in vitro antibiotic synergy against Acinetobacter baumannii

The aim of the work was to describe the different in vitro models for testing synergism of antibiotics and gather the results of antibiotic synergy against multidrug-resistant Acinetobacter baumannii (MDR-Ab). The different original articles were obtained from different web sites. In order to compare the results obtained by the different methods for synergy testing, the Pearson chi-square and the Fischer tests were used. Moreover, non-parametric chi-square test was used in order to compare the frequency distribution in each analysed manuscript. In the current meta-analysis 24 manuscripts, which encompassed 2016 tests of in vitro synergism of different antimicrobials against MDR-Ab, were revised. Checkerboard synergy testing was used in 11 studies, which encompasses 1086 tests (53.9%); time-kill assays were applied in 12 studies, which encompass 359 tests (17.8%); gradient diffusion methods were used in seven studies, encompassing 293 tests (14.5%). And, finally, time-kill plus checkerboard were applied in two studies, encompassing 278 tests (13.8%). By comparing these data, checkerboard and time-kill methods were significantly more used than gradient diffusion methods (p<0.005). Regarding synergy rates obtained on the basis of the applied method, checkerboard provided 227 tests (20.9%) with a synergistic effect; time-kill assays yielded 222 tests (61.8%) with a synergistic effect; gradient diffusion methods only provided 29 tests (9.9%) with a synergistic effect; and, finally, time-kill plus checkerboard yielded just 15 tests (5.4%) with a synergistic effect. When comparing these percentages, synergy rates reported by time-kill methods were significantly higher than that obtained by checkerboard and gradient diffusion methods (p<0.005). On the basis of the revised data, the combinations of a bactericidal antibiotic plus Tigecycline, Vancomycin or Teicoplanin are not recommended. The best combinations of antibiotics are those which include bactericidal antibiotics such as Carbapenems, Fosfomycin, Amikacin, Polymyxins, Rifampicin and Ampicillin/Sulbactam.

Efficacy of ciprofloxacin-clarithromycin combination against drug-resistant Pseudomonas aeruginosa mature biofilm using in vitro experimental model

Pseudomonas aeruginosa is the main cause of mortality in cystic fibrosis patients and eradication of its biofilm represents a substantial problem clinically. In this study, biofilm of a cystic fibrosis strain P. aeruginosa PACI22 was established and confocal laser scanning microscopy was utilized for biofilm visualization. A quantitative time-kill biofilm model was implemented in vitro to assess the biocidal effect of ciprofloxacin, clarithromycin, and their combination at concentration levels ranged from 0.5× to 64× minimum biofilm inhibitory concentrations (MBIC) against the biofilm and the mean log bacterial densities (Log CFU/ml) retrieved from the biofilm were monitored by frequent sampling at 0, 3, 6, 9, 12, and 24 hr throughout the experiment. The results revealed that none of the tested antibiotics alone could completely eradicate the biofilm-ensconced bacteria at 0.5-64× MBIC values after 24 hr of treatment. Conversely, ciprofloxacin-clarithromycin combination at 32-64× MBIC entirely exterminated the biofilm. Furthermore, a substantial in vitro synergism between ciprofloxacin and clarithromycin against the biofilm was experimentally verified. This promising synergism affords scientific rationale for further in vivo investigations to evaluate the therapeutic potential of this combination for treatment of chronic pulmonary infections caused by P. aeruginosa biofilms.

Empiric antimicrobial therapy in severe sepsis and septic shock: optimizing pathogen clearance

Mortality and morbidity in severe sepsis and septic shock remain high despite significant advances in critical care. Efforts to improve outcome in septic conditions have focused on targeted, quantitative resuscitation strategies utilizing intravenous fluids, vasopressors, inotropes, and blood transfusions to correct disease-associated circulatory dysfunction driven by immune-mediated systemic inflammation. This review explores an alternate paradigm of septic shock in which microbial burden is identified as the key driver of mortality and progression to irreversible shock. We propose that clinical outcomes in severe sepsis and septic shock hinge upon the optimized selection, dosing, and delivery of highly potent antimicrobial therapy.

Colistin for lung infection: an update

Increasing incidence of resistance of gram-negative bacteria against even newer antibiotic including carbapenem has generated interest in the old antibiotic colistin, which are being used as salvage therapy in the treatment of multidrug resistant infection. Colistin has excellent bactericidal activity against most gram-negative bacilli. It has shown persist level in the liver, kidney, heart, and muscle; while it is poorly distributed to the bones, cerebrospinal fluid, lung parenchyma, and pleural cavity. Being an old drug, colistin was never gone through the drug development process needed for compliance with competent regulatory authorities that resulted in very much limited understanding of pharmacokinetic (PK) and pharmacodynamic (PD) parameters, such as C_max/MIC ratio, AUC/MIC and T > MIC that could predict the efficacy of colistin. In available PK/PD studies of colistin, mean maximum serum concentration (C_max) of colistin were found just above the MIC breakpoint at steady states that would most probably lead to suboptimal for killing the bacteria, even at dosages of 3.0 million international units (MIU) i.e., 240 mg of colistimethate sodium (CMS) intravenously every 8 h. These finding stresses to use high loading as well as high maintenance dose of intravenous colistin. It is not only suboptimal plasma concentration of colistin but also poor lung tissue concentration, which has been demonstrated in recent studies, poses major concern in using intravenous colistin. Combination therapy mainly with carbapenems shows synergistic effect. In recent studies, inhaled colistin has been found promising in treatment of lung infection due to MDR gram-negative bacteria. New evidence shows less toxicity than previously reported.

Successful management of an outbreak due to carbapenem-resistant Acinetobacter baumannii in a neonatal intensive care unit

The investigation and successful management of a monoclonal Acinetobacter baumannii outbreak in a neonatal intensive care unit are described. Upon the first clustered carbapenem-resistant A. baumannii (CRAB) infections, a bundle of actions were taken, including enhanced infection control, active surveillance (weekly stool samples), case-control study, staff education, daily audits and discontinuation of new admissions. Between September and December 2011, eight neonates developed 10 CRAB infections (five blood, four respiratory and one eye). A total of 216 active surveillance cultures were obtained from 96 neonates (43 % had ≥2 samples). During weeks 12, 16 and 17, active surveillance detected 3, 1 and 2 new CRAB acquisitions, respectively. Prevalence of infections/colonizations decreased, and no event occurred after 20th week. A colonized neonate developed CRAB sepsis and died. All CRAB isolates harboured bla OXA-58 and the intrinsic chromosomal bla OXA-51 carbapenemase genes.

CONCLUSION

Active surveillance and enhanced infection control measures effectively contained spread of CRAB clone in the neonatal intensive care unit.

In vitro efficacy of various antibiotic combinations against Pseudomonas aeruginosa isolates

OBJECTIVE

Pseudomonas aeruginosa is one of the leading causes of nosocomial infection. The present study tested the in vitro efficacy of ceftazidime or imipenem combined with amikacin, levofloxacin and colistin in P.aeruginosa isolates.

METHODS

P.aeruginosa strains, isolated from clinical samples, were assessed for antibiotic susceptibility using the disc diffusion method. Antibiotic combination tests were performed using minimum inhibitory concentration (MIC) test strips and the sum of the Fractional Inhibitory Concentration (ΣFIC) index was used to assess synergy.

RESULTS

Out of 60 isolated P.aeruginosa strains, 100% were susceptible to colistin and 26.7% (16 strains) were multidrug resistant. MIC50 and MIC90 values were 2 and 32 µg/ml for imipenem; 1.5 and 24 µg/ml for ceftazidime; 3 and 8 µg/ml for amikacin; 0.38 and 32 µg/ml for levofloxacin; 1 and 1.5 µg/ml for colistin, respectively. Antagonism was not found in any of the antibiotic combinations tested. The amikacin-ceftazidime combination was found to have a synergistic effect in 15% of the strains, but no synergistic effect was detected for other combinations.

CONCLUSIONS

In Pseudomonas infection, alternative treatment options using different antibiotic combinations should be tested in vitro and findings should be confirmed by clinical studies.

Activities of antibiotic combinations against resistant strains of Pseudomonas aeruginosa in a model of infected THP-1 monocytes

Antibiotic combinations are often used for treating Pseudomonas aeruginosa infections but their efficacy toward intracellular bacteria has not been investigated so far. We have studied combinations of representatives of the main antipseudomonal classes (ciprofloxacin, meropenem, tobramycin, and colistin) against intracellular P. aeruginosa in a model of THP-1 monocytes in comparison with bacteria growing in broth, using the reference strain PAO1 and two clinical isolates (resistant to ciprofloxacin and meropenem, respectively). Interaction between drugs was assessed by checkerboard titration (extracellular model only), by kill curves, and by using the fractional maximal effect (FME) method, which allows studying the effects of combinations when dose-effect relationships are not linear. For drugs used alone, simple sigmoidal functions could be fitted to all concentration-effect relationships (extracellular and intracellular bacteria), with static concentrations close to (ciprofloxacin, colistin, and meropenem) or slightly higher than (tobramycin) the MIC and with maximal efficacy reaching the limit of detection in broth but only a 1 to 1.5 (colistin, meropenem, and tobramycin) to 2 to 3 (ciprofloxacin) log10 CFU decrease intracellularly. Extracellularly, all combinations proved additive by checkerboard titration but synergistic using the FME method and more bactericidal in kill curve assays. Intracellularly, all combinations proved additive only based on both FME and kill curve assays. Thus, although combinations appeared to modestly improve antibiotic activity against intracellular P. aeruginosa, they do not allow eradication of these persistent forms of infections. Combinations including ciprofloxacin were the most active (even against the ciprofloxacin-resistant strain), which is probably related to the fact this drug was the most effective alone intracellularly.

In vitro synergy of polymyxins with other antibiotics for Acinetobacter baumannii: a systematic review and meta-analysis

In order to provide preliminary guidance for rational antibiotic combination therapy in the clinic, a systematic review and meta-analysis was performed to evaluate the in vitro synergistic activity of polymyxins combined with other antibiotics against Acinetobacter baumannii. An extensive literature search was undertaken without restriction according to region, publication type or language. All available in vitro synergy tests on antibiotic combinations consisting of polymyxins were included. The primary outcome assessed was the in vitro activity of combination therapy on bacterial kill or inhibition. In total, 70 published studies and 31 conference proceedings reporting testing of polymyxins in combination with 11 classes consisting of 28 antibiotic types against 1484 A. baumannii strains were included in the analysis. In time-kill studies, high in vitro synergy and bactericidal activity were found for polymyxins combined with several antibiotic classes such as carbapenems and glycopeptides. Carbapenems or rifampicin combination could efficiently suppress the development of colistin resistance and displayed a >50% synergy rate against colistin-resistant strains. Synergy rates of chequerboard microdilution and Etest methods in most antibiotic combinations were generally lower than those of time-kill assays. The benefits of these antibiotic combinations should be further demonstrated by well-designed clinical studies.

Structural changes and differentially expressed genes in Pseudomonas aeruginosa exposed to meropenem-ciprofloxacin combination

The effect of a meropenem-ciprofloxacin combination (MCC) on the susceptibility of multidrug-resistant (MDR) Pseudomonas aeruginosa (MRPA) clinical isolates was determined using checkerboard and time-kill curve techniques. Structural changes and differential gene expression that resulted from the synergistic action of the MCC against one of the P. aeruginosa isolates (1071-MRPA]) were evaluated using electron microscopy and representational difference analysis (RDA), respectively. The differentially expressed, SOS response-associated, and resistance-associated genes in 1071-MRPA exposed to meropenem, ciprofloxacin, and the MCC were monitored by quantitative PCR. The MCC was synergistic against 25% and 40.6% of MDR P. aeruginosa isolates as shown by the checkerboard and time-kill curves, respectively. The morphological and structural changes that resulted from the synergistic action of the MCC against 1071-MRPA were a summation of the effects observed with each antimicrobial alone. One exception included outer membrane vesicles, which were seen in a greater amount upon ciprofloxacin exposure but were significantly inhibited upon MCC exposure. Cell wall- and DNA repair-associated genes were differentially expressed in 1071-MRPA exposed to meropenem, ciprofloxacin, and the MCC. However, some of the RDA-detected, resistance-associated, and SOS response-associated genes were expressed at significantly lower levels in 1071-MRPA exposed to the MCC. The MCC may be an alternative for the treatment of MDR P. aeruginosa. The effect of this antimicrobial combination may be not only the result of a summation of the effects of meropenem and ciprofloxacin but also a result of differential action that likely inhibits protective mechanisms in the bacteria.