Synergy testing of multidrug resistant Acinetobacter baumanii against tigecycline and polymyxin using an E-test methodology

Additivity vs Synergism: Investigation of the Additive Interaction of Cinnamon Bark Oil and Meropenem in Combinatory Therapy

Combinatory therapies have been commonly applied in the clinical setting to tackle multi-drug resistant bacterial infections and these have frequently proven to be effective. Specifically, combinatory therapies resulting in synergistic interactions between antibiotics and adjuvant have been the main focus due to their effectiveness, sidelining the effects of additivity, which also lowers the minimal effective dosage of either antimicrobial agent. Thus, this study was undertaken to look at the effects of additivity between essential oils and antibiotic, via the use of cinnamon bark essential oil (CBO) and meropenem as a model for additivity. Comparisons between synergistic and additive interaction of CBO were performed in terms of the ability of CBO to disrupt bacterial membrane, via zeta potential measurement, outer membrane permeability assay and scanning electron microscopy. It has been found that the additivity interaction between CBO and meropenem showed similar membrane disruption ability when compared to those synergistic combinations which was previously reported. Hence, results based on our studies strongly suggest that additive interaction acts on a par with synergistic interaction. Therefore, further investigation in additive interaction between antibiotics and adjuvant should be performed for a more in depth understanding of the mechanism and the impacts of such interaction.

In Vitro Activity of Various Antibiotics in Combination with Tigecycline Against Acinetobacter baumannii: A Systematic Review and Meta-Analysis

Given that tigecycline-based combination therapy is recognized as a valuable option for the treatment of tigecycline-resistant Acinetobacter baumannii, we conducted this systematic review and meta-analysis to assess the overall evidence of its effectiveness. The synergy rate was defined as the primary outcome that was calculated separately for time-kill, Etest, and checkerboard microdilution methods. The secondary outcomes were bactericidal activity and the efficacy of combination treatment on the development of resistance. In total, 37 published papers and 16 conference proceedings were included. Nine classes consisting of 22 antibiotic types in combination with tigecycline against 1,159 A. baumannii strains were reported in the analysis. For the time-kill studies, combination therapy showed a synergy rate of 37.9% (95% confidence interval [CI], 30.7-46.5); the highest synergy rate was 67.4% (95% CI, 27.3-91.9) for tigecycline in combination with colistin. Moreover, combination with amikacin or colistin could efficiently inhibit the development of tigecycline resistance. Compared with checkerboard microdilution and Etest methods, time-kill studies always showed higher synergy rates. Altogether, these results suggest that the in vitro tigecycline-based combinations resulted in moderate synergy rates and that several combinations could suppress the resistance of A. baumannii to tigecycline, which should be further confirmed in animal models and clinical trials.

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.

In vitro synergistic activity of colistin with tigecycline or β-lactam antibiotic/β-lactamase inhibitor combinations against carbapenem-resistant Acinetobacter baumannii

OBJECTIVE

Nosocomial infection caused by carbapenem-resistant Acinetobacter baumannii is a worldwide problem and treatment options remain controversial. This study investigated the in vitro effect of various antibiotic combinations against carbapenem-resistant A. baumannii strains.

METHODS

Antibiotic susceptibility of A. baumannii strains was analysed. In vitro synergistic efficacy of colistin combined with tigecycline, cefoperazone/sulbactam or piperacillin/tazobactam was tested against carbapenem-resistant A. baumannii strains. Synergy studies were performed using an eplisometer test-strip method.

RESULTS

Of the 50 carbapenem-resistant A. baumannii strains tested, 96% were susceptible to colistin and 64% were susceptible to tigecycline. Colistin-tigecycline, colistin-cefoperazone/sulbactam and colistin-piperacillin/tazobactam combinations were found to have synergistic effects against six (12%), two (4%), and one (2%), respectively, of the strains tested.

CONCLUSIONS

Colistin combined with tigecycline, cefoperazone/sulbactam or piperacillin/tazobactam revealed synergistic effects in some carbapenem-resistant A. baumannii strains. These results, together with the shortage of treatment options and the risk of developing resistance to colistin, suggest that clinicians should use colistin combined with other antibiotics or β-lactamase inhibitors when treating carbapenem-resistant A. baumannii infection.

Efficacy and safety of polymyxins for the treatment of Acinectobacter baumannii infection: a systematic review and meta-analysis

BACKGROUND

Multi-drug resistance among Acinetobacter baumannii increases the need for polymyxins. We conducted a meta-analysis aimed to assess the efficacy and safety of polymyxins for the treatment of Acinetobacter baumannii infection.

METHODS

We searched PUBMED, EMBASE, the Cochrane Central Register of Controlled Trials (CENTRAL), CNKI, Chinese Biomedical Literature Database up to November 1, 2013, to identify published studies, and we searched clinical trial registries to identify completed unpublished studies. Randomized controlled trials and cohort studies were considered for inclusion. Data were extracted on clinical response, microbiological response, mortality, length of stay and adverse events.

RESULTS

12 controlled studies, comparing 677 patients, were included. Although clinical (odds ratio 1.421, 95% confidence interval 0.722-2.797) and microbiological (OR 1.416, 95% CI 0.369-5.425) response rates favored the polymyxins group, these differences were not significant. Treatment with polymyxins vs. controls did not affect hospital mortality (OR 0.506, 95% CI 0.101-2.536), lengths of hospital stay (standard mean difference -0.221, 95% CI 0.899-0.458) or nephrotoxicity (OR 1.192, 95% CI 0.436-3.261). The combination of polymyxins with other antibiotics achieved similar clinical response rates to its monotherapy regimen (OR 0.601, 95% CI 0.320-1.130).

CONCLUSIONS

Our results suggest that polymyxins may be as safe and as efficacious as standard antibiotics for the treatment of A. baumannii infection. There is no strong evidence that combination regimen of polymyxins is superior to monotherapy regimen.

Synergistic effects and antibiofilm properties of chimeric peptides against multidrug-resistant Acinetobacter baumannii strains

The increasing prevalence of drug-resistant pathogens highlights the need to identify novel antibiotics. Here we investigated the efficacies of four new antimicrobial peptides (AMPs) for potential drug development. The antibacterial activities, synergistic effects, and antibiofilm properties of the four chimeric AMPs were tested against Acinetobacter baumannii, an emerging Gram-negative, nosocomial, drug-resistant pathogen. Nineteen A. baumannii strains resistant to ampicillin, cefotaxime, ciprofloxacin, tobramycin, and erythromycin were isolated at a hospital from patients with cholelithiasis. All four peptides exhibited significant antibacterial effects (MIC=3.12 to 12.5 μM) against all 19 strains, whereas five commercial antibiotics showed little or no activity against the same pathogens. An exception was polymyxin, which was effective against all of the strains tested. Each of the peptides showed synergy against one or more strains when administered in combination with cefotaxime, ciprofloxacin, or erythromycin. The peptides also exhibited an ability to prevent biofilm formation, which was not seen with cefotaxime, ciprofloxacin, or erythromycin, though polymyxin also inhibited biofilm formation. Indeed, when administered in combination with ciprofloxacin, the AMP HPMA exerted a potent synergistic effect against A. baumannii biofilm formation. Collectively, our findings indicate that the AMPs tested have no cytotoxicity but possess potent antibacterial and antibiofilm activities and may act synergistically with commercial antibiotics.

Epidemiology, clinical characteristics and outcomes of extensively drug-resistant Acinetobacter baumannii infections among solid organ transplant recipients

BACKGROUND

Extensively drug-resistant Acinetobacter baumannii (XDR-Ab) has emerged as a major nosocomial pathogen, but optimal treatment regimens are unknown. Although solid organ transplant (SOT) recipients are particularly susceptible to XDR-Ab infections, studies in this population are limited. Our objectives were to determine the epidemiology, clinical characteristics and outcomes of XDR-Ab infections among SOT patients.

METHODS

A retrospective study of SOT recipients at our center who were colonized or infected with XDR-Ab between November 2006 and December 2011 was conducted. Among infected patients, the primary outcome was survival at 28 days. Secondary outcomes included survival at 90 days and clinical success at 28 days, and XDR-Ab infection recurrence.

RESULTS

XDR-Ab was isolated from 69 SOT patients, of whom 41% (28) and 59% (41) were colonized and infected, respectively. Infections were significantly more common among cardiothoracic than abdominal transplant recipients (p=0.0004). Ninety-eight percent (40/41) of patients had respiratory tract infections, most commonly ventilator-associated pneumonia (VAP; 88% [36/41]). Survival rates at 28 and 90 days were 54% (22/41) and 46% (19/41), respectively. Treatment with a colistin-carbapenem regimen was an independent predictor of 28-day survival (p=0.01; odds ratio=7.88 [95% CI: 1.60-38.76]). Clinical success at 28 days was achieved in 49% (18/37) of patients who received antimicrobial therapy, but 44% (8/18) of successes were associated with infection recurrence within 3 months. Colistin resistance emerged in 18% (2/11) and 100% (3/3) of patients treated with colistin-carbapenem and colistin-tigecycline, respectively (p=0.03).

CONCLUSIONS

XDR-Ab causes VAP and other respiratory infections following SOT that are associated with significant recurrence and mortality rates. Cardiothoracic transplant recipients are at greatest risk. Results from this retrospective study suggest that colistin-carbapenem combinations may result in improved clinical responses and survival compared to other regimens and may also limit the emergence of colistin resistance.

In vitro evaluation of antibiotic synergy for polymyxin B-resistant carbapenemase-producing Klebsiella pneumoniae

Since carbapenemase-producing Klebsiella pneumoniae strains were first reported in North Carolina, these highly resistant organisms have been isolated with increasing frequency, especially in the New York City area. Polymyxin B is one of the few antimicrobials that retain reliable activity against these organisms. However, polymyxin B MICs are elevated against K. pneumoniae isolates with increasing frequency, leaving clinicians with few therapeutic options. We investigated several antimicrobial agents for potential synergy with polymyxin B against 12 clinical strains of carbapenemase-producing K. pneumoniae. A broth microdilution assay using a 96-well plate was developed in which graded dilutions of polymyxin B and the study drug were incubated with resistant isolates in a checkerboard pattern. Polymyxin B was studied in combination with cefazolin, ceftriaxone, cefepime, imipenem, gentamicin, tigecycline, doxycycline, and rifampin. All K. pneumoniae strains tested positive for K. pneumoniae carbapenemase (KPC) genes by real-time PCR and had elevated polymyxin B MIC values ranging from 16 to 128 μg/ml. Synergy was observed with the combination of polymyxin B and rifampin as well as with polymyxin B and doxycycline, resulting in at least a 4-fold decrease in the polymyxin B MIC. For both combinations, this effect occurred at physiologically achievable concentrations. Less pronounced synergy was noted with tigecycline and polymyxin B. No synergy was observed at physiologic concentrations with the other antimicrobials studied. These results suggest that rifampin, doxycycline, and tigecycline may be useful additions to polymyxin B in the treatment of infections caused by highly resistant carbapenemase-producing K. pneumoniae. Further studies are warranted to determine if these in vitro findings translate into clinical efficacy.

Multidrug-resistant Gram-negative infections: what are the treatment options?

The emergence of multidrug-resistant (MDR) Gram-negative bacilli creates a challenge in the treatment of nosocomial infections. While the pharmaceutical pipeline is waning, two revived old antibacterials (colistin and fosfomycin), a newer one (tigecycline) and an 'improved' member of an existing class (doripenem) are the only therapeutic options left. The class of polymyxins, known since 1947 and represented mostly by polymyxin B and polymyxin E (colistin), has recently gained a principal role in the treatment of the most problematic MDR Gram-negative pathogens (such as Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae and Stenotrophomonas maltophilia). Future prospective studies are needed to answer important clinical questions, such as the possible benefit of combination with other antimicrobials versus monotherapy, the efficacy of colistin in neutropenic hosts and the role of inhaled colistin. As new pharmacokinetic data emerge, clarification of the pharmacokinetic/pharmacodynamic (PK/PD) profile of colistin as well as appropriate dosing seems urgent, while development of resistance must be carefully monitored. Fosfomycin tromethamine, a synthetic salt of fosfomycin discovered in 1969, has regained attention because of its in vitro activity against extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae and MDR P. aeruginosa. Although in use for decades in oral and parenteral formulations for a variety of infections without significant toxicity, its clinical utility in MDR infections remains to be explored in future studies. Tigecycline, the first representative of the new class of glycylcyclines, holds promise in infections from MDR K. pneumoniae (K. pneumoniae carbapenemase [KPC]- and ESBL-producing strains) and Enterobacteriaceae with various mechanisms of resistance. The in vitro activity of tigecycline against A. baumannii makes it a tempting option, as it is currently the most active compound against MDR strains along with colistin. However, the usual minimum inhibitory concentration values of this pathogen are approximately 2 mg/L and compromise clinical outcomes based on PK/PD issues. Its advantageous penetration into various tissues is useful in infections of the skin and soft tissues as well as intra-abdominal infections (official indications), whereas low serum concentrations compromise its use in bloodstream infections. Therefore, prospective studies with dose escalation are urgently needed, as well as clarification of its role in nosocomial pneumonia, after poor results in the study of ventilator-associated pneumonia. Finally, doripenem, the recently licensed member of the carbapenems (without significant spectrum alterations from the ascendant members) seems to possess a lower potential for resistance selection and a more favourable pharmacokinetic profile when given as an extended infusion. The latter strategy could prove helpful in overcoming low level resistance of A. baumannii and P. aeruginosa strains.

In vitro activity of tigecycline in combination with various antimicrobials against multidrug resistant Acinetobacter baumannii

Background

Infections sustained by multidrug-resistant (MDR) and pan-resistant Acinetobacter baumannii have become a challenging problem in Intensive Care Units. Tigecycline provided new hope for the treatment of MDR A. baumannii infections, but isolates showing reduced susceptibility have emerged in many countries, further limiting the therapeutic options. Empirical combination therapy has become a common practice to treat patients infected with MDR A. baumannii, in spite of the limited microbiological and clinical evidence supporting its efficacy. Here, the in vitro interaction of tigecycline with seven commonly used anti-Acinetobacter drugs has been assessed.

Methods

Twenty-two MDR A. baumannii isolates from Intensive Care Unit (ICU) patients and two reference strains for the European clonal lineages I and II (including 3, 15 and 6 isolates that were resistant, intermediate and susceptible to tigecycline, respectively) were tested. Antimicrobial agents were: tigecycline, levofloxacin, piperacillin-tazobactam, amikacin, imipenem, rifampicin, ampicillin-sulbactam, and colistin. MICs were determined by the broth microdilution method. Antibiotic interactions were determined by chequerboard and time-kill assays. Only antibiotic combinations showing synergism or antagonism in both chequerboard and time-kill assays were accepted as authentic synergistic or antagonistic interactions, respectively.

Results

Considering all antimicrobials in combination with tigecycline, chequerboard analysis showed 5.9% synergy, 85.7% indifference, and 8.3% antagonism. Tigecycline showed synergism with levofloxacin (4 strains; 16.6%), amikacin (2 strains; 8.3%), imipenem (2 strains; 8.3%) and colistin (2 strains; 8.3%). Antagonism was observed for the tigecycline/piperacillin-tazobactam combination (8 strains; 33.3%). Synergism was detected only among tigecycline non-susceptible strains. Time-kill assays confirmed the synergistic interaction between tigecycline and levofloxacin, amikacin, imipenem and colistin for 5 of 7 selected isolates. No antagonism was confirmed by time-kill assays.

Conclusion

This study demonstrates the in vitro synergistic activity of tigecycline in combination with colistin, levofloxacin, amikacin and imipenem against five tigecycline non-susceptible A. baumannii strains, opening the way to a more rationale clinical assessment of novel combination therapies to combat infections caused by MDR and pan-resistant A. baumannii.

Tigecycline in combination with other antimicrobials: a review of in vitro, animal and case report studies

Tigecycline has been investigated in combination with other antibacterials against a wide range of susceptible and multiresistant Gram-positive and Gram-negative bacteria. Combinations have been analysed in vitro, in animal models and in human case reports. In vitro, tigecycline combined with other antimicrobials produces primarily an indifferent response (neither synergy nor antagonism). Nevertheless, synergy occurred when tigecycline was combined with rifampicin against 64-100% of Enterococcus spp., Streptococcus pneumoniae, Enterobacter spp. and Brucella melitensis isolates. Combinations of tigecycline with amikacin also showed synergy for 40-100% of Enterobacter spp., Klebsiella pneumoniae, Proteus spp. and Stenotrophomonas maltophilia isolates. Moreover, bactericidal synergisms occurred with tigecycline plus amikacin against problematic Acinetobacter baumannii and Proteus vulgaris, and with colistin against K. pneumoniae. Data from animal experiments and case reports, although limited, displayed consistent beneficial activity of tigecycline in combination with other antibacterials against multiresistant organisms, including vancomycin against penicillin-resistant S. pneumoniae in experimental meningitis, gentamicin against Pseudomonas aeruginosa in experimental pneumonia, daptomycin against Enterococcus faecium endocarditis, and colistin against K. pneumoniae bacteraemia and P. aeruginosa osteomyelitis. Antagonism was extremely rare in vitro and was not reported in vivo. Thus, tigecycline may be combined with a second antimicrobial as part of a combination regimen.

Acinetobacter baumannii: emergence of a successful pathogen

Acinetobacter baumannii has emerged as a highly troublesome pathogen for many institutions globally. As a consequence of its immense ability to acquire or upregulate antibiotic drug resistance determinants, it has justifiably been propelled to the forefront of scientific attention. Apart from its predilection for the seriously ill within intensive care units, A. baumannii has more recently caused a range of infectious syndromes in military personnel injured in the Iraq and Afghanistan conflicts. This review details the significant advances that have been made in our understanding of this remarkable organism over the last 10 years, including current taxonomy and species identification, issues with susceptibility testing, mechanisms of antibiotic resistance, global epidemiology, clinical impact of infection, host-pathogen interactions, and infection control and therapeutic considerations.

Tigecycline for the treatment of multidrug-resistant (including carbapenem-resistant) Acinetobacter infections: a review of the scientific evidence

OBJECTIVES

New antibacterial agents are required for the treatment of infections caused by multidrug-resistant (MDR) Acinetobacter spp. Whether tigecycline constitutes an effective treatment option or not, is not well established. We sought to evaluate the available evidence regarding the microbiological activity and clinical effectiveness of tigecycline for MDR (including the subset of carbapenem-resistant) Acinetobacter spp.

METHODS

We searched PubMed for relevant articles and extracted/evaluated the available evidence.

RESULTS

We identified 22 microbiological studies reporting data for 2384 Acinetobacter spp. (1906 Acinetobacter baumannii). Susceptibility of at least 90% of the Acinetobacter isolates to tigecycline (with an MIC breakpoint of susceptibility < or =2 mg/L) was noted in 9/18 studies reporting data on MDR Acinetobacter and in 7/15 studies reporting specific data on carbapenem-resistant Acinetobacter. In an additional study reporting data for both resistance categories, adequate susceptibility of Acinetobacter spp. was observed by one (broth microdilution) of the methods employed. The effectiveness of tigecycline for MDR Acinetobacter infections was evaluated in eight identified clinical studies, reporting retrospective data regarding 42 severely ill patients, among whom 31 had respiratory tract infection (in 4 cases with secondary bacteraemia) and 4 had bacteraemia. Tigecycline therapy (in combination with other antibiotics in 28 patients) was effective in 32/42 cases. In three cases, resistance to tigecycline developed during treatment.

CONCLUSIONS

Tigecycline showed considerable, though not consistent, antimicrobial activity against MDR (including carbapenem-resistant) Acinetobacter spp. However, data to support its clinical use, particularly for ventilator-associated pneumonia or bacteraemia, caused by these pathogens, are still limited.