Effect of ceftazidime-avibactam combined with different antimicrobials against carbapenem-resistant Klebsiella pneumoniae

This study aimed to assess the in vitro efficacy of ceftazidime-avibactam (CZA) in combination with various antimicrobial agents against carbapenem-resistant Klebsiella pneumoniae (CRKP). We selected 59 clinical CRKP isolates containing distinct drug resistance mechanisms. The minimum inhibitory concentrations (MICs) of meropenem (MEM), colistin (COL), eravacycline (ERA), amikacin (AK), fosfomycin (FOS), and aztreonam (ATM), both individually and in combination with CZA, were tested using the checkerboard method. The interactions of antimicrobial agent combinations were assessed by fractional inhibitory concentration index (FICI) and susceptible breakpoint index (SBPI). The time-kill curve assay was employed to dynamically evaluate the effects of these drugs alone and in combination format. In the checkerboard assay, the combination of CZA+MEM showed the highest level of synergistic effect against both KPC-producing and carbapenemase-non-producing isolates, with synergy rates of 91.3% and 100%, respectively. Following closely was the combination of FOS+CZA . For metallo-beta-lactamases (MBLs) producing strains, ATM+CZA displayed complete synergy, while the combination of MEM+CZA showed a synergy rate of only 57.14% for NDM-producing strains and 91.67% for IMP-producing strains. In the time-kill assay, MEM+CZA also demonstrated significant synergistic effects against the two KPC-2-producing isolates (Y070 and L70), the two carbapenemase-non-producing isolates (Y083 and L093), and the NDM-1-producing strain L13, with reductions in log10 CFU/mL exceeding 10 compared to the control. Against the IMP-producing strain Y047, ATM+CZA exhibited the highest synergistic effect, resulting in a log10 CFU/mL reduction of 10.43 compared to the control. The combination of CZA and MEM exhibited good synergistic effects against KPC-producing and non-enzyme-producing strains, followed by the FOS+CZA combination. Among MBL-producing strains, ATM+CZA demonstrated the most pronounced synergistic effect. However, the combinations of CZA with ERA, AK, and COL show irrelevant effects against the tested clinical isolates.

IMPORTANCE

Our study confirmed the efficacy of the combination CZA+MEM against KPC-producing and non-carbapenemase-producing strains. For metalloenzyme-producing strains, CZA+ATM demonstrated the most significant synergy. Additionally, CZA exhibited a notable synergy effect when combined with FOS. These combination therapies present promising new options for the treatment of CRKP infection.

Antimicrobial Synergy Testing: Comparing the Tobramycin and Ceftazidime Gradient Diffusion Methodology Used in Assessing Synergy in Cystic Fibrosis-Derived Multidrug-Resistant Pseudomonas aeruginosa

The need for synergy testing is driven by the necessity to extend the antimicrobial spectrum, reducing drug dosage/toxicity and the development of resistance. Despite the abundance of synergy testing methods, there is the absence of a gold standard and a lack of synergy correlation among methods. The most popular method (checkerboard) is labor-intensive and is not practical for clinical use. Most clinical laboratories use several gradient synergy methods which are quicker/easier to use. This study sought to evaluate three gradient synergy methods (direct overlay, cross, MIC:MIC ratio) with the checkerboard, and compare two interpretative criteria (the fractional inhibitory concentration index (FICI) and susceptibility breakpoint index (SBPI)) regarding these methods. We tested 70 multidrug-resistant Pseudomonas aeruginosa, using a tobramycin and ceftazidime combination. The agreement between the checkerboard and gradient methods was 60 to 77% for FICI, while agreements for SBPI that ranged between 67 and 82.86% were statistically significant (p ≤ 0.001). High kappa agreements were observed using SBPI (Ƙ > 0.356) compared to FICI (Ƙ < 0.291) criteria, and the MIC:MIC method demonstrated the highest, albeit moderate, intraclass correlation coefficient (ICC = 0.542) estimate. Isolate resistance profiles suggest method-dependent synergism for isolates, with ceftazidime susceptibility after increased exposure. The results show that when interpretative criteria are considered, gradient diffusion (especially MIC:MIC) is a valuable and practical method that can inform the treatment of cystic fibrosis patients who are chronically infected with P. aeruginosa.

Trends of Antimicrobial Resistance and Combination Susceptibility Testing of Multidrug-Resistant Pseudomonas aeruginosa Isolates from Cystic Fibrosis Patients: a 10-Year Update

Antimicrobial combination therapy is a time/resource-intensive procedure commonly employed in the treatment of cystic fibrosis (CF) pulmonary exacerbations caused by Pseudomonas aeruginosa Ten years ago, the most promising antimicrobial combinations were proposed, but there has since been the introduction of new β-lactam plus β-lactamase inhibitor antimicrobial combinations. The aims of this study were to (i) compare in vitro activity of these new antimicrobials with other antipseudomonal agents and suggest their most synergistic antimicrobial combinations and (ii) determine antimicrobial resistance rates and study inherent trends of antimicrobials over 10 years. A total of 721 multidrug-resistant P. aeruginosa isolates from 183 patients were collated over the study period. Antimicrobial susceptibility and combination testing were carried out using the Etest method. The results were further assessed using the fractional inhibitory concentration index (FICI) and the susceptible breakpoint index (SBPI). Resistance to almost all antimicrobial agents maintained a similar level during the studied period. Colistin (P < 0.001) and tobramycin (P = 0.001) were the only antimicrobials with significant increasing isolate susceptibility, while an increasing resistance trend was observed for levofloxacin. The most active antimicrobials were colistin, ceftolozane-tazobactam, ceftazidime-avibactam, and gentamicin. All combinations with β-lactam plus β-lactamase inhibitors produced some synergistic results. Ciprofloxacin plus ceftolozane-tazobactam (40%) and amikacin plus ceftazidime (36.7%) were the most synergistic combinations, while colistin combinations gave the best median SBPI (50.11). This study suggests that effective fluoroquinolone stewardship should be employed for CF patients. It also presents in vitro data to support the efficacy of novel combinations for use in the treatment of chronic P. aeruginosa infections.

Synergistic Effect of Cefazolin Plus Fosfomycin Against Staphylococcus aureus in vitro and in vivo in an Experimental Galleria mellonella Model

Objectives: This study investigated the synergistic in vitro and in vivo activity of cefazolin plus fosfomycin against methicillin-susceptible and methicillin-resistant S. aureus (MSSA and MRSA) to provide the basis for a potential treatment alternative.

Methods: Antimicrobial susceptibility and in vitro synergy tests were performed with five MSSA and five MRSA isolates using the broth microdilution and chequerboard assays, respectively. The in vivo efficacy of cefazolin plus fosfomycin for the treatment of MRSA infections was assessed using the Galleria mellonella survival assay.

Results: Using fractional inhibitory concentration index (FICI), the evaluated combination of cefazolin plus fosfomycin showed synergistic in vitro activity against all MSSA and MRSA isolates tested. In addition, cefazolin susceptibility was recovered in all MRSA isolates except one fosfomycin-resistant strain when combined with fosfomycin at readily achievable concentrations. The G. mellonella survival assay demonstrated highly synergistic in vivo activity of cefazolin plus fosfomycin, resulting in a 44–52% reduction in mortality when compared to cefazolin-alone and fosfomycin-alone, respectively.

Conclusion: If susceptibility to fosfomycin is either confirmed or can be assumed based on local resistance patterns, combination therapy with cefazolin plus fosfomycin could be a valuable treatment option for empirical as well as targeted therapy of S. aureus and MRSA infections. Future studies proving the clinical significance of this combination therapy are therefore warranted.

Longitudinal Surveillance and Combination Antimicrobial Susceptibility Testing of Multidrug-Resistant Achromobacter Species from Cystic Fibrosis Patients

Achromobacter spp. are recognized as emerging pathogens in patients with cystic fibrosis (CF). Though recent works have established species-level identification using nrdA sequencing, there is a dearth in knowledge relating to species-level antimicrobial susceptibility patterns and antimicrobial combinations, which hampers the use of optimal antimicrobial combinations for the treatment of chronic infections. The aims of this study were to (i) identify at species-level referred Achromobacter isolates, (ii) describe species-level antimicrobial susceptibility profiles, and (iii) determine the most promising antimicrobial combination for chronic Achromobacter infections. A total of 112 multidrug-resistant (MDR) Achromobacter species isolates from 39 patients were identified using nrdA sequencing. Antimicrobial susceptibility and combination testing were carried out using the Etest method. We detected six species of Achromobacter and found that Achromobacter xylosoxidans was the most prevalent species. Interestingly, sequence analysis showed it was responsible for persistent infection (18/28 patients), followed by Achromobacter ruhlandii (2/3 patients). Piperacillin-tazobactam (70.27%) and co-trimoxazole (69.72%) were the most active antimicrobials. Differences were observed in species-level susceptibility to ceftazidime, carbapenems, ticarcillin-clavulanate, and tetracycline. Antimicrobial combinations with co-trimoxazole or tobramycin demonstrate the best synergy, while co-trimoxazole gave the best susceptibility breakpoint index values. This study enriches the understanding of MDR Achromobacter spp. epidemiology and confirms prevalence and chronic colonization of A. xylosoxidans in CF lungs. It presents in vitro data to support the efficacy of new combinations for use in the treatment of chronic Achromobacter infections.

In Vitro Activity of Ceftazidime-Avibactam Alone and in Combination with Amikacin Against Colistin-Resistant Gram-Negative Pathogens

Aims: Colistin became the critical treatment option for multidrug-resistant Gram-negative bacteria (GNB); however, resistance to colistin is increasingly being reported among clinical isolates. New therapy strategies should be considered nowadays. The aim of this study was to investigate the in vitro activity of a novel β-lactam/β-lactamases inhibitor ceftazidime-avibactam (CZA) alone and in combination with amikacin against colistin-resistant Gram-negative pathogens. Results: Among all the colistin-resistant GNB strains, 30.4% (21/69) were resistant to CZA, which was similar to the resistance rate of 25.4% (35/138) in colistin-susceptible strains (p > 0.05), displaying a relatively lower resistance rate compared with other antimicrobial agents (except amikacin). A majority of CZA-resistant GNB isolates (33/56) produced NDM carbapenemase. The fractional inhibitory concentration index method revealed synergistic (47.6%, 10/21) or additive (52.4%, 11/21) effects of CZA in combination with amikacin against colistin- and CZA-resistant GNB isolates, wherein the synergistic activity was found against all tested Klebsiella pneumoniae isolates (four) and Pseudomonas aeruginosa isolates (two). The time-killing curve assay verified the synergistic activity of CZA and amikacin in K. pneumoniae (FK2778) and P. aeruginosa (TL2294). The susceptible breakpoint index values showed that CZA in combination with amikacin reduced the MIC to less than the susceptibility breakpoint among 71.4% (15/21) of all tested strains. Conclusion: CZA may be a new alternative for colistin-resistant Gram-negative infections and pending clinical studies combining CZA with amikacin should be considered against these pathogens, particularly for K. pneumoniae and P. aeruginosa.

Pharmacodynamics Of Linezolid-Plus-Fosfomycin Against Vancomycin-Susceptible And -Resistant Enterococci In Vitro And In Vivo Of A Galleria mellonella Larval Infection Model

Objectives

To explore the in vitro and in vivo antibacterial activity of linezolid/fosfomycin combination against vancomycin-susceptible and -resistant enterococci (VSE and VRE), and provide a theoretical basis for the treatment of VRE.

Methods

The checkerboard method and time-kill curve study were used to evaluate the efficacy of linezolid combined with fosfomycin against VSE and VRE. The transmission electron microscopy (TEM) was employed to observe the cell morphology of bacteria treated with each drug alone or in combination, which further elucidate the mechanism of action of antibiotic combination therapy. The Galleria mellonella infection model was constructed to demonstrate the in vivo efficacy of linezolid plus fosfomycin for VSE and VRE infection.

Results

The fractional inhibitory concentration index (FICI) values of all strains suggested that linezolid showed synergy or additivity in combination with fosfomycin against five of the six strains. Time-kill experiments demonstrated that the combination of linezolid-fosfomycin at 1×MIC or 2×MIC led to higher degree of bacterial killing without regrowth for all isolates tested than each monotherapy. TEM images showed that the combination treatment damaged the bacterial cell morphology more obviously than each drug alone. In the Galleria mellonella infection model, the enhanced survival rate of the combination treatment compared with linezolid monotherapy (P<0.05) was revealed.

Conclusion

Our data manifested that the combination of linezolid and fosfomycin was a potential therapeutic regimen for VRE infection. The combination displayed excellent bacterial killing and inhibited amplification of fosfomycin-resistant subpopulations.

Activity of Ceftazidime-Avibactam Alone and in Combination with Ertapenem, Fosfomycin, and Tigecycline Against Carbapenemase-Producing Klebsiella pneumoniae

The aim of this study was to investigate the synergy between ceftazidime-avibactam, ertapenem, fosfomycin, and tigecycline against carbapenemase-producing Klebsiella pneumoniae using the E test MIC:MIC (minimum inhibitory concentration) ratio synergy method. The results were interpreted using fractional inhibitory concentration index (FICI) to describe the effects of antimicrobial combinations in vitro. To assess the clinical significance of each antibiotic combination, the susceptible breakpoint index (SBPI) was calculated for each combination, and within each strain. The FICI method revealed that the most synergistic combinations against carbapenemase-producing K. pneumoniae were ceftazidime-avibactam with ertapenem and ceftazidime-avibactam with fosfomycin. This effect was demonstrated in 47% (9/19) of all tested clinical K. pneumoniae isolates. Considering the effects of all drug combinations in K. pneumoniae harboring bla_KPC, bla_NDM, and bla_OXA-48 genes, we observed that the combination of ceftazidime-avibactam with fosfomycin was the most synergistic in New Delhi metallo-β-lactamase (NDM)-producing K. pneumoniae, and the combination of ceftazidime-avibactam with ertapenem was the most synergistic in K. pneumoniae carbapenemase (KPC)-producing K. pneumoniae. In addition, all tested combinations were synergistic against oxacillinase (OXA)-48-producing K. pneumoniae, except the combination of ceftazidime-avibactam with tigecycline. The SBPI index showed that ceftazidime-avibactam in combination with fosfomycin reduced the MIC to less than the susceptibility breakpoint among all tested carbapenemase-producing K. pneumoniae. Moreover, the combinations of ceftazidime-avibactam with ertapenem, and ceftazidime-avibactam with tigecycline were able to reduce the MIC to less than the susceptibility breakpoint in all KPC- and OXA-48-producing K. pneumoniae.

Pharmacokinetic-pharmacodynamic modelling to investigate in vitro synergy between colistin and fusidic acid against MDR Acinetobacter baumannii

Objectives

The potential for synergy between colistin and fusidic acid in the treatment of MDR Acinetobacter baumannii has recently been shown. The aim of this study was to perform an extensive in vitro characterization of this effect using pharmacokinetic-pharmacodynamic modelling (PKPD) of time–kill experiments in order to estimate clinical efficacy.

Methods

For six clinical strains, 312 individual time–kill experiments were performed including 113 unique pathogen–antimicrobial combinations. A wide range of concentrations (0.25–8192 mg/L for colistin and 1–8192 mg/L for fusidic acid) were explored, alone and in combination. PKPD modelling sought to quantify synergistic effects.

Results

A PKPD model confirmed synergy in that colistin EC₅₀ was found to decrease by 83% in the presence of fusidic acid, and fusidic acid maximum increase in killing rate (E_max) also increased 58% in the presence of colistin. Simulations indicated, however, that at clinically achievable free concentrations, the combination may be bacteriostatic in colistin-susceptible strains, but growth inhibition probability was <20% in a colistin-resistant strain.

Conclusions

Fusidic acid may be a useful agent to add to colistin in a multidrug combination for MDR Acinetobacter baumannii.

The FICI paradigm: Correcting flaws in antimicrobial in vitro synergy screens at their inception

Antibiotics have become the corner stone of modern medicine. However, our society is currently facing one of the greatest challenges of its time: the emergence of antimicrobial resistance. It is estimated that if no new therapies are implemented by 2050, 10 million people will die worldwide every year as a result of infections caused by bacteria resistant to current antibiotics; new antimicrobials are thus urgently needed. However, drug development is a tedious and very costly endeavor of hundreds of millions that can take up to 15-20 years from the bench discovery to the bedside. Under this scenario, drug repurposing, which consists in identifying new uses for old, clinically approved drugs, has gathered momentum within the pharmaceutical industry. Because most of these drugs have safety and toxicity information packages available, clinical evaluation could be done in a much shorter period than standard timelines. Synergistic combinations of these clinically approved drugs could also be a promising approach to identify novel antimicrobial therapies that might provide rational choices of available drugs to shorten treatment, increase efficacy, reduce toxicity, prevent resistance and treat infections caused by drug-resistant strains. However, although simple in its conception, translating results from in vitro synergy screens into in vivo efficacy or the clinical practice has proven to be a paramount challenge. In this Commentary, we will discuss common flaws at the inception of synergy research programs, with a special focus on the use of the Fractional Inhibitory Concentration Index (FICI), and evaluate potential interventions that can be made at different developmental pre-clinical stages in order to improve the odds of translation from in vitro studies.

Synergistic effect of linezolid with fosfomycin against Staphylococcus aureus in vitro and in an experimental Galleria mellonella model

BACKGROUND/PURPOSES

Treatment of Staphylococcus aureus infections is challenging owing to widespread multidrug resistance. There is now considerable interest in the potential of combination therapies. Although linezolid/fosfomycin combination appears to be a promising treatment option based on in vitro data, further preclinical work is needed. In this study, the 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 S. aureus infections.

METHODS

The antimicrobial activity of linezolid and fosfomycin alone and in combination was assessed versus four S. aureus. Synergy studies were performed using the microtitre plate chequerboard assay and time-kill methodology. The in vivo activity of linezolid/fosfomycin combination was assessed using a G. mellonella larvae model.

RESULTS

The combination of linezolid and fosfomycin was synergistic and bacteriostatic against four tested strains. Treatment of G. mellonella larvae infected with lethal doses of S. aureus resulted in significantly enhanced survival rates when low-dose of combination has no significant differences with high-dose combination (P > 0.05), G. mellonella hemolymph burden of S. aureus suggest that combination therapy with rapid and sustained bacteriostatic activity compared monotherapy.

CONCLUSION

This work indicated that linezolid combination with fosfomycin has synergistic effect against S. aureus in vitro and in an experimental G. mellonella model, and it suggests that high-dose of linezolid and fosfomycin may not necessary.

Combination antimicrobial susceptibility testing of Burkholderia cepacia complex: significance of species

The Burkholderia cepacia complex (Bcc) is notorious for the life-threatening pulmonary infections it causes in patients with cystic fibrosis. The multidrug-resistant nature of Bcc and differing infective Bcc species make the design of appropriate treatment regimens challenging. Previous synergy studies have failed to take account of the species of Bcc isolates. Etest methodology was used to facilitate minimum inhibitory concentration (MIC) and antimicrobial combination testing on 258 isolates of Bcc, identified to species level by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF/MS). The most active antimicrobials were trimethoprim/sulphamethoxazole, doxycycline and minocycline (52.5%, 46.4% and 45.9% of isolates susceptible, respectively). Synergy was observed in 9.2% of the 1799 combinations tested; the most common synergistic combinations were tobramycin + ceftazidime, meropenem + tobramycin and levofloxacin + piperacillin/tazobactam (35.4%, 32.3% and 22.2% synergy, respectively). Antimicrobial susceptibility analysis revealed differences between Burkholderia cenocepacia and Burkholderia multivorans. Disparity in clinical outcome during infection with these two micro-organisms necessitates further investigation into the clinical outcomes of treatment regimens in light of species identification and in vitro antimicrobial susceptibility studies.

Investigation of C-5 alkynyl (alkynyloxy or hydroxymethyl) and/or N-3 propynyl substituted pyrimidine nucleoside analogs as a new class of antimicrobial agents

The resurgence of mycobacterial infections and the emergence of drug-resistant strains urgently require a new class of agents that are distinct than current therapies. A group of 5-ethynyl (6-10), 5-(2-propynyloxy) (16, 18, 20, 22, 24), 5-(2-propynyloxy)-3-N-(2-propynyl) (17, 19, 21, 23, 25) and 5-hydroxymethyl-3-N-(2-propynyl) (30-33) derivatives of pyrimidine nucleosides were synthesized and evaluated against mycobacteria [Mycobacterium tuberculosis (Mtb), Mycobacterium bovis (BCG) and Mycobacterium avium], gram-positive bacteria (Staphylococcus aureus and Enterococcus faecalis) and gram-negative bacteria (Escherichia coli, Salmonella typhimurium and Pseudomonas aeruginosa) alone and in combination with existing drugs in in vitro assays. Although several compounds exhibited marked inhibitory activity at a higher concentration against Mtb, M. bovis, S. aureus and E. faecalis, they displayed unexpected synergistic and additive interactions at their lower concentrations with antitubercular drugs isoniazid and rifampicin, and antibacterial drug gentamicin. The active analogues were also found to inhibit intracellular Mtb in a human monocytic cell line infected with H37Ra. Oral administration of 5-hydroxymethyl-3-N-(2-propynyl)-3'-azido-2',3'-dideoxyuridine (32) and 5-hydroxymethyl-3-N-(2-propynyl)-2',3'-dideoxyuridine (33) at a dose of 100mg/kg for two weeks showed promising in vivo effects in mice infected with Mtb (H37Ra). No in vitro cytotoxicity of the test compounds was observed up to the highest concentration tested (CC50>300μg/mL).

Enhanced efficacy of imipenem-colistin combination therapy against multiple-drug-resistant Enterobacter cloacae: in vitro activity and a Galleria mellonella model

BACKGROUND/PURPOSE

To investigate the in vitro and in vivo activity of imipenem-colistin combination against multidrug-resistant Enterobacter cloacae infections in order to determine whether it should be explored further.

METHODS

The antimicrobial activity of colistin alone and in combination with imipenem was assessed versus an imipenem-susceptible isolate, E. cloacae GN1059, or an imipenem-resistant strain, E. cloacae GN0791, isolated in Anhui, China. The potential synergy of imipenem-colistin was evaluated using a checkerboard assay, as well as static time-kill experiments at 1× and 2× minimum inhibitory concentration (MIC). A simple invertebrate model (Galleria mellonella) was developed to assess the in vivo efficacy of imipenem-colistin in treating E. cloacae infection.

RESULTS

In checkerboard assays, synergy (defined as a fractional inhibitory concentration index of ≤ 0.5) was observed between imipenem and colistin for both isolates tested. In time-kill assays, the combination of imipenem-colistin at 1× or 2× MIC resulted in complete killing of both strains. In the G. mellonella larvae model infected with lethal doses of E. cloacae, the combination therapy led to significantly increased survival of the larvae as compared with imipenem or colistin monotherapy alone (p < 0.05).

CONCLUSION

This is the first report demonstrating the efficacy of antimicrobial agents in the G. mellonella larvae model of infections caused by E. cloacae. Our study suggested that imipenem-colistin combination was highly active against E. cloacae both in vitro and in the simple invertebrate model, and provided preliminary in vivo evidence that such combination might be useful therapeutically.

Should daptomycin-rifampin combinations for MSSA/MRSA isolates be avoided because of antagonism?

PURPOSE

There is increasing clinical evidence from observational studies, that combination therapy of daptomycin with rifampin is a valuable treatment option for biofilm-associated difficult to treat Staphylococcus aureus infections such as osteomyelitis, prosthetic joint infection and endocarditis. However, two studies analyzing a limited number of S. aureus isolates reported an antagonism of those two drugs questioning the benefit of this combination.

METHODS

To estimate the frequency of this possible antagonism, we performed in vitro checkerboard assays on 58 consecutive clinical isolates of S. aureus (MSSA n = 9, MRSA n = 49). We determined the fractional inhibitory concentration index (FICI) and the susceptible breakpoint index (SBPI). All isolates were characterized by a microprobe array detecting 336 different genes/alleles to ensure their non-clonal origin.

RESULTS

For all isolates, the FICI was between 1.00 and 1.25 indicating additive effects for the daptomycin/rifampin combination. Neither antagonism nor synergism as defined by the FICI was found for any of the isolates.

CONCLUSION

Based on these data, there is no evidence to advise against the daptomycin/rifampin combination therapy.

Activity of levofloxacin in combination with colistin against Acinetobacter baumannii: In vitro and in a Galleria mellonella model

BACKGROUND/PURPOSE

Treatment of Acinetobacter baumannii infections is challenging owing to widespread multidrug-resistant A. baumannii (MDR-AB) and the lack of novel agents. Although recent data suggest that levofloxacin (LVX) may have unique activity against MDR-AB in combination with colistin (CST), further preclinical work is needed.

METHODS

We used a A. baumannii type strain ATCC19606, a CST-resistant strain AB19606R, and two clinical isolates (GN0624 and GN1115) of MDR-AB to investigate the in vitro and in vivo efficacy of LVX-CST combination. Synergy studies were performed using the microtiter plate chequerboard assay and time-kill methodology. Inhibitory activity of antibiotics against biofilms and the mutant prevention concentrations were also studied in vitro. A simple invertebrate model (Galleria mellonella) has been used to assess the in vivo activity of antimicrobial therapies.

RESULTS

The LVX-CST combination was bactericidal against the CST-susceptible clinical isolate (GN0624). In checkerboard assays, synergy (defined as a fractional inhibitory concentration index of < 0.5) was observed between CST and LVX in GN0624. The combination had antibiofilm properties on the preformed biofilms of four tested strains and could prevent the emergence of CST-resistant A. baumanni. Treatment of G. mellonella larvae infected with lethal doses of A. baumannii resulted in significantly enhanced survival rates when LVX was given with CST compared with CST treatment alone (p < 0.05).

CONCLUSION

In summary, a synergistic or additive effect between CST and LVX was observed in vitro and in vivo against CST-susceptible A. baumannii strains, although not against CST-resistant ones.

Colistin and Fusidic Acid, a Novel Potent Synergistic Combination for Treatment of Multidrug-Resistant Acinetobacter baumannii Infections

The spread of multidrug-resistant Acinetobacter baumannii (MDRAB) has led to the renaissance of colistin (COL), often the only agent to which MDRAB remains susceptible. Effective therapy with COL is beset with problems due to unpredictable pharmacokinetics, toxicity, and the rapid selection of resistance. Here, we describe a potent synergistic interaction when COL was combined with fusidic acid (FD) against A. baumannii. Synergy in vitro was assessed against 11 MDRAB isolates using disc diffusion, checkerboard methodology (fractional inhibitory concentration index [FICI] of ≤ 0.5, susceptibility breakpoint index [SBPI] of >2), and time-kill methodology (≥2 log10 CFU/ml reduction). The ability of FD to limit the emergence of COL resistance was assessed in the presence and absence of each drug alone and in combination. Synergy was demonstrated against all strains, with an average FICI and SBPI of 0.064 and 78.85, respectively. In time-kill assays, COL-FD was synergistic and rapidly bactericidal, including against COL-resistant strains. Fusidic acid prevented the emergence of COL resistance, which was readily selected with COL alone. This is the first description of a novel COL-FD regimen for the treatment of MDRAB. The combination was effective at low concentrations, which should be therapeutically achievable while limiting toxicity. Further studies are warranted to determine the mechanism underlying the interaction and the suitability of COL-FD as an unorthodox therapy for the treatment of multidrug-resistant Gram-negative infections.

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.

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.

In vitro activity of curcumin in combination with epigallocatechin gallate (EGCG) versus multidrug-resistant Acinetobacter baumannii

BACKGROUND

Acinetobacter baumannii is an opportunistic human pathogen often associated with life-threatening infections in the immunocompromised and the critically ill. Strains are often multidrug-resistant (MDR) and due to the lack of new synthetic antimicrobials in development for treatment, attention is increasingly focused on natural compounds either as stand-alone or adjunctive agents. Curcumin (CCM) is a natural polyphenol found in turmeric and isolated from the plant, Curcuma longa. Curcumin has been found to possess many biological properties, including antibacterial activity. In this study the antimicrobial activity of CCM and synergistic effects with epigallocatechin gallate (EGCG) against multidrug-resistant strains of A. baumannii were investigated and assessed via checkerboard and time-kill assays.

RESULTS

The MIC of CCM was >256 μg/mL against all strains of A. baumannii whilst those for EGCG ranged from 128-1024 μg/mL. In checkerboard studies synergy was observed against 5/9 isolates, with an additive effect noted in the remaining 4. The addition of EGCG reduced the MIC of CCM by 3- to 7-fold, with the greatest interaction resulting in a CCM MIC of 4 μg/mL. Time-kill curves indicated that a CCM-EGCG (1:8 and 1:4) combination was bactericidal with a 4 to 5-log reduction in viable counts after 24 h compared to the most effective polyphenol alone.

CONCLUSIONS

This study demonstrates that despite little antibacterial activity alone, CCM activity is greatly enhanced in the presence of EGCG resulting in antibacterial activity against MDR A. baumannii. The combination may have a potential use in medicine as a topical agent to prevent or treat A. baumannii infections.

Activity of colistin in combination with tigecycline or rifampicin against multidrug-resistant Stenotrophomonas maltophilia

The antimicrobial treatment of Stenotrophomonas maltophilia infections is complicated by intrinsic multidrug resistance and a lack of reliable susceptibility data. We assessed the activity of colistin (COL), rifampicin (RIF) and tigecycline (TGC) alone and in combination using a range of in vitro susceptibility testing methodologies and a simple invertebrate model of S. maltophilia infection (Galleria mellonella). Synergy [fractional inhibitory concentration indices (FICIs) ≤0.5] between COL and either RIF or TGC was observed against 92 % and 88 % of 25 S. maltophilia isolates, respectively, despite resistance to one or another of the single agents alone. In time-kill assays, COL combined with either RIF or TGC was superior to single agents, but only the COL/RIF regimen was reliably bactericidal. The in vitro findings correlated with treatment outcomes in G. mellonella, with heightened survival observed for larvae treated with COL/RIF or COL/TGC compared with COL, RIF or TGC alone. COL combined with RIF was the most effective combination overall in both in vitro and in vivo (p < 0.05) assays. Given the difficulty in selecting appropriate therapy for S. maltophilia infections, regimens consisting of COL combined with RIF or TGC could be considered for clinical use.

In vitro and in vivo activities of tigecycline-colistin combination therapies against carbapenem-resistant Enterobacteriaceae

We assessed the activity of tigecycline (TGC) combined with colistin (COL) against carbapenem-resistant enterobacteria. Synergy occurred in vitro against the majority of isolates, with the exception of Serratia marcescens. In a simple animal model (Galleria mellonella), TGC-COL was superior (P < 0.01) in treating Escherichia coli, Klebsiella pneumoniae, and Enterobacter infections, including those with TGC-COL resistance. Clinical studies are needed to determine whether TGC-COL regimens may be a viable option.

Systematic review and meta-analysis of in vitro synergy of polymyxins and carbapenems

Our objective was to examine the evidence of in vitro synergy of polymyxin-carbapenem combination therapy against Gram-negative bacteria (GNB). A systematic review and meta-analysis were performed. All studies examining in vitro interactions of antibiotic combinations consisting of any carbapenem with colistin or polymyxin B against any GNB were used. A broad search was conducted with no language, date, or publication status restrictions. Synergy rates, defined as a fractional inhibitory concentration index of ≤0.5 or a >2-log reduction in CFU, were pooled separately for time-kill, checkerboard, and Etest methods in a mixed-effect meta-analysis of rates. We examined whether the synergy rate depended on the testing method, type of antibiotic, bacteria, and resistance to carbapenems. Pooled rates with 95% confidence intervals (CI) are shown. Thirty-nine published studies and 15 conference proceeding were included, reporting on 246 different tests on 1,054 bacterial isolates. In time-kill studies, combination therapy showed synergy rates of 77% (95% CI, 64 to 87%) for Acinetobacter baumannii, 44% (95% CI, 30 to 59%) for Klebsiella pneumoniae, and 50% (95% CI, 30 to 69%) for Pseudomonas aeruginosa, with low antagonism rates for all. Doripenem showed high synergy rates for all three bacteria. For A. baumannii, meropenem was more synergistic than imipenem, whereas for P. aeruginosa the opposite was true. Checkerboard and Etest studies generally reported lower synergy rates than time-kill studies. The use of combination therapy led to less resistance development in vitro. The combination of a carbapenem with a polymyxin against GNB, especially A. baumannii, is supported in vitro by high synergy rates, with low antagonism and less resistance development. These findings should be examined in clinical studies.

In vivo efficacy of telavancin/colistin combination therapy in a Galleria mellonella model of Acinetobacter baumannii infection

Treatment of Acinetobacter baumannii infections is challenging owing to widespread multidrug resistance and the lack of novel agents. There is now considerable interest in the potential of unorthodox combination therapies such as colistin and glycopeptides (e.g. vancomycin and teicoplanin), since potent synergy can be demonstrated in vitro. A simple invertebrate model (Galleria mellonella) has been developed to assess the in vivo activity of antimicrobial therapies and was used to investigate the efficacy of colistin combined with the lipoglycopeptide telavancin in the treatment of A. baumannii infection. Galleria mellonella larvae were inoculated with 10⁵ CFU/larvae of A. baumannii type strain ATCC 19606 or the multidrug-resistant clinical isolate AB210. Infected caterpillars were treated with either telavancin (10 mg/kg), colistin (2.5 mg/kg) or a telavancin/colistin combination. Larvae were incubated at 37 °C for up to 96 h and were scored daily. Survival curves were plotted and analysed using the log-rank test. The telavancin/colistin combination was effective in the treatment of larvae infected with both strains but was superior to colistin monotherapy in the treatment of A. baumannii AB210 (P<0.001). The combination of telavancin and colistin was effective in a simple invertebrate model of A. baumannii infection. This is in agreement with a previous in vitro study and provides preliminary in vivo evidence that such a combination might be useful therapeutically.

Antimicrobial susceptibility and synergy studies of cystic fibrosis sputum by direct sputum sensitivity testing

Standard disc diffusion antimicrobial susceptibility testing (C+S) on individual Pseudomonas aeruginosa colonial morphotypes cultured from cystic fibrosis (CF) sputum has questionable clinical relevance. Direct sputum sensitivity testing (DSST) is a whole-sputum susceptibility test that removes bias associated with selecting individual colonial morphotypes. We sought to determine whether, in principle, the results from DSST support the possibility of improved clinical relevance compared with C+S. Individual (DSSTi) and combination (DSST) susceptibility to gentamicin, tobramycin, ceftazidime and meropenem were determined on 130 sputum samples referred from CF subjects with antibiotic-resistant chronic Gram-negative endobronchial infection. DSSTi and concurrent C+S were compared for categorical susceptibility, synergistic combinations were evaluated and the combination DSST efficacy index (DEI) calculated. Meropenem and tobramycin were the most active individual antibiotics by DSSTi on 89 P. aeruginosa-predominant samples, with 62 % of samples sensitive to each. C+S and DSSTi showed poor agreement (κ ranging from 0.02 to 0.6), discordance ranging from 20 % (meropenem) to 49 % (tobramycin), with DSSTi demonstrating both increased susceptibility and increased resistance. The combination that most frequently had the highest DEI was tobramycin + meropenem, occurring in 76 % of samples. DSSTi appears to be reproducible, yields different antimicrobial susceptibility results from C+S without simply identifying the most resistant isolates and DSST identifies the most effective in vitro antibiotic combinations, providing preliminary proof of concept of the potentially improved clinical relevance of whole-sputum testing. Future studies will determine whether these potential theoretical advantages translate into clinical benefits.

Combination antimicrobial susceptibility testing of multidrug-resistant Stenotrophomonas maltophilia from cystic fibrosis patients

Stenotrophomonas maltophilia is increasingly being isolated from the respiratory tract of individuals with cystic fibrosis, and, because of its multidrug-resistant nature, the selection of suitable treatment regimens can be problematical. Etest methodology was used to facilitate MIC and antimicrobial combination testing on 80 isolates of S. maltophilia cultured from the respiratory tract of Scottish individuals with cystic fibrosis between 2001 and 2010. The overall rate of susceptibility for the 1,410 MIC tests was 23.1%, and resistance was 68.9%. The most active antimicrobials were minocycline, co-trimoxazole, and doxycycline, with 92.4%, 87.3%, and 58.8% of isolates being susceptible, respectively. Of the 517 combinations, 13.2% were synergistic, with the most synergistic being ticarcillin/clavulanate plus aztreonam (91.7% synergistic), ticarcillin/clavulanate plus colistin (40%), and ticarcillin/clavulanate plus levofloxacin (19.4%). Colistin plus tobramycin was the only antagonistic combination (0.2%). By the median susceptible breakpoint index, the most active combinations were minocycline plus co-trimoxazole (median index, 20), minocycline plus piperacillin-tazobactam (median, 20), and co-trimoxazole plus ceftazidime (median, 16.5). The increasing problem of multidrug resistance in organisms recovered from the respiratory tracts of individuals with cystic fibrosis is not going to go away. Current susceptibility testing methods do not address the slow-growing organisms associated with chronic infection, and interpretive standards are based on achievable blood levels of antimicrobials. Addressing these issues specifically for organisms recovered from the respiratory tracts of individuals with cystic fibrosis should lead to better therapeutic outcomes and improved wellbeing of individuals with cystic fibrosis.

In vitro activity of telavancin in combination with colistin versus Gram-negative bacterial pathogens

The treatment of Gram-negative infections is increasingly compromised by the spread of resistance. With few agents currently in development, clinicians are now considering the use of unorthodox combination therapies for multidrug-resistant strains. Here we assessed the in vitro activity of the novel lipoglycopeptide telavancin (TLV) when combined with colistin (COL) versus 13 Gram-negative type strains and 66 clinical isolates. Marked synergy was observed in either checkerboard (fractional inhibitory concentration index [FICI], <0.5; susceptibility breakpoint index [SBPI], >2) or time-kill assays (>2-log reduction in viable counts compared with starting inocula at 24 h) versus the majority of COL-susceptible enterobacteria, Stenotrophomonas maltophilia, and Acinetobacter baumannii isolates, but only limited effects were seen against Pseudomonas aeruginosa or strains with COL resistance. Using an Etest/agar dilution method, the activity of TLV was potentiated by relatively low concentrations of COL (0.25 to 0.75 μg/ml), reducing the MIC of TLV from >32 μg/ml to ≤ 1 μg/ml for 35% of the clinical isolates. This provides further evidence that glycopeptide-polymyxin combinations may be a useful therapeutic option in the treatment of Gram-negative infections.

Novel concepts in evaluating antimicrobial therapy for bacterial lung infections in patients with cystic fibrosis

Cystic fibrosis (CF) patients suffer typically from bacterial infections of their airways. Whilst current antibiotic-based treatment of these infections has brought much benefit to patients, it has been difficult to make either direct or indirect assessments of the in vivo efficacy of any specific treatment used. Traditional culture-based assessment has for example been rarely used to determine the direct impact of therapy on the bacteria in the airways. Instead, the "success" of a treatment is most often gauged through measures of respiratory and general health. New culture-independent approaches though are emerging that offer much promise here however in allowing a more comprehensive evaluation of antimicrobial efficacy. These new methods offer an opportunity to examine bacterial outcomes rather than host outcomes alone. Application of these novel techniques in a systematic way will lead to the rationalisation and, likely greater still individualisation, of therapy for CF patients. This review discusses host and microbiological factors that may influence antibiotic efficacy. Moreover, the degree to which the inherent complexity of CF respiratory infections complicates the process of determining treatment impact and the need to identify more robust microbiological outcome measures will also be reviewed.

In vitro activity of teicoplanin combined with colistin versus multidrug-resistant strains of Acinetobacter baumannii

OBJECTIVES

Antimicrobial treatment of multidrug-resistant Acinetobacter baumannii (MDRAB) remains an important therapeutic challenge. With isolates resistant to all conventional agents now reported, clinicians are increasingly forced to turn to unorthodox combination treatments in the hope that these may be efficacious. Although a potent interaction between vancomycin and colistin has been demonstrated, there are concerns regarding the inherent toxicity of combining these agents in clinical practice. As teicoplanin has less nephrotoxic potential than vancomycin, we assessed whether a colistin/teicoplanin combination would have similar antimicrobial activities in vitro.

METHODS

The antimicrobial activity of colistin alone and in combination with teicoplanin was assessed versus a collection of MDRAB belonging to a number of epidemic lineages present in the UK. Synergy studies were undertaken using microtitre plate chequerboard assays, an Etest agar dilution method and standard time-kill methodology.

RESULTS

The combination of teicoplanin and colistin was bactericidal versus all of the strains tested. In chequerboard assays, fractional inhibitory concentration indices of <0.5 were obtained, consistent with significant in vitro synergy. Using the Etest method the MIC of teicoplanin fell from >256 mg/L to ≤2 mg/L in the presence of subinhibitory concentrations of colistin.

CONCLUSIONS

Significant synergy was observed when colistin was combined with teicoplanin versus MDRAB in vitro. This may represent a useful therapeutic combination for the treatment of A. baumannii infections, especially when renal toxicity is a significant concern.

Current concepts in antimicrobial therapy against resistant gram-negative organisms: extended-spectrum beta-lactamase-producing Enterobacteriaceae, carbapenem-resistant Enterobacteriaceae, and multidrug-resistant Pseudomonas aeruginosa

The development of antimicrobial resistance among gram-negative pathogens has been progressive and relentless. Pathogens of particular concern include extended-spectrum β-lactamase-producing Enterobacteriaceae, carbapenem-resistant Enterobacteriaceae, and multidrug-resistant Pseudomonas aeruginosa. Classic agents used to treat these pathogens have become outdated. Of the few new drugs available, many have already become targets for bacterial mechanisms of resistance. This review describes the current approach to infections due to these resistant organisms and elaborates on the available treatment options.

Potent synergy and sustained bactericidal activity of a vancomycin-colistin combination versus multidrug-resistant strains of Acinetobacter baumannii

Multidrug-resistant Acinetobacter baumannii (MDRAB) presents an increasing challenge to health care. Although colistin has been used as a treatment of last resort, there is concern regarding its potential for toxicity and the emergence of resistance. The mechanism of action of colistin, however, raises the possibility of synergy with compounds that are normally inactive against Gram-negative organisms by virtue of the impermeability of the bacterial outer membrane. This study evaluated the effect of colistin combined with vancomycin on 5 previously characterized epidemic strains and 34 MDRAB clinical isolates by using time-kill assay, microdilution, and Etest methods. For all the isolates, significant synergy was demonstrated by at least one method, with reductions in the MIC of vancomycin from >256 μg/ml to ≤48 μg/ml for all strains after exposure to 0.5 μg/ml colistin. This raises the possibility of the clinical use of this combination for infections due to MDRAB, with the potential for doses lower than those currently used.

Early Pseudomonas aeruginosa infection in individuals with cystic fibrosis: is susceptibility testing justified?

OBJECTIVES

To test the presumption that Pseudomonas aeruginosa isolates responsible for initial lung infection in individuals with cystic fibrosis (CF) are invariably susceptible to antipseudomonal agents.

METHODS

Antibiotic susceptibility was determined (MIC and Etest) in two populations of P. aeruginosa associated with initial lung infection. Population 1: environmental isolates (n=78). Population 2: clinical isolates responsible for first infection in previously non-infected patients (85 isolates from 85 patients). Susceptibility or resistance was determined using current BSAC guidelines; ninth version (2009).

RESULTS

The majority (≥ 90%) of isolates in both bacterial populations were susceptible to the front-line antipseudomonal agents; colistin, ciprofloxacin, tobramycin, ceftazidime, amikacin and meropenem. Up to 10% of isolates were resistant to one or more antibiotics. A single isolate from each population would be defined as resistant to tobramycin based on a breakpoint (>128 mg/L) that has been suggested for use in patients receiving inhaled therapy.

CONCLUSIONS

The high prevalence of susceptibility found in P. aeruginosa isolates associated with initial infection contrasts with the high prevalence of resistance found in isolates from chronic CF lung infection. However, susceptibility in early isolates cannot be presumed. Until further data are obtained from clinically based studies, susceptibility tests should continue to be performed to assist the choice of antibiotics for treatment of early infection.