In Vitro Synergistic Antimicrobial Activity of Combinations of Meropenem, Colistin, Tigecycline, Rifampin, and Ceftolozane/Tazobactam Against Carbapenem-Resistant Acinetobacter Baumannii

The aim of this study was to investigate the in vitro activity of various antimicrobial combinations against carbapenem-resistant Acinetobacter baumannii (CRAB) isolates producing OXA-23 carbapenemases.In vitro activity of six two-drug combinations against CRAB isolates collected from patients with CRAB bacteremia was evaluated using the checkerboard method and time-kill assay [0.5 ×, 1 ×, 2 × minimum inhibitory concentrations (MIC)], to identify potential synergistic and bactericidal two-drug combinations against CRAB isolates, using meropenem, colistin, tigecycline, rifampin, and ceftolozane/tazobactam. All 10 CRAB isolates in our study carried the OXA-58-type and OXA-23-type carbapenem-hydrolyzing oxacillinase. The colistin-ceftolozane/tazobactam combination demonstrated a synergistic effect in both the time-kill assay (using an antibiotic concentration of 1 × MIC) and the checkerboard method, while simultaneously showing a bactericidal effect in the time-kill assay. For all 10 CRAB isolates, time-kill curves showed a significant synergistic bactericidal activity of the colistin-ceftolozane/tazobactam combination at 0.5 × MIC. Overall, there is substantial discordance of synergistic activity between the checkerboard microdilution and time-kill assay (with a concordance of 35%). Our study demonstrated that the two-drug combinations of colistin and ceftolozane/tazobactam can be a potential alternative for treating CRAB infections. The effect of these antibiotic combinations should be evaluated through clinical trials.

Systematic Review of Antimicrobial Combination Options for Pandrug-Resistant Acinetobacter baumannii

Antimicrobial combinations are at the moment the only potential treatment option for pandrug-resistant A. baumannii. A systematic review was conducted in PubMed and Scopus for studies reporting the activity of antimicrobial combinations against A. baumannii resistant to all components of the combination. The clinical relevance of synergistic combinations was assessed based on concentrations achieving synergy and PK/PD models. Eighty-four studies were retrieved including 818 eligible isolates. A variety of combinations (n = 141 double, n = 9 triple) were tested, with a variety of methods. Polymyxin-based combinations were the most studied, either as double or triple combinations with cell-wall acting agents (including sulbactam, carbapenems, glycopeptides), rifamycins and fosfomycin. Non-polymyxin combinations were predominantly based on rifampicin, fosfomycin, sulbactam and avibactam. Several combinations were synergistic at clinically relevant concentrations, while triple combinations appeared more active than the double ones. However, no combination was consistently synergistic against all strains tested. Notably, several studies reported synergy but at concentrations unlikely to be clinically relevant, or the concentration that synergy was observed was unclear. Selecting the most appropriate combinations is likely strain-specific and should be guided by in vitro synergy evaluation. Furthermore, there is an urgent need for clinical studies on the efficacy and safety of such combinations.

Chemotherapy and Mechanisms of Action of Antimicrobial Agent

Pseudomonas aeruginosa is a widespread opportunistic pathogen that causes bloodstream, urinary tract, burn wounds infections and is one of the largest pathogens that infect cystic fibrosis patients’ airways and can be life-threatening for P. aeruginosa infections. In addition, P. aeruginosa remains one of the most significant and difficult nosocomial pathogens to handle. Increasingly, multi-drug resistance (MDR) strains are identified and the option of therapy is often very limited in these cases, particularly when searching for antimicrobial combinations to treat serious infections. The fact that no new antimicrobial agents are active against the MDR strains of P. aeruginosa is an additional matter of concern. In recent decades, bacterial drug resistance has increased, but the rate of discovery of new antibiotics has decreased steadily. The fight for new, powerful antibacterial agents has therefore become a top priority. This chapter illustrates and explores the current state of several innovative therapeutic methods that can be further discussed in clinical practice in the treatment of P. aeruginosa infections.

Losing the Battle but Winning the War: Can Defeated Antibacterials Form Alliances to Combat Drug-Resistant Pathogens?

Despite the recent development of antibacterials that are active against multidrug-resistant pathogens, drug combinations are often necessary to optimize the killing of difficult-to-treat organisms. Antimicrobial combinations typically are composed of multiple agents that are active against the target organism; however, many studies have investigated the potential utility of combinations that consist of one or more antibacterials that individually are incapable of killing the relevant pathogen. The current review summarizes in vitro, in vivo, and clinical studies that evaluate combinations that include at least one drug that is not active individually against Pseudomonas aeruginosa, Klebsiella pneumoniae, Acinetobacter baumannii, or Staphylococcus aureus. Polymyxins were often included in combinations against all three of the Gram-negative pathogens, and carbapenems were commonly incorporated into combinations against K. pneumoniae and A. baumannii. Minocycline, sulbactam, and rifampin were also frequently investigated in combinations against A. baumannii, whereas the addition of ceftaroline or another β-lactam to vancomycin or daptomycin showed promise against S. aureus with reduced susceptibility to vancomycin or daptomycin. Although additional clinical studies are needed to define the optimal combination against specific drug-resistant pathogens, the large amount of in vitro and in vivo studies available in the literature may provide some guidance on the rational design of antibacterial combinations.

In vitro efficacy of gentamicin against multidrug-resistant Neisseria gonorrhoeae: synergy of three gentamicin antimicrobial combinations

ABSTRACTObjectivesTo determine in vitro activities of gentamicin alone and in combination with ceftriaxone, ertapenem and azithromycin against multidrug-resistant (MDR) N. gonorrhoeae isolates.Methods407 isolates from Nanjing, China, obtained in 2016 and 2017, had minimum inhibitory concentrations (MICs) determined for gentamicin using the agar dilution method. Antimicrobial combinations were also tested in 97 MDR strains using the antimicrobial gradient epsilometer test (Etest); results ranging from synergy to antagonism were interpreted using the fractional inhibitory concentration (FICI).ResultsAll 407 gonococcal isolates were susceptible to gentamicin. MICs ranged from 2 mg/L to 16 mg/L. Synergy was demonstrated in 16.5%(16/97), 27.8%(27/97) and 8.2%(8/97) MDR strains when gentamicin was combined with ceftriaxone [geometric mean (GM) FICI; 0.747], ertapenem (GM FICI; 0.662) and azithromycin (GM FICI; 1.021), respectively. No antimicrobial antagonism was observed with any combination. The three antimicrobial combinations were indifferent overall. The overall GM MICs of gentamicin were reduced by 2.63-, 3.80- and 1.98-fold when tested in combination with ceftriaxone, ertapenem and azithromycin, respectively. The GM MICs of the three antimicrobials by themselves were reduced by 3-, 2.57- and 1.98-fold respectively, when each was tested in combination with gentamicin. No antimicrobial antagonism was observed with any combination.ConclusionsGentamicin alone was effective in vitro against MDR N. gonorrhoeae and in combination with ceftriaxone, ertapenem or azithromycin. Combination testing of resistant strains, overall, showed lower effective MICs against gentamicin itself and each of the three antimicrobials when used in combination with gentamicin.

Trends of antimicrobial resistance and combination susceptibility testing of cystic fibrosis multidrug-resistant Pseudomonas aeruginosa: A ten-year update

Background: Antimicrobial combination therapy is a time/resource- intensive procedure commonly employed in the treatment of cystic fibrosis (CF) pulmonary exacerbations caused by P. aeruginosa. Ten years ago the most promising antimicrobial combinations were proposed, but there has since been the introduction of new β-lactam+β-lactamase inhibitor antimicrobial combinations. The aims of this study were i) to compare in vitro activity of these new antimicrobials with other anti-pseudomonals agents and suggest their most synergistic antimicrobial combinations. ii) to determine antimicrobial resistance rates and study inherent trends of antimicrobials over ten years. Methods: 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). Results: 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+β-lactamase inhibitors produced some synergistic results. Ciprofloxacin+ceftolozane/tazobactam (40%) and amikacin+ceftazidime (36.7%) were the most synergistic combinations while colistin combinations gave the best median SPBI (50.11). Conclusions: 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.

Ocular Toxoplasmosis: An Update on Diagnosis, Multimodal Imaging and Therapy

Ocular toxoplasmosis remains to be the most common cause of infectious uveitis in immunocompetent individuals with highly variable prognosis. The transmission mode can be either congenital or acquired. A precise diagnosis of the disease is necessary to opt effective and rapid treatment. While ocular toxoplasmosis usually presents in the classic form, it may as well present in variable clinical spectrum. The diagnosis can be suspected by the ocular inflammatory clinical presentation as well as multimodal imaging. However, serologic tests including intraocular fluid testing may be needed. Treatment includes combination of systemic antiparasitic and anti-inflammatory drugs with variable effectivity. More recently, intravitreally antimicrobials may be used. The chapter aims to layout the different clinical presentations and complications of ocular toxoplasmosis. Diagnostic techniques and different antimicrobial combinations for treatment will also be discussed.