Clinical utility of synergy testing for multidrug-resistant Pseudomonas aeruginosa isolated from patients with cystic fibrosis: 'the motion for'

Drug ratio-dependent antagonism: a new category of multidrug resistance and strategies for its circumvention

A newly identified form of multidrug resistance (MDR) in tumor cells is presented, pertaining to the commonly encountered resistance of cancer cells to anticancer drug combinations at discrete drug:drug ratios. In vitro studies have revealed that whether anticancer drug combinations interact synergistically or antagonistically can depend on the ratio of the combined agents. Failure to control drug ratios in vivo due to uncoordinated pharmacokinetics could therefore lead to drug resistance if tumor cells are exposed to antagonistic drug ratios. Consequently, the most efficacious drug combination may not occur at the typically employed maximum tolerated doses of the combined drugs if this leads to antagonistic ratios in vivo after administration and resistance to therapeutic effects of the drug combination. Our approach to systematically screen a wide range of drug ratios and concentrations and encapsulate the drug combination in a liposomal delivery vehicle at identified synergistic ratios represents a means to mitigate this drug ratio-dependent MDR mechanism. The in vivo efficacy of the improved agents (CombiPlex formulations) is demonstrated and contrasted with the decreased efficacy when drug combinations are exposed to tumor cells in vivo at antagonistic ratios.

The glycerol-3-phosphate permease GlpT is the only fosfomycin transporter in Pseudomonas aeruginosa

Fosfomycin is transported into Escherichia coli via both glycerol-3-phosphate (GlpT) and a hexose phosphate transporter (UhpT). Consequently, the inactivation of either glpT or uhpT confers increased fosfomycin resistance in this species. The inactivation of other genes, including ptsI and cyaA, also confers significant fosfomycin resistance. It has been assumed that identical mechanisms are responsible for fosfomycin transport into Pseudomonas aeruginosa cells. The study of an ordered library of insertion mutants in P. aeruginosa PA14 demonstrated that only insertions in glpT confer significant resistance. To explore the uniqueness of this resistance target in P. aeruginosa, the linkage between fosfomycin resistance and the use of glycerol-3-phosphate was tested. Fosfomycin-resistant (Fos-R) mutants were obtained in LB and minimal medium containing glycerol as the sole carbon source at a frequency of 10(-6). However, no Fos-R mutants grew on plates containing fosfomycin and glycerol-3-phosphate instead of glycerol (mutant frequency, < or = 5 x 10(-11)). In addition, 10 out of 10 independent spontaneous Fos-R mutants, obtained on LB-fosfomycin, harbored mutations in glpT, and in all cases the sensitivity to fosfomycin was recovered upon complementation with the wild-type glpT gene. The analysis of these mutants provides additional insights into the structure-function relationship of glycerol-3-phosphate the transporter in P. aeruginosa. Studies with glucose-6-phosphate and different mutant derivatives strongly suggest that P. aeruginosa lacks a specific transport system for this sugar. Thus, glpT seems to be the only fosfomycin resistance mutational target in P. aeruginosa. The high frequency of Fos-R mutations and their apparent lack of fitness cost suggest that Fos-R variants will be obtained easily in vivo upon the fosfomycin treatment of P. aeruginosa infections.

Cystic fibrosis pulmonary guidelines: treatment of pulmonary exacerbations

The natural history of cystic fibrosis lung disease is one of chronic progression with intermittent episodes of acute worsening of symptoms frequently called acute pulmonary exacerbations These exacerbations typically warrant medical intervention. It is important that appropriate therapies are recommended on the basis of available evidence of efficacy and safety. The Cystic Fibrosis Foundation therefore established a committee to define the key questions related to pulmonary exacerbations, review the clinical evidence using an evidence-based methodology, and provide recommendations to clinicians. It is hoped that these guidelines will be helpful to clinicians in the treatment of individuals with cystic fibrosis.

Comparison of methods to test antibiotic combinations against heterogeneous populations of multiresistant Pseudomonas aeruginosa from patients with acute infective exacerbations in cystic fibrosis

Multiresistant Pseudomonas aeruginosa isolates can chronically infect patients with cystic fibrosis. Acute infective exacerbations are treated with combinations of two antipseudomonal antibiotics. Patients may respond clinically even if the bacteria are resistant, possibly due to antimicrobial synergy. The challenge for testing for synergy in vitro is that there is no standardized method, and the antibiotic susceptibility in a population of P. aeruginosa isolates in a single sputum sample can vary. We therefore compared (i) antibiotic combinations with different examples of resistant bacteria from the same sputum sample and (ii) the results of synergy testing by different methods. Antibiotic synergy was tested by using resistant P. aeruginosa isolates recovered from sputum samples taken just before the start of treatment for an acute infective exacerbation. Several examples of each morphotype of P. aeruginosa were tested by cidal checkerboard, time-kill curve, and multiple-combination bactericidal testing. The isolates were typed by pulsed-field gel electrophoresis (PFGE). The results were compared with the clinical and microbiological responses to 14 days of antibiotic treatment. Forty-four resistant isolates from nine patients were tested. Some P. aeruginosa isolates with the same morphotype and PFGE pulsotype had different results by synergy testing. There was a poor correlation between the results of the different methods of synergy testing, and no one method would have predicted the response to treatment in all patients. The in vitro effects of antibiotic combinations against different isolates from the same sputum sample can vary, and the results depend on the methodology used. The role of combination testing for the treatment of antibiotic-resistant P. aeruginosa in acute exacerbations of chronic infection in patients with cystic fibrosis needs to be reviewed.

Cystic fibrosis infections: treatment strategies and prospects

Pseudomonas aeruginosa and Burkholderia cepacia are the two major Gram-negative rods that colonize/infect the lungs of patients with cystic fibrosis (CF). These organisms may cause progressive respiratory failure, although occasionally more rapid infections result in the 'Cepacia' syndrome. Many antibiotics have been used against Pseudomonas and Burkholderia, but once chronic colonization has been established, eradication of these organisms is rare. Drug therapy for CF patients is compromised by a number of bacterial factors that render the infectious agents resistant to antibiotics, including efflux pumps that remove antibiotics, lack of penetration of antibiotics into bacterial biofilms, and changes in the cell envelope that reduce the permeability of antibiotics. Any combination of these mechanisms increases the likelihood of bacterial survival. Therefore, combinations of antibiotics or of antibiotic and nonantibiotic compounds are currently being tested against Pseudomonas and Burkholderia. However, progress has been slow, with only occasional combinations showing promise for the eradication of persistent Gram-negative rods in the airways of CF patients. This review will summarize the current knowledge of CF infections and speculate on potential future pathways to treat these chronic infections.

Synergism between natural products and antibiotics against infectious diseases

Antibiotics have been effective in treating infectious diseases, but resistance to these drugs has led to the emergence of new and the reemergence of old infectious diseases. One strategy employed to overcome these resistance mechanisms is the use of combination of drugs, such as beta-lactams together with beta-lactamase inhibitors. Several plant extracts have exhibited synergistic activity against microorganisms. This review describes in detail, the observed synergy and mechanism of action between natural products including flavonoids and essential oils and synthetic drugs in effectively combating bacterial, fungal and mycobacterial infections. The mode of action of combination differs significantly than that of the same drugs acting individually; hence isolating a single component may lose its importance thereby simplifying the task of pharma industries.

Airway epithelial control of Pseudomonas aeruginosa infection in cystic fibrosis

Defective expression or function of the cystic fibrosis transmembrane conductance regulator (CFTR) underlies the hypersusceptibility of cystic fibrosis (CF) patients to chronic airway infections, particularly with Pseudomonas aeruginosa. CFTR is involved in the specific recognition of P. aeruginosa, thereby contributing to effective innate immunity and proper hydration of the airway surface layer (ASL). In CF, the airway epithelium fails to initiate an appropriate innate immune response, allowing the microbe to bind to mucus plugs that are then not properly cleared because of the dehydrated ASL. Recent studies have identified numerous CFTR-dependent factors that are recruited to the epithelial plasma membrane in response to infection and that are needed for bacterial clearance, a process that is defective in CF patients hypersusceptible to infection with this organism.