Determination of activities of levofloxacin, alone and combined with gentamicin, ceftazidime, cefpirome, and meropenem, against 124 strains of Pseudomonas aeruginosa by checkerboard and time-kill methodology

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

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

Dose Optimization of Cefpirome Based on Population Pharmacokinetics and Target Attainment during Extracorporeal Membrane Oxygenation

To obtain the optimal dosage regimen in patients receiving extracorporeal membrane oxygenation (ECMO), we developed a population pharmacokinetics model for cefpirome and performed pharmacodynamic analyses. This prospective study included 15 patients treated with cefpirome during ECMO. Blood samples were collected during ECMO (ECMO-ON) and after ECMO (ECMO-OFF) at predose and 0.5 to 1, 2 to 3, 4 to 6, 8 to 10, and 12 h after cefpirome administration. The population pharmacokinetic model was developed using nonlinear mixed effects modeling and stepwise covariate modeling. Monte Carlo simulation was used to assess the probability of target attainment (PTA) and cumulative fraction of response (CFR) according to the MIC distribution. Cefpirome pharmacokinetics were best described by a two-compartment model. Covariate analysis indicated that serum creatinine concentration (SCr) was negatively correlated with clearance, and the presence of ECMO increased clearance and the central volume of distribution. The simulations showed that patients with low SCr during ECMO-ON had lower PTA than patients with high SCr during ECMO-OFF; so, a higher dosage of cefpirome was required. Cefpirome of 2 g every 8 h for intravenous bolus injection or 2 g every 12 h for extended infusion over 4 h was recommended with normal kidney function receiving ECMO. We established a population pharmacokinetic model for cefpirome in patients with ECMO, and appropriate cefpirome dosage regimens were recommended. The impact of ECMO could be due to the change in patient status on consideration of the small population and uncertainty in covariate relationships. Dose optimization of cefpirome may improve treatment success and survival in patients receiving ECMO. (This study has been registered at ClinicalTrials.gov under identifier NCT02581280.).

5-Benzylidene-4-Oxazolidinones Are Synergistic with Antibiotics for the Treatment of Staphylococcus aureus Biofilms

The failure of frontline antibiotics in the clinic is one of the most serious threats to human health and requires a multitude of novel therapeutics and innovative approaches to treatment so as to curtail the growing crisis. In addition to traditional resistance mechanisms resulting in the lack of efficacy of many antibiotics, most chronic and recurring infections are further made tolerant to antibiotic action by the presence of biofilms. Herein, we report an expanded set of 5-benzylidene-4-oxazolidinones that are able to inhibit the formation of Staphylococcus aureus biofilms, disperse preformed biofilms, and, in combination with common antibiotics, are able to significantly reduce the bacterial load in a robust collagen-matrix model of biofilm infection.

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

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

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

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

Evaluation of the best method to assess antibiotic potentiation by phytochemicals against Staphylococcus aureus

The increasing occurrence of bacterial resistance to antibiotics has now reached a critical level. Finding antibiotic coadjuvants capable to inhibit the bacterial resistance mechanisms would be a valuable mid-term solution, until new classes of antibiotics are discovered. Selected plant alkaloids were combined with 5 antibiotics against 10 Staphylococcus aureus strains, including strains expressing distinct efflux pumps and methicillin-resistant S. aureus strains. The efficacy of each combination was assessed using the microdilution checkerboard, time-kill, Etest, and disc diffusion methods. The cytotoxicity of the alkaloids was evaluated in a mouse fibroblast cell line. Potentiation was obtained in 6% of all 190 combinations, especially with the combination of: ciprofloxacin with reserpine (RES), pyrrolidine (PYR), and quinine (QUIN); tetracycline with RES; and erythromycin with PYR. The highest cytotoxicity values were found for QUIN (half maximal inhibitory concentration [IC50] = 25 ± 2.2 mg/L) and theophylline (IC50 = 100 ± 4.7 mg/L).

Combination antibiotic therapy for empiric and definitive treatment of gram-negative infections: insights from the Society of Infectious Diseases Pharmacists

The widespread emergence of antibiotic-resistant gram-negative organisms has compromised the utility of current treatment options for severe infections caused by these pathogens. The rate of gram-negative multidrug resistance is worsening, threatening the effectiveness of newer broad-spectrum antibiotic agents. Infections associated with multidrug-resistant Pseudomonas aeruginosa, Acinetobacter baumannii, and Enterobacteriaceae are having a substantial impact on hospital costs and mortality rates. The potential for these resistant gram-negative nosocomial pathogens must always be a primary consideration when selecting antibiotic therapy for critically ill patients. Empiric combination therapy directed at gram-negative pathogens is a logical approach for patients with suspected health care-associated infections, particularly those with risk factors for infections caused by multidrug-resistant pathogens. Although in vitro synergy tests have shown potential benefits of continued combination therapy, convincing clinical data that demonstrate a need for combination therapy once susceptibilities are known are lacking. Thus, deescalation to a single agent once susceptibilities are known is recommended for most patients and pathogens. Use of polymyxins, often in combination with other antimicrobials, may be necessary for salvage therapy.

Should multivisceral transplantation be considered in patients colonized with multidrug-resistant Pseudomonas aeruginosa?

This report describes two subsequent liver-small bowel-pancreas-kidney (multivisceral) transplantations in a child colonized with multidrug-resistant Pseudomonas aeruginosa. We discuss the dilemma concerning the transplantation of patients colonized with multidrug-resistant Pseudomonas spp., its potential consequences, and the peri and postoperative management of these patients.

Carbapenems: past, present, and future

In this review, we summarize the current "state of the art" of carbapenem antibiotics and their role in our antimicrobial armamentarium. Among the β-lactams currently available, carbapenems are unique because they are relatively resistant to hydrolysis by most β-lactamases, in some cases act as "slow substrates" or inhibitors of β-lactamases, and still target penicillin binding proteins. This "value-added feature" of inhibiting β-lactamases serves as a major rationale for expansion of this class of β-lactams. We describe the initial discovery and development of the carbapenem family of β-lactams. Of the early carbapenems evaluated, thienamycin demonstrated the greatest antimicrobial activity and became the parent compound for all subsequent carbapenems. To date, more than 80 compounds with mostly improved antimicrobial properties, compared to those of thienamycin, are described in the literature. We also highlight important features of the carbapenems that are presently in clinical use: imipenem-cilastatin, meropenem, ertapenem, doripenem, panipenem-betamipron, and biapenem. In closing, we emphasize some major challenges and urge the medicinal chemist to continue development of these versatile and potent compounds, as they have served us well for more than 3 decades.

Norfloxacin and ursolic acid: in vitro association and postantibiotic effect against Staphylococcus aureus

AIMS

We investigated the effectiveness in vitro of the association between norfloxacin (NOR) and ursolic acid (UA) against Staphylococcus aureus.

METHODS AND RESULTS

The minimal inhibitory concentrations (MICs), the minimal bactericidal concentrations, the bacterial killing and the postantibiotic effect (PAE) of NOR and UA were determined both singly and in combination. A synergistic interaction was observed against Staph. aureus ATCC 29213: the mean PAEs were 3 h for NOR, -1.2 h for UA (1 × MIC) and 2.0 h for UA (2 × MIC). Synergism was observed with longer PAEs and postantibiotic sub-MIC effects after NOR/UA exposure. UA was also active against clinical isolates and methicillin-resistant Staph. aureus.

CONCLUSIONS

The application of antimicrobial combinations may address the rising resistance to established classes of both systemic and topical agents.

SIGNIFICANCE AND IMPACT OF THE STUDY

In vitro interactions between NOR and UA may contribute to the development of novel topical agents for the treatment of skin infections as well as for topical formulations.

Antimicrobial potential of polyphenols extracted from almond skins

AIMS

To evaluate the antimicrobial properties of flavonoid-rich fractions derived from natural and blanched almond skins, the latter being a by-product from the almond processing industry.

METHODS AND RESULTS

Almond skin extracts were tested against Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa, Salmonella enterica, Serratia marcescens), Gram-positive bacteria (Listeria monocytogenes, Enterococcus hirae, Staphylococcus aureus, Enterococcus durans) and the yeast Candida albicans. Almond skin fractions were found to have antimicrobial activity against L. monocytogenes and Staph. aureus in the range 250-500 microg ml(-1), natural skins showing antimicrobial potential against the Gram-negative Salm. enterica. The interactions between three almond skin flavonoids were also evaluated with isobolograms.

CONCLUSIONS

Pairwise combinations of protocatechuic acid, naringenin and epicatechin showed both synergistic and indifferent interactions against Salm. enterica and Staph. aureus. Antagonism was observed against L. monocytogenes with all combinations tested. Further studies need to be performed to understand the mechanisms responsible for these interactions.

SIGNIFICANCE AND IMPACT OF THE STUDY

Almond skins are a potential source of natural antimicrobials.

Assessing the emergence of resistance: the absence of biological cost in vivo may compromise fosfomycin treatments for P. aeruginosa infections

BACKGROUND

Fosfomycin is a cell wall inhibitor used efficiently to treat uncomplicated urinary tract and gastrointestinal infections. A very convenient feature of fosfomycin, among others, is that although the expected frequency of resistant mutants is high, the biological cost associated with mutation impedes an effective growth rate, and bacteria cannot offset the obstacles posed by host defenses or compete with sensitive bacteria. Due to the current scarcity of new antibiotics, fosfomycin has been proposed as an alternative treatment for other infections caused by a wide variety of bacteria, particularly Pseudomonas aeruginosa. However, whether fosfomycin resistance in P. aeruginosa provides a fitness cost still remains unknown.

PRINCIPAL FINDINGS

We herein present experimental evidence to show that fosfomycin resistance cannot only emerge easily during treatment, but that it is also cost-free for P. aeruginosa. We also tested if, as has been reported for other species such as Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis, fosfomycin resistant strains are somewhat compromised in their virulence. As concerns colonization, persistence, lung damage, and lethality, we found no differences between the fosfomycin resistant mutant and its sensitive parental strain. The probability of acquisition in vitro of resistance to the combination of fosfomycin with other antibiotics (tobramycin and imipenem) has also been studied. While the combination of fosfomycin with tobramycin makes improbable the emergence of resistance to both antibiotics when administered together, the combination of fosfomycin plus imipenem does not avoid the appearance of mutants resistant to both antibiotics.

CONCLUSIONS

We have reached the conclusion that the use of fosfomycin for P. aeruginosa infections, even in combined therapy, might not be as promising as expected. This study should encourage the scientific community to assess the in vivo cost of resistance for specific antibiotic-bacterial species combinations, and therefore avoid reaching universal conclusions from single model organisms.

Synergism and postantibiotic effect of tobramycin and Melaleuca alternifolia (tea tree) oil against Staphylococcus aureus and Escherichia coli

The application of antimicrobial combinations may address the rising resistance to established classes of both systemic and topical agents and their clinical relevance is related to the presence of a significant postantibiotic effect (PAE). We investigated the effectiveness in vitro of the association between tobramycin and tea tree oil (TTO) against Gram-positive and Gram-negative bacteria. The minimal inhibitory concentrations, the bacterial killing and the PAE of tobramycin and TTO were determined both singly and in combination against Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 29213. A synergistic interaction was observed against both strains tested: the mean PAEs were 1.3 and 1.7h for tobramycin against E. coli and S. aureus respectively, 10.8h for tobramycin and TTO (0.05%) against E. coli, 10.4h and 17.4h against S. aureus for tobramycin and TTO (0.25 and 0.50%, respectively). Longer PASMEs were observed with S. aureus after TTO/tobramycin exposure. In vitro interactions can improve the antimicrobial effectiveness of the antibiotic and may contribute for the development of novel topical agents for the treatment of skin lesions including conjunctiva and respiratory infections by inhalation.

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.

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.

Synergy of gatifloxacin with cefoperazone and cefoperazone-sulbactam against resistant strains of Pseudomonas aeruginosa

Chequerboard and time-kill methods were used to compare the in vitro efficacies of the combinations gatifloxacin (GAT) with cefoperazone (CFP) and GAT with cefoperazone-sulbactam (CFP-SUL) against 58 clinical isolates of Pseudomonas aeruginosa. The combinations GAT+CFP and GAT+CFP-SUL were shown to be synergistic for 36.2 and 58.6 % of isolates tested, respectively, using the chequerboard method. Time-kill studies with 11 strains showed synergy in 54.5 % for the GAT+CFP combination and 72.7 % for the GAT+CFP-SUL combination. The agreement between these two methods was found to be 72-81 %. There was a significant difference in synergy between the two combinations tested (P=0.011).

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

Time-kill synergy studies showed that at 24 h, subinhibitory meropenem and ciprofloxacin concentrations of 0.06 to 128 and 0.03 to 32 microg/ml, respectively, showed synergy against 34/51 Pseudomonas aeruginosa strains; subinhibitory concentrations of meropenem (0.06 to 8 microg/ml) and colistin (0.12 to 1 microg/ml) showed synergy against 13 isolates. Subinhibitory meropenem and ciprofloxacin concentrations of 0.25 to 2 and 0.12 to 16 microg/ml, respectively, showed synergy against 18/52 Acinetobacter baumannii strains at 24 h. Subinhibitory meropenem and colistin concentrations of 0.03 to 64 and 0.06 to 8 microg/ml, respectively, showed synergy against 49 strains at 24 h.

Population pharmacokinetics and pharmacodynamics of cefpirome in critically ill patients against Gram-negative bacteria

OBJECTIVES

To develop a population pharmacokinetics model for cefpirome in ICU patients, to assess pharmacokinetic-pharmacodynamic profiles vs. MIC distribution of likely ICU pathogens, and to assess their expected cumulative fraction of response (CFR).

DESIGN AND SETTING

Prospective observational study in a multidisciplinary ICU.

MEASUREMENTS AND RESULTS

Twelve patients received 2g cefpirome intravenously over 12h. Thirteen blood samples were taken on two occasions. Demographic and creatinine clearance data were collected. Based on the final covariate model obtained using NONMEM, Monte Carlo simulations were undertaken to simulate free-drug concentrations for two administration methods: intermittent bolus administration (IBA) and continuous infusion (CI) with a loading dose of 0.5 g. Concentration-time profiles were evaluated by the probability of achieving free-drug concentrations above the MIC for more than 65% of dosing interval. Using MIC distributions from the EUCAST programme the CFR for each method was evaluated. A three-compartment model with zero-order input best described the concentration-time data. The CFR for Escherichia coli and Klebsiella spp. was greater than 97% in all IBA and CI doses but for Pseudomonas aeruginosa, and Acinetobacter spp. achieved target concentrations of 56% and 46%, respectively. High-dose CI cefpirome (6g/day) for P. aeruginosa and Acinetobacter spp. was required to achieve CFR of 89%.

CONCLUSION

Measured creatinine clearance appears to be a good marker of cefpirome clearance and potentially could be used to individualise cefpirome therapy. When given as IBA or CI for E. coli and Klebsiella spp., cefpirome should be successful. Cefpirome fails to achieve the bactericidal target even when administered at high-doses such as 6g/day for P. aeruginosa and Acinetobacter spp. Prospective clinical studies are needed to conclusively validate these findings.

Pneumonia due to Pseudomonas aeruginosa: the levofloxacin clinical trials experience

Respiratory infections caused by Pseudomonas aeruginosa present significant treatment challenges, including that of overcoming intrinsic and adaptive resistance by these organisms. The fluoroquinolones may provide an effective option for treating these infections. In this analysis, we report on the efficacy of levofloxacin in the treatment of community-acquired pneumonia (CAP) and nosocomial pneumonia caused by P. aeruginosa using information from nine clinical studies supported by Johnson & Johnson Pharmaceutical Research and Development (Raritan, NJ) or Ortho-McNeil Pharmaceutical (Raritan, NJ). From these studies, a total of 36 patients were identified with pneumonia caused by P. aeruginosa and treated with levofloxacin (750 mg or 500 mg). For patients diagnosed with nosocomial pneumonia, levofloxacin treatment achieved a 64.7% (11/17) clinical success rate, compared with 41.2% (7/17) with comparator treatment (imipenem/cilastatin followed by ciprofloxacin) in the microbiologically evaluable population. Eradication rates were 58.8% with levofloxacin treatment vs. 29.4% with comparator (95% CI, -64.2 to 5.4). For levofloxacin-treated CAP patients with P. aeruginosa infections (n = 19), clinical success and microbiological eradication rates in the microbiologically evaluable population were 89.5% and 78.9%, respectively. Several limitations of this analysis exist including that this was a retrospective evaluation that pooled data from multiple studies with varying protocols, the number of patients included was limited, and the nosocomial pneumonia patients used adjunctive therapy with an antipseudomonal beta-lactam in most cases. Nonetheless, these findings suggest that levofloxacin may play a role in the treatment of these difficult respiratory infections.

Nosocomial infections due to multidrug-resistant Pseudomonas aeruginosa: epidemiology and treatment options

Pseudomonas aeruginosa is one of the leading gram-negative organisms associated with nosocomial infections. The increasing frequency of multi-drug-resistant Pseudomonas aeruginosa (MDRPA) strains is concerning as efficacious antimicrobial options are severely limited. By searching MEDLINE from January 1966-February 2005 and relevant journals for abstracts, we reviewed the frequency, risk factors, and patient outcomes of MDRPA nosocomial infections in critically ill patients, determined the available antimicrobial therapies, and then provided recommendations for clinicians. The definition of MDRPA was established as isolates intermediate or resistant to at least three drugs in the following classes: beta-lactams, carbapenems, aminoglycosides, and fluoroquinolones. Reported rates of MDRPA varied from 0.6-32% according to geographic location and type of surveillance study. Risk factors for MDRPA infection included prolonged hospitalization, exposure to antimicrobial therapy, and immunocompromised states such as human immunodeficiency virus infection. Emergence of MDRPA isolates during therapy was reported in 27-72% of patients with initially susceptible P. aeruginosa isolates. Patients with severe MDRPA infections should be treated with combination therapy, consisting of an antipseudomonal beta-lactam with an aminoglycoside or fluoroquinolone rather than aminoglycoside and fluoroquinolone combinations, to provide adequate therapy and improve patient outcomes. Synergy has been observed when resistant antipseudomonal drugs were combined in vitro against MDRPA with successful clinical application reported in two centers. Colistin with adjunctive therapy, such as a beta-lactam or rifampin, may be a useful agent in MDRPA when antimicrobial options are limited, but patients should be monitored closely for toxicities associated with this agent. Standardization of terminology for MDRPA isolates is needed for consistency and comparability of surveillance and institutional reports. Clinical studies are needed to identify risk factors for MDRPA development and to determine the economic impact of these infections, as well as to determine the most efficacious antimicrobial regimens and duration of therapy to maximize outcomes in the treatment of MDRPA infections.

In vitro and in vivo synergy of levofloxacin or amikacin both in combination with ceftazidime against clinical isolates of Pseudomonas aeruginosa

The aim of the present study was to explore the antibacterial activity of the levofloxacin (LVX) and ceftazidime (CAZ) combination compared with the amikacin (AMK)/CAZ combination against Pseudomonas aeruginosa. Minimum inhibitory concentrations (MICs) were determined according to NCCLS. FIC indices (Fl) were calculated by the checkerboard technique. CAZ combined with LVX or AMK yielded Fls indicating synergism (Fl < or = 0.5) for 71/102 (69.6%) and 81/102 (79.4%) (p = 0.108), indifference (FI > 0.5-4) for 24/102 (23.5%) and 12/102 (11.7%) (p = 0.027), and antagonism (Fl > 4) for 7/102 (6.8%) and 9/102 (8.8%) (p = 0.602) strains, respectively. In vivo, CAZ/LVX was as bactericidal as CAZ/AMK combination. Our results support the potential role of LVX as an alternative to AMK in the combination therapy with CAZ in the treatment of P. aeruginosa severe infections. Anyway, further investigations and clinical trials are awaited until any definitive conclusions can be drawn.

Synergistic potential of ceftazidime plus amikacin or levofloxacin against Pseudomonas aeruginosa as determined using a checkerboard and a disk diffusion technique

The synergistic potential of ceftazidime plus amikacin or levofloxacin was assessed against 61 Pseudomonas aeruginosa isolates with variable susceptibility patterns to the 3 antibiotics. A checkerboard broth method and a disk diffusion method were used and compared. The latter, also easy to perform as a triple-disk assay, could be a helpful laboratory screening tool for drug synergism to drive possible combination treatments.

In vitro selection of resistance in Pseudomonas aeruginosa and Acinetobacter spp. by levofloxacin and ciprofloxacin alone and in combination with β-lactams and amikacin

OBJECTIVES

The aim of this study was to evaluate the ability of levofloxacin and ciprofloxacin alone and in combination with either ceftazidime, cefepime, imipenem, piperacillin-tazobactam or amikacin to select for antibiotic-resistant mutants of Pseudomonas aeruginosa and Acinetobacter spp.

METHODS

Clinical strains of P. aeruginosa (n = 5) and Acinetobacter spp. (n = 5) susceptible to all the drugs used in the study were assayed. Development of resistance was determined by multi-step and single-step methodologies. For multi-step studies, MICs were determined after five serial passages on antibiotic-gradient plates containing each antibiotic alone or in combination with levofloxacin or ciprofloxacin. Acquisition of resistance was defined as an increase of >or=4-fold from the starting MIC. In single-step studies, the frequency of spontaneous mutations was calculated after a passage on plates containing antibiotics alone and in combinations at concentrations equal to the highest NCCLS breakpoints.

RESULTS

Serial passages on medium containing single antibiotics resulted in increased MICs for each antibiotic; MIC increases were limited by antibiotics in combination. A decrease in the number of strains with MICs above the NCCLS breakpoints occurred when fluoroquinolones were combined with a second antibiotic for both P. aeruginosa and Acinetobacter spp. isolates. Frequencies of mutation were higher for antibiotics alone than for combinations.

CONCLUSIONS

Use of combinations of fluoroquinolones with beta-lactams and amikacin reduces the risk for in vitro selection of resistant P. aeruginosa and Acinetobacter spp.

In vitro activities of antibiotic combinations against clincal isolates of Pseudomonas aeruginosa

Combination therapy has been recommended to treat Pseudomonas aeruginosa infections worldwide. The purpose of the present study was to determine the in vitro activities of piperacillin, cefepime, aztreonam, amikacin, and ciprofloxacin alone and in combination against 100 clinical isolates of P. aeruginosa from one medical center in southern Taiwan. The combination susceptibility assay was performed using the checkerboard technique. The percentage of resistance of P. aeruginosa to single agents in our study was relatively high for the Asia-Pacific area, except to aztreonam. Piperacillin plus amikacin exhibited the highest potential for synergy (59/100) in this study. Moreover, a high percentage of synergism was also noted with amikacin combined with cefepime (7/100) or aztreonam (16/100). The combination of two beta-lactams, such as cefepime with piperacillin, and aztreonam with cefepime or piperacillin, showed synergistic effects against some P. aeruginosa isolates. Although ciprofloxacin is a good anti-pseudomonal agent, a very low potential for synergy with other antibiotics was demonstrated in this study. No antagonism was exhibited by any combination in our study. Among piperacillin-resistant strains, there was synergy with a beta-lactam plus amikacin, including the combination of piperacillin and amikacin. However, the combination of two beta-lactams, such as piperacillin and cefepime or aztreonam, did not have any synergistic activity against these strains. In summary, the combinations of amikacin with the tested beta-lactams (piperacillin, aztreonam, cefepime) had a greater synergistic effect against P. aeruginosa, even piperacillin-resistant strains, than other combinations. Understanding the synergistic effect on clinical strains may help clinicians choose better empirical therapy in an area with high prevalence of multidrug-resistant P. aeruginosa.

Levofloxacin: a review of its use in the treatment of bacterial infections in the United States

Levofloxacin (Levaquin) is a fluoroquinolone antibacterial agent with a broad spectrum of activity against Gram-positive and Gram-negative bacteria and atypical respiratory pathogens. It is active against both penicillin-susceptible and penicillin-resistant Streptococcus pneumoniae. The prevalence of S. pneumoniae resistance to levofloxacin is <1% overall in the US.A number of randomised comparative trials in the US have demonstrated the efficacy of levofloxacin in the treatment of infections of the respiratory tract, genitourinary tract, skin and skin structures. Sequential intravenous to oral levofloxacin 750mg once daily for 7-14 days was as effective in the treatment of nosocomial pneumonia as intravenous imipenem/cilastatin 500-1000mg every 6-8 hours followed by oral ciprofloxacin 750mg twice daily in one study. In patients with mild to severe community-acquired pneumonia (CAP), intravenous and/or oral levofloxacin 500mg once daily for 7-14 days achieved clinical and bacteriological response rates similar to those with comparator agents, including amoxicillin/clavulanic acid, clarithromycin, azithromycin, ceftriaxone and/or cefuroxime axetil and gatifloxacin. A recent study indicates that intravenous or oral levofloxacin 750mg once daily for 5 days is as effective as 500mg once daily for 10 days, in the treatment of mild to severe CAP. Exacerbations of chronic bronchitis and acute maxillary sinusitis respond well to treatment with oral levofloxacin 500mg once daily for 7 and 10-14 days, respectively. Oral levofloxacin was as effective as ofloxacin in uncomplicated urinary tract infections and ciprofloxacin or lomefloxacin in complicated urinary tract infections. In men with chronic bacterial prostatitis treated for 28 days, oral levofloxacin 500mg once daily achieved similar clinical and bacteriological response rates to oral ciprofloxacin 500mg twice daily. Uncomplicated skin infections responded well to oral levofloxacin 500mg once daily for 7-10 days, while in complicated skin infections intravenous and/or oral levofloxacin 750mg for 7-14 days was at least as effective as intravenous ticarcillin/clavulanic acid (+/- switch to oral amoxicillin/clavulanic acid) administered for the same duration. Levofloxacin is generally well tolerated, with the most frequently reported adverse events being nausea and diarrhoea; in comparison with some other quinolones it has a low photosensitising potential and clinically significant cardiac and hepatic adverse events are rare.

CONCLUSION

Levofloxacin is a broad-spectrum antibacterial agent with activity against a range of Gram-positive and Gram-negative bacteria and atypical organisms. It provides clinical and bacteriological efficacy in a range of infections, including those caused by both penicillin-susceptible and -resistant strains of S. pneumoniae. Levofloxacin is well tolerated, and is associated with few of the phototoxic, cardiac or hepatic adverse events seen with some other quinolones. It also has a pharmacokinetic profile that is compatible with once-daily administration and allows for sequential intravenous to oral therapy. The recent approvals in the US for use in the treatment of nosocomial pneumonia and chronic bacterial prostatitis, and the introduction of a short-course, high-dose regimen for use in CAP, further extend the role of levofloxacin in treating bacterial infections.

Levofloxacin disposition in cerebrospinal fluid in patients with external ventriculostomy

In vitro levofloxacin exhibits both potent or intermediate activity against most of the pathogens frequently responsible for acute bacterial meningitis and synergistic activity with some beta-lactams. Since levofloxacin was shown to penetrate the cerebrospinal fluid (CSF) during meningeal inflammation both in animals and in humans, the disposition of levofloxacin in CSF was studied in 10 inpatients with external ventriculostomy because of communicating hydrocephalus related to subarachnoid occlusion due to cerebral accidents who were treated with 500 mg of levofloxacin intravenously twice a day because of extracerebral infections. Plasma and CSF concentration-time profiles and pharmacokinetics were assessed at steady state. Plasma and CSF levofloxacin concentrations were analyzed by high-pressure liquid chromatography. The peak concentration of levofloxacin at steady state (C(max ss))was 10.45 mg/liter in plasma and 4.06 mg/liter in CSF, respectively, with the ratio of the C(max ss) in CSF to the C(max ss) in plasma being 0.47. The areas under the concentration-time curves during the 12-h dosing interval (AUC(0-tau)s) were 47.69 mg. h/liter for plasma and 33.42 mg. h/liter for CSF, with the ratio of the AUC(0-tau) for CSF to the AUC(0-tau) for plasma being 0.71. The terminal-phase half-life of levofloxacin in CSF was longer than that in plasma (7.02 +/- 1.57 and 5.51 +/- 1.36 h, respectively; P = 0.034). The ratio of the levofloxacin concentration in CSF to the concentration in plasma progressively increased with time, from 0.30 immediately after dosing to 0.99 at the end of the dosing interval. In the ventricular CSF of patients with uninflamed meninges, levofloxacin was shown to provide optimal exposure, which approximately corresponded to the level of exposure of the unbound drug in plasma. The findings provide support for trials of levofloxacin with twice-daily dosing in combination with a reference beta-lactam for the treatment of bacterial meningitis in adults. This cotreatment could be useful both for overcoming Streptococcus pneumoniae resistance and for enabling optimal exposure of the CSF to at least one antibacterial agent for the overall treatment period.

Activity of combinations of ceftazidime, imipenem and pefloxacin against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa

The chequerboard technique was used to look for synergistic combinations of ceftazidime, imipenem and pefloxacin. The synergistic combinations were used in vivo in mice experimentally infected with Escherichia coli, Salmonella typhimurium and Pseudomonas aeruginosa. In vitro ceftazidime/imipenem, ceftazidime/pefloxacin and pefloxacin/imipenem combinations showed synergistic effects against Staphylococcus aureus and S. typhimurium and additive effects against P. aeruginosa. Only the ceftazidime/pefloxacin combination was synergistic against E. coli while the ceftazidime/imipenem and pefloxacin/imipenem combinations resulted in additive effects. In vivo, combination of ceftazidime/imipenem against E. coli infection and the pefloxacin/imipenem combination against S. typhimurium infection were protective.

Current and future perspectives for levofloxacin in severe Pseudomonas aeruginosa infections

The question of whether levofloxacin includes Pseudomonas aeruginosa in its spectrum of clinical activity is discussed by reviewing the major findings on this issue, mainly those published in Italy. The in vitro activity of levofloxacin against P. aeruginosa is now documented on thousands of strains worldwide. The pharmacodynamic properties of levofloxacin allow for the treatment of pseudomonal infections. The levofloxacin clinical results extrapolated from published studies document the efficacy of levofloxacin in the treatment of infections sustained by P. aeruginosa.

Combination effects of cephalexin and gentamicin on Edwardsiella tarda and Streptococcus iniae

The objective of this study was to determine the in vitro activity of cephalexin-gentamicin combination by a microbroth chequerboard technique against clinical isolates of Edwardsiella tarda and Streptococcus iniae. Gentamicin was shown more susceptible than cephalexin against both bacteria. The effect of cephalexin-gentamicin combination against both bacteria represented additive interaction. The combination even showed synergic interaction (22%) against E. tarda, with a FIC index of <0.5 as a borderline. No antagonism for cephalexin-gentamicin combination was observed for any bacterial strain.

In vitro evaluation of the activity of two doses of Levofloxacin alone and in combination with other agents against Pseudomonas aeruginosa

P. aeruginosa is one of the most difficult to treat pathogens that generally requires combination therapy to prevent the development of resistance. This study evaluated the in vitro activity of two concentrations of levofloxacin (modeled for the 500 mg and 750 mg daily dose) in combination with ceftazidime, cefepime, piperacillin/tazobactam, imipenem, and tobramycin against P. aeruginosa. MICs and time-kill studies were performed against 12 non-duplicate clinical isolates of P. aeruginosa. The percent susceptible for levofloxacin, ceftazidime, cefepime, piperacillin/tazobactam, imipenem, and tobramycin were 67%, 58%, 58%, 67%, 75%, and 100%, respectively. Tobramycin was the most active single agent, killing and maintaining > or =99.9% killing over a 24 h period against all isolates. Levofloxacin 4 microg/mL(750 mg/day) alone reached 99.9% killing and maintain this killing over the time period more often than levofloxacin 2 microg/mL (500 mg/day). No combination was antagonistic and all combinations with tobramycin were indifferent. Overall, levofloxacin 2 microg/mL plus a beta-lactam was synergistic (65%) more often than levofloxacin 4 microg/mL combinations (46%). This was not unexpected due to the increased activity of levofloxacin 4 microg/mL. However, levofloxacin 4 microg/mL combinations maintained a > or =99.9% killing over the entire 24 h period more often than levofloxacin 2 microg/mL combinations (94% vs 83%). The findings from this work suggest that levofloxacin 750 mg/day in combination with another agent active against P. aeruginosa may prove to be clinically beneficial and superior to combinations using lower doses of levofloxacin. In vivo studies are needed to evaluate the clinical significance of these findings.

In vitro activities of linezolid combined with other antimicrobial agents against Staphylococci, Enterococci, Pneumococci, and selected gram-negative organisms

The activities of linezolid, an oxazolidinone antibacterial agent active against gram-positive organisms, alone and in combination with 35 antimicrobial agents were tested in vitro against methicillin-sensitive (n = 1 to 2 strains) and methicillin-resistant (n = 8 to 10) Staphylococcus aureus strains; vancomycin-sensitive (n = 6) and vancomycin-resistant (n = 6 to 8) Enterococcus faecalis strains; vancomycin-sensitive (n = 5) and vancomycin-resistant (n = 6) Enterococcus faecium strains; penicillin-sensitive (n = 2 to 5), penicillin-intermediate (n = 5 to 6), and penicillin-resistant (n = 5 to 6) Streptococcus pneumoniae strains; Escherichia coli (n = 6); and Klebsiella pneumoniae (n = 6). The fractional inhibitory concentration indices of linezolid in combination with other antimicrobial agents for the organisms tested were generated on checkerboard broth microdilution plates prepared by a semiautomated method. Of 1,380 organism-drug combinations, 1,369 (99.2%) combinations of linezolid with 28 antimicrobial drugs were indifferent, 9 combinations (0.65%) of linezolid with 6 drugs (amoxicillin, erythromycin, imipenem, sparfloxacin, teicoplanin, and tetracycline) were synergistic, and 2 combinations (0.15%) of linezolid with 2 drugs (ofloxacin and sparfloxacin) were antagonistic. Overall, the in vitro data demonstrated that linezolid combined with other antimicrobial agents primarily produces an indifferent response, with infrequent occurrences of synergism and antagonism.

Levofloxacin compared with imipenem/cilastatin followed by ciprofloxacin in adult patients with nosocomial pneumonia: a multicenter, prospective, randomized, open-label study

BACKGROUND

Therapy of nosocomial pneumonia is usually empiric and includes > or = 1 broad-spectrum antimicrobial agent. When considering the use of fluoroquinolones in these difficult-to-treat infections--in which drug delivery to the site of infection may be impaired or organisms with higher minimum inhibitory concentrations may be present--an agent should be chosen whose pharmacodynamics ensure maximal drug exposure. Use of the 750-mg dose of levofloxacin should enhance therapeutic benefit in patients with nosocomial pneumonia.

OBJECTIVE

The goal of this study was to compare the efficacy and safety of levofloxacin 750 mg and imipenem/cilastatin followed by ciprofloxacin in adult patients with nosocomial pneumonia.

METHODS

This was a multicenter, prospective, randomized, open-label trial conducted in North America. Patients were randomly assigned to 1 of 2 treatment arms: levofloxacin 750 mg QD given i.v. and then orally for 7 to 15 days or imipenem/cilastatin 500 mg to 1 g i.v. every 6 to 8 hours, followed by oral ciprofloxacin 750 mg every 12 hours for 7 to 15 days. Adjunctive antibacterial therapy was mandatory in patients with documented or suspected Pseudomonas aeruginosa or methicillin-resistant Staphylococcus aureus infection. The primary predefined outcome measure was the clinical response (cure, improvement, failure, or unable to evaluate) in microbiologically evaluable patients 3 to 15 days after the end of therapy.

RESULTS

The study enrolled 438 adult patients (315 men, 123 women; mean [SD] age, 55.7 [20.04] years). Two hundred twenty patients received levofloxacin, and 218 received the comparator regimen. Demographic and baseline clinical characteristics were similar in the intent-to-treat and clinically evaluable populations. In patients evaluable for microbiologic efficacy, clinical success (cure or improvement) was achieved in 58.1% (54/93) of patients who received levofloxacin, compared with 60.6% (57/94) of patients who received the comparator regimen (95% CI, -12.0 to 17.2). Similar clinical results were seen in patients evaluable for clinical efficacy and in the intent-to-treat population. In the 187 patients evaluable for microbiologic efficacy, eradication was achieved in 66.7% (62/93) of patients receiving levofloxacin and 60.6% (57/94) of patients receiving the comparator regimen (95% CI, -20.3 to 8.3).

CONCLUSION

In this study, levofloxacin was at least as effective and was as well tolerated as imipenem/cilastatin followed by ciprofloxacin in adult patients with nosocomial pneumonia, as demonstrated by comparable clinical and microbiologic success rates.

In vitro activity of beta-lactams in combination with other antimicrobial agents against resistant strains of Pseudomonas aeruginosa

Using the chequerboard titration method, the activity in combination of beta-lactams, fluoroquinolones and aminoglycosides was investigated against 24 Pseudomonas aeruginosa isolates resistant to these antibiotics. Synergy was detected with one or more antimicrobial combinations against 15 of 24 (63%) isolates and partial synergy was detected with one or more combinations against all 24 isolates. No antagonism was seen with any combination. Ceftazidime and cefepime with aztreonam, amikacin and isepamicin showed synergy or partial synergy against 12-20 (50-80%) isolates. Imipenem and meropenem with amikacin and isepamicin showed synergy or partial synergy against eight to 12 (33-50%) isolates. The results of this study indicate that against P. aeruginosa, synergy may occur between beta-lactams, fluoroquinolones and aminoglycosides although the strains are resistant to the individual antibiotics.

The comparison of in the vitro effect of imipenem or meropenem combined with ciprofloxacin or levofloxacin against multidrug-resistant Pseudomonas aeruginosa strains

This study evaluated the in vitro effects of the combination of a carbapenem (imipenem or meropenem) with a quinolone (ciprofloxacin or levofloxacin) using a microbroth dilution chequerboard technique and multidrug-resistant Pseudomonas aeruginosa strains. The ciprofloxacin and meropenem combination was only synergistic against 2 strains (6.2%) and ciprofloxacin and imipenem against 1 strain (3.1%). Levofloxacin and imipenem or meropenem were not synergistic for against any strain. None of the combinations showed an antagonistic effect.

Cefepime, piperacillin/tazobactam, gentamicin, ciprofloxacin, and levofloxacin alone and in combination against Pseudomonas aeruginosa

A beta-lactam plus an aminoglycoside is the standard for treating severe Pseudomonas aeruginosa infections. However, the fluoroquinolones are safer and have been widely used as an alternative to the aminoglycosides in this setting. In this study we compared the synergistic activities of piperacillin/tazobactam and cefepime when either drug was combined with gentamicin, ciprofloxacin, or levofloxacin against P. aeruginosa. Susceptibility testing and time-kill curves were performed against 12 clinical isolates of P. aeruginosa. All combinations were bactericidal and retained this activity over the 24 hr period except for piperacillin/tazobactam in combination with levofloxacin or ciprofloxacin against 2 isolates and cefepime in combination with levofloxacin against 1 isolate. None of the combinations were antagonistic. No statistical difference in the frequency of synergy exists between the beta-lactam plus gentamicin (79%) and the beta-lactams plus either ciprofloxacin or levofloxacin combinations (58%, 67%). Furthermore, no differences in synergistic activity were noted between ciprofloxacin combinations (58%) and levofloxacin combinations (67%). In conclusion, the degree of synergy between a beta-lactam plus aminoglycoside and a beta-lactam plus fluoroquinolone seem to be comparable. Furthermore, there is a similar rate of synergy among different fluoroquinolone-based combinations. However, faster killing, less regrowth, and decrease in the development of resistance were seen with the beta-lactam plus aminoglycoside combination.

Effectiveness of fosfomycin combined with other antimicrobial agents against multidrug-resistant Pseudomonas aeruginosa isolates using the efficacy time index assay

We investigated the effectiveness of fosfomycin combined with other antibiotics, such as piperacillin, cefepime, ceftazidime, imipenem, meropenem, aztreonam, gentamicin, or levofloxacin, against 30 Pseudomonas aeruginosa strains, including multidrug-resistant strains, isolated from clinical specimens, using the efficacy time index (ETI) assay. The assay refers to the result of pharmacokinetics obtained from adult men volunteers, and yields an ETI to evaluate the effect of a combination of antimicrobial agents. With the ETI, based on serum concentration 3 h after the administration of two antimicrobial agents, the effectiveness of antimicrobial combinations was evaluated as follows: poor, ETI < 0.5; fair, 0.5 < or = ETI < 1; good, 1 < or = ETI < 8; and excellent, ETI > or = 8. The combination of fosfomycin and cefepime (efficacy rate [excellent plus good], 76.7%) and fosfomycin/aztreonam (efficacy rate, 76.7%) appeared to be the most effective, followed by fosfomycin/meropenem (efficacy rate, 76.6%), fosfomycin/imipenem (efficacy rate, 73.3%), fosfomycin/ceftazidime (efficacy rate, 70%), fosfomycin/gentamicin (efficacy rate, 70%), fosfomycin/piperacillin (efficacy rate, 66.7%), and fosfomycin/levofloxacin (efficacy rate, 66.7%). Fosfomycin/cefepime, fosfomycin/aztreonam, and fosfomycin/meropenem may be clinically useful in selected patients, particularly for P. aeruginosa. The ETI assay provided information on the minimum inhibitory concentration (MIC) of many pairs of combined antimicrobial agents simultaneously. The ETI assay may be a useful technique with which to investigate the effect of combinations of antimicrobial agents against P. aeruginosa, including multidrug-resistant strains.

In vitro synergy testing of levofloxacin, ofloxacin, and ciprofloxacin in combination with aztreonam, ceftazidime, or piperacillin against Pseudomonas aeruginosa

The synergistic potential of levofloxacin, ofloxacin and ciprofloxacin combined with aztreonam, ceftazidime, or piperacillin was compared using 24 strains of Pseudomonas aeruginosa with varying susceptibility profiles. Levofloxacin and ciprofloxacin demonstrated similar in vitro activity, with ofloxacin demonstrating less activity compared to the other agents. Predominantly additive effects were seen with all combinations, with no significant differences detected between the fluoroquinolone agents.

The new antibiotics

It is easy to become overwhelmed by the amount of information available on the new antibiotics and difficult to keep abreast of the appropriate indications for each of them. For most patients with community-acquired infections, the first-line agent is usually not one of the newer agents, but a standard regimen, or at times, no antibiotic at all. The development of resistance is likely to parallel the extent to which these agents are prescribed. They should be used only when standard treatment fails, when compliance with treatment is a real and serious issue, or when the patient has a real allergic reaction to the standard regimen.

Anti-anaerobic activity of levofloxacin alone and combined with clindamycin and metronidazole

Microdilution MICs of levofloxacin against twelve anaerobes ranged between 0.5-8.0 microg/ml and those of clindamycin and metronidazole between 0.008-2.0 and 0.25->16.0 microg/ml, respectively. Combination of levofloxacin with clindamycin and/or metronidazole in time-kill tests led to synergy at levofloxacin concentrations at or below the MIC in 7/12 strains.

Summary of Canadian guidelines for the initial management of community-acquired pneumonia: an evidence-based update by the Canadian Infectious Disease Society and the Canadian Thoracic Society

Community-acquired pneumonia (CAP) is a serious illness with a significant impact on individual patients and society as a whole. Over the past several years, there have been significant advances in the knowledge and understanding of the etiology of the disease, and an appreciation of problems such as mixed infections and increasing antimicrobial resistance. The development of additional fluoroquinolone agents with enhanced activity against Streptococcus pneumoniae has been important as well.It was decided that the time had come to update and modify the previous CAP guidelines, which were published in 1993. The current guidelines represent a joint effort by the Canadian Infectious Diseases Society and the Canadian Thoracic Society, and they address the etiology, diagnosis and initial management of CAP. The diagnostic section is based on the site of care, and the treatment section is organized according to whether one is dealing with outpatients, inpatients or nursing home patients.

Activity of piperacillin/tazobactam in combination with amikacin, ciprofloxacin, and trovafloxacin against Pseudomonas aeruginosa by time-kill

Pseudomonal infections have a high rate of morbidity and mortality, thus combination therapy is often recommended. We compared the activity of piperacillin/tazobactam in combination with amikacin, ciprofloxacin, or trovafloxacin at different concentrations against P. aeruginosa using time-kill methodology. MICs were determined for 4 clinical isolates of P. aeruginosa. Time-kill studies were conducted over 24 h. Each drug was tested alone and in combination using the following concentrations: 2 and 1/4, 1/4 and 2, and 1/4 and 1/4xMIC of piperacillin/tazobactam and amikacin, ciprofloxacin, or trovafloxacin. Combinations were classified as synergistic, indifferent, or antagonistic. Synergy was defined as > or = 2-log(10) decrease in CFU/mL at 24 h with the combination when compared to the most active single agent and the number of surviving organisms for the antimicrobial combination was > or =2-log(10) less than the initial inoculum. The MICs for piperacillin/tazobactam, amikacin, ciprofloxacin, and trovafloxacin, ranged from 4/4-512/4, 0.5-4, 0.125-4, and 0.5-8 microg/mL, respectively. Fifty eight percent of the combinations using concentrations of 1/4xMIC of piperacillin/tazobactam and 2xMIC of amikacin, ciprofloxacin, and trovafloxacin or 2xMIC of piperacillin/tazobactam and 1/4xMIC of amikacin, ciprofloxacin, and trovafloxacin were synergistic. Although no differences existed in synergistic activity between the two combinations, the 1/4 and 2xMIC maintained colony counts below the limit of quantification for 24 h for a significantly greater percentage of isolates than the 2 and 1/4xMIC combinations (75 and 25%, respectively; p = 0.04). Overall, synergy was most frequently (42%) noted with the piperacillin/tazobactam and amikacin combinations followed by 33 and 8% of the piperacillin/tazobactam and trovafloxacin and ciprofloxacin combinations. No combination demonstrated antagonism. Further more extensive studies are necessary to determine clinical significance.

Activities of clinafloxacin, alone and in combination with other compounds, against 45 gram-positive and -negative organisms for which clinafloxacin MICs are high

Time-kill studies indicated that clinafloxacin showed synergy after 24 h with ceftazidime, amikacin, and imipenem against 12, 8, and 10 of 33 gram-negative rods, respectively; with vancomycin, teicoplanin, cefotaxime, and amikacin against 3, 3, 1, and 1 of 9 staphylococci and enterococci, respectively; and with vancomycin, penicillin, and cefotaxime against 0, 2, and 2 of 3 pneumococci, respectively. The MICs of clinafloxacin alone for most strains were >/=1 microg/ml.