The effect of rifampicin on the in-vitro activity of cefpirome or ceftazidime in combination with aminoglycosides against Pseudomonas aeruginosa

The in-vitro activity of cefpirome and ceftazidime when combined with aminoglycosides (gentamicin, amikacin, and tobramycin) in the presence and in the absence of rifampicin was evaluated against 32 isolates of Pseudomonas aeruginosa by two methods. Agar dilution susceptibilities demonstrated a marked reduction in synergy (FIC less than or equal to 0.5) when rifampicin was added to the combination. Synergy rates decreased from 59.4-84.4% without to 3.1-9.4% with the addition of rifampicin. In contrast, kill curve tests performed on two P. aeruginosa strains demonstrated synergy at 24 h when rifampicin was added to cefpirome, ceftazidime, gentamicin or a beta-lactam agent plus gentamicin combination. The addition of rifampicin to the combinations of cefpirome or ceftazidime plus gentamicin achieved a 2-log10 lower bacterial count at 24 h than that of the beta-lactam and gentamicin combination alone. When rifampicin was added to the combination cefpirome or ceftazidime plus gentamicin at different times during incubation, a greater bactericidal effect was observed when rifampicin was added at 0 and 1 h of incubation than when added later. No antagonism was observed with rifampicin when used in combination with beta-lactam agents and/or aminoglycosides.     

Cefoperazone against carbenicillin-resistant isolates of Pseudomonas aeruginosa: comparison with other newer cephalosporins and N-formimidoyl thienamycin

The in-vitro activities of cefoperazone, cefotaxime, ceftriaxone, latamoxef (moxalactam), ceftazidime and N-formimidoyl-thienamycin (N-f-thienamycin) were compared against clinical isolates of Pseudomonas aeruginosa that were resistant to carbenicillin and gentamicin, or against those which were susceptible. N-f-thienamycin and ceftazidime were more active than cefotaxime, ceftriaxone and latamoxef only against carbenicillin-susceptible isolates: isolates resistant to carbenicillin exhibited markedly increased resistance to cefoperazone, but not to N-f-thienamycin or ceftazidime. A partial cross-resistance was observed between carbenicillin and cefoperazone. This was due to the presence in these Pseudomonas isolates of a plasmid-mediated beta-lactamase which was active against carbenicillin and cefoperazone but not against cefotaxime, ceftriazone and latamoxef.     

Comparative activities of ciprofloxacin, ticarcillin, and tobramycin against experimental Pseudomonas aeruginosa pneumonia.

The therapeutic efficacy of ciprofloxacin, an investigational quinoline derivative, was compared with those of ticarcillin and tobramycin in guinea pigs with experimental Pseudomonas aeruginosa pneumonia. Guinea pigs challenged with tracheal instillations of 10(8) CFU of P. aeruginosa developed acute pneumonia, for which survival rates were: controls, 0%; ticarcillin treatment, 37%; ciprofloxacin treatment, 57%; and tobramycin treatment, 69%. Intrapulmonary killing of P. aeruginosa was greater (P less than 0.05) in animals treated with ciprofloxacin or tobramycin than in groups treated with ticarcillin. A more chronic, nonfatal form of bronchopneumonia caused by P. aeruginosa was induced with agar beads impregnated with bacteria for pulmonary challenge. In this model, ciprofloxacin treatment resulted in significantly (P less than 0.001) greater intrapulmonary killing than did any other therapy. These data suggest that ciprofloxacin may be useful in the treatment of acute and more-chronic forms of pulmonary infection caused by P. aeruginosa.     

Comparative in-vitro activities of azlocillin, ticarcillin and ticarcillin plus clavulanic acid against Staphylococcus aureus, Klebsiella pneumoniae and Pseudomonas aeruginosa

Azlocillin (5 g), ticarcillin (5 g) and ticarcillin (5 g) plus clavulanic acid (200 mg) were administered intravenously on separate days to human volunteers. Serum levels 1 h after the infusion were (mean +/- S.E.): 263 +/- 40 mg/1 for azlocillin, 238 +/- 56 mg/l for ticarcillin and 5.5 +/- 1.9 mg/l for clavulanic acid. Serum withdrawn at 0.5 h was titrated against ten strains of Staphylococcus aureus, ten Klebsiella pneumoniae and nine Pseudomonas aeruginosa of which four were resistant to ticarcillin. Against Staph. aureus and K. pneumoniae, median serum bactericidal activity (SBA) and percentage of sera with SBA greater than or equal to 1:8 were greater with ticarcillin plus clavulanic acid (median SBA 1:32 and 85% of sera with SBA greater than or equal to 1:8) than with the other two regimens. There were no differences in activity against Ps. aeruginosa.     

In vitro activity of E1040 against imipenem-resistant Pseudomonas aeruginosa strains.

E1040 showed the most potent activity (MIC for 90% of strains, 6.25 micrograms/ml) among beta-lactams tested against 70 strains of imipenem-resistant Pseudomonas aeruginosa. Two strains showed high-level resistance to E1040; one strain produced a type II oxyiminocephalosporin-hydrolyzing beta-lactamase (group 3), and the other produced an enzyme similar to a type II penicillinase (OXA-1). Both beta-lactamases contributed to resistance to E1040.     

Comparative activities of N-formimidoyl thienamycin, ticarcillin, and tobramycin against experimental Pseudomonas aeruginosa pneumonia.

The therapeutic efficacies of disodium ticarcillin, tobramycin sulfate, and N-formimidoyl thienamycin (MK0787) were compared in guinea pigs with experimentally induced Pseudomonas aeruginosa pneumonia. Survival rates were 35% for ticarcillin, 80% for tobramycin, and 75% for N-formimidoyl thienamycin. Numbers of viable Pseudomonas organisms in lungs approximately 3 h after the first dose of drug were nearly 10-fold fewer in tobramycin- or N-formimidoyl thienamycin-treated animals than in ticarcillin-treated animals. Our data suggest that N-formimidoyl thienamycin may have therapeutic efficacy against respiratory infections with P. aeruginosa equivalent to that of tobramycin.     

Comparative activity of cefepime, alone and in combination, against clinical isolates of Pseudomonas aeruginosa and Pseudomonas cepacia from cystic fibrosis patients.

The comparative in vitro activity and synergy of cefepime were evaluated with clinical isolates of Pseudomonas aeruginosa and Pseudomonas cepacia from cystic fibrosis patients. The activity of cefepime, both alone and in combination, was comparable to those of other antibiotics. The clinical efficacy of cefepime in cystic fibrosis patients merits investigation.     

Comparative in vivo efficacy of meropenem, imipenem, and cefepime against Pseudomonas aeruginosa expressing MexA-MexB-OprM efflux pumps

To identify the optimal pharmacodynamic exposures of meropenem, imipenem, and cefepime, and the emergence of resistance in vivo for Pseudomonas aeruginosa overexpressing MexA-MexB-OprM efflux pumps, we used the murine thigh model. Mice were challenged with P. aeruginosa isolates: PAO1 (K767 wild type), K767+ (MexA-MexB-OprM efflux mutant), and DeltaK767 (knockout strain). Efficacy (Delta log colony-forming unit [CFU]) was determined at various exposures of %T > MIC at both standard (10(5) CFU/thigh) and high (10(7) CFU/thigh) inoculums. At 10(5) CFU/thigh, meropenem and imipenem produced a maximal activity against PAO1 (-2.82, -1.88) and K767+ (-2.24, -2.68) at 40%T > MIC; cefepime at 70%T > MIC produced a comparable kill (-2.74 and -2.19, respectively). Similar magnitudes of kill were observed at the 10(7) inocula. Except for DeltaK767 with cefepime, no development of resistance emerged at various %T > MIC. All agents exhibited reduced activity against DeltaK767. DeltaK767 cefepime-resistant strains were isolated up to 100%T > MIC. The overexpression of MexA-MexB-OprM efflux pumps did not result in the loss of efficacy of the antibiotics tested regardless of the amount of bacterial inocula; however, their presence also did not lead to increased selection for resistance. The effects of efflux mechanisms on beta-lactam agents in vivo warrant further research.     

Comparative antibacterial activities of new beta-lactam antibiotics against Pseudomonas aeruginosa

In vitro activity of nine new cephalosporins and penicillins was determined against 417 isolates of Pseudomonas aeruginosa. Carbenicillin, ticarcillin, gentamicin, tobramycin and netilmicin were also included in the study. Imipenem showed highest activity. More than 90% of the isolates were susceptible to ticarcillin, piperacillin, azlocillin, cefoperazone, cefsulodin and to tobramycin. 57 isolates included in the study were resistant to gentamicin (MIC greater than 4 mg/l); of these, none were resistant to imipenem, and more than 80% were susceptible to piperacillin, azlocillin, cefoperazone and cefsulodin.     

Comparison of cefepime, cefpirome, and cefaclidine binding affinities for penicillin-binding proteins in Escherichia coli K-12 and Pseudomonas aeruginosa SC8329.

The relative binding affinities of the extended-spectrum cephalosporins cefepime, cefpirome, and cefaclidine for the penicillin-binding proteins (PBPs) of Escherichia coli K-12 and Pseudomonas aeruginosa SC8329 were determined. Affinities were calculated from competition experiments between these antibiotics and [3H]benzylpenicillin in isolated membrane preparations. The concentrations which reduced binding to a PBP by 50% (IC50s) were determined. For E. coli, all three antibiotics displayed good PBP 3 binding (IC50s of 0.5 microgram/ml or less), and MICs roughly correlated with these values. Cefepime had a greater than 20-fold-lower IC50 for PBP 2 of E. coli than the other antibiotics. For P. aeruginosa, all of the antibiotics bound poorly (greater than 25 micrograms/ml) to PBP 2 but showed excellent pseudomonal (less than 0.0025 microgram/ml) PBP 3 binding. No correlations were seen between IC50s and MICs for P. aeruginosa. Despite differences in PBP binding, cefepime, cefpirome, and cefaclidine all displayed similar bactericidal activity for E. coli K-12 over the initial 3 h after antibiotic addition. All three caused E. coli to form filaments at values close to the MICs. In addition, cefepime induced "bleb" formation along the filaments at concentrations greater than 10x the MIC.     

Comparative in vitro exoenzyme-suppressing activities of azithromycin and other macrolide antibiotics against Pseudomonas aeruginosa.

The inhibitory effects of azithromycin (AZM), a new 15-membered macrolide antibiotic, on the production of exotoxin A, total protease, elastase, and phospholipase C by Pseudomonas aeruginosa were determined, and the virulence-suppressing effects of AZM were compared with those of erythromycin (EM), roxithromycin (RXM), and rokitamycin (RKM). The effect of exposure of P. aeruginosa PA103 or B16 in cultures to sub-MICs of these macrolide antibiotics on the production of exoenzymes was determined. AZM suppressed the in vitro production of extracellular and intracellular exotoxin A by P. aeruginosa PA103 more than did EM, even at a concentration of only 2 micrograms/ml. At concentrations of between 4 and 32 micrograms/ml, AZM also inhibited total protease, elastase, and phospholipase C production by P. aeruginosa B16 more than did EM, RXM, and RKM. AZM was effective in suppressing exotoxin A and total protease production through 24 h of incubation in the presence of drug at sub-MICs, but it had no significant effect on either the growth of P. aeruginosa or its total protein production. Moreover, at a concentration of 4 micrograms/ml, AZM suppressed exoenzyme production by other strains of P. aeruginosa more than did EM. These findings indicate that AZM, EM, RXM, and RKM each has an inhibitory effect on exoenzyme production separate from the antimicrobial effect and that, of these macrolides, AZM has the strongest virulence-suppressing effect.     

Comparative in vitro bactericidal activity between cefepime and ceftazidime, alone and associated with amikacin, against carbapenem-resistant Pseudomonas aeruginosa strains

Fifteen unique isolates of carbapenem-resistant Pseudomonas aeruginosa were selected for time-kill studies to assess the bactericidal activity of cefepime (CFP) and ceftazidime (CZD) (at 4 and 16 microg/mL), alone and associated with amikacin (AMK) (4 microg/mL). CFP proved more active than CZD (p < 0.05, Student's t test). Bactericidal activity after 24-h incubation was only achieved by the combination of CFP (16 microg/mL) plus AMK. The higher in vitro activity of cefepime over that of ceftazidime against imipenem-resistant P. aeruginosa strains highlights the differences of these drugs beyond Enterobacterspp. and Staphylococcus aureus.     

Activity of cefepime against ceftazidime- and cefotaxime-resistant gram-negative bacteria and its relationship to beta-lactamase levels.

One hundred clinical isolates resistant to ceftazidime and/or cefotaxime were examined for susceptibility to cefepime. The most frequently encountered ceftazidime-cefotaxime-resistant strains belonged to the genera Enterobacter, Pseudomonas, and Citrobacter. Among these strains, 92% were resistant to cefoperazone, 91% were resistant to cefotaxime, 84% were resistant to ceftazidime, and 6% were resistant to cefepime. Of the members of the family Enterobacteriaceae, 57% were resistant to ceftriaxone. The six strains resistant to cefepime were all Pseudomonas aeruginosa and were resistant to both cefotaxime and ceftazidime. Cefepime-resistant P. aeruginosa strains had exceptionally high levels of beta-lactamase activity, higher than the levels found in strains resistant to ceftazidime but susceptible to cefepime. The beta-lactamases from the cefepime-resistant strains were type I (Richmond-Sykes), were constitutively produced, and did not have increased affinity or hydrolytic activity for cefepime. Thus, cefepime was active against most gram-negative bacteria which have developed resistance to the broad-spectrum cephalosporins, and resistance to cefepime in P. aeruginosa appears to be associated with higher beta-lactamase levels than in cefepime-susceptible strains.     

In vitro activity of combination therapy with cefepime, piperacillin-tazobactam, or meropenem with ciprofloxacin against multidrug-resistant Pseudomonas aeruginosa strains

The prevalence of multidrug-resistant Pseudomonas aeruginosa strains has been increasing every year, and treatment with various antimicrobial combinations has been offered alternatively in the clinical practice. The aim of this study was to evaluate the in vitro effects of combinations of meropenem, cefepime, or piperacillin-tazobactam with ciprofloxacin against multidrug-resistant P. aeruginosa strains by the time-kill method. The results show that both the combination of two beta lactams with ciprofloxacin and three beta lactams with ciprofloxacin against 4 of 5 multidrug-resistant P. aeruginosa strains has no effect in vitro. The combination of one beta lactam (meropenem, cefepime, piperacillin-tazobactam) with ciprofloxacin has a synergistic effect against one strain of multidrug-resistant P. aeruginosa that is ciprofloxacin susceptible and resistant to meropenem, cefepime, and piperacillin-tazobactam. None of the combinations had an antagonistic effect against these multidrug-resistant strains.     

Synergy of new C-3 substituted cephalosporins and tobramycin against Pseudomonas aeruginosa and Pseudomonas cepacia

The effects of the combination of E-1040, a new cephalosporin, ceftazidime, cefpirome, or cefepime with tobramycin against 40 Pseudomonas aeruginosa and 16 P. cepacia isolates from cystic fibrosis patients were examined. Synergy fractional inhibitory concentration less than or equal to 0.5 was found against P. aeruginosa when tobramycin was combined with E-1040 32%, cefpirome 32%, cefepime 22%, and ceftazidime 22%. Fifty-two percent of isolates showed an additive response for E-1040, 60% for cefpirome, 45% cefepime, and 55% ceftazidime, p greater than 0.05. The differences were not statistically significant. Synergy was not more likely to be achieved if the isolates were susceptible or resistant to either the aminoglycoside or cephalosporin. None of the E-1040-resistant isolates, all of which were tobramycin-resistant, became susceptible when tested with an aminoglycoside, whereas 15 of 34 cefpirome-resistant, 13 of 30 cefepime-resistant, and seven of 14 ceftazidime-resistant isolates became susceptible. Synergy of aminoglycoside and the cephalosporins against P. cepacia was found for 25% with E-1040, 44% with cefpirome, 38% with cefepime, and 31% with ceftazidime. These differences were not statistically significant.     

Comparative in vitro activities of beta-lactam-tobramycin combinations against Pseudomonas aeruginosa and multidrug-resistant gram-negative enteric bacilli.

Piperacillin was more consistently active than tobramycin, carbenicillin, moxalactam, or ceftriaxone against strains of Pseudomonas aeruginosa isolated from blood cultures and against multidrug-resistant strains. Moxalactam and ceftriaxone were more consistently active than tobramycin, carbenicillin, or piperacillin against multidrug-resistant Enterobacteriaceae. Synergy between beta-lactam antibiotics and tobramycin was frequently observed against strains of P. aeruginosa isolated from blood cultures but not against multidrug-resistant organisms. Piperacillin plus tobramycin was the most active antibiotic combination against P. aeruginosa. Moxalactam plus tobramycin and ceftriaxone plus tobramycin were the most active antibiotic combinations against Enterobacteriaceae.     

Synergic activity of cephalosporins plus fluoroquinolones against Pseudomonas aeruginosa with resistance to one or both drugs

Owing to increasing resistance in Pseudomonas aeruginosa, empirical drug regimens may include agents to which some strains may be resistant. The purpose of this study was to evaluate the in vitro activities of different combinations of cephalosporin plus fluoroquinolone against P. aeruginosa isolates with varying susceptibility to the study drugs. Broth microdilution susceptibility testing was performed with 10 clinical isolates of P. aeruginosa. The bactericidal activity of cefepime or ceftazidime alone and in combination with ciprofloxacin, levofloxacin, gatifloxacin or moxifloxacin was evaluated using time-kill methods. Colony counts were determined at 0, 4, 8 and 24 h, using antimicrobial concentrations of 0.5 x MIC. All procedures were performed in duplicate. Synergy was defined as a >2-log decrease in cfu/mL at 24 h compared with the single most active agent. The MICs for tested strains were: ceftazidime 0.75-32, cefepime 0.125-8, ciprofloxacin 0.0078-8, levofloxacin 0.023-16, gatifloxacin 0.023-16 and moxifloxacin 0.0521-32 mg/L. Four strains were susceptible to all drugs, two strains were cephalosporin susceptible and fluoroquinolone resistant, and two strains were cephalosporin resistant and fluoroquinolone susceptible. Two strains were resistant or intermediately susceptible to all drugs. Various cephalosporin and fluoroquinolone combinations were synergic against P. aeruginosa, including strains resistant to one or both agents in combination. No synergy was observed in two strains susceptible to all drugs. There were no differences noted between different cephalosporin and fluoroquinolone combinations. Concentrations used in this study are clinically achievable with recommended regimens in most cases.