In vitro activity of piperacillin, ticarcillin, and mezlocillin alone and in combination with aminoglycosides against Pseudomonas aeruginosa.

A total of 103 isolates of Pseudomonas aeruginosa were studied to compare the in vitro effectiveness of three beta-lactam antibiotics (piperacillin, ticarcillin, and mezlocillin) when used alone and in combination with four aminoglycosides (tobramycin, gentamicin, amikacin, and netilmicin). All drugs were tested as single agents against a standard inoculum (5 X 10(5) CFU/ml). The three antipseudomonal penicillins were also tested against the isolates at a higher inoculum concentration (10(7) CFU/ml). Synergy testing was performed by the two-dimensional checkerboard method and was defined by a fractional bactericidal index of less than or equal to 0.5 and bacterial killing accomplished at antibiotic concentrations no greater than those achievable in serum. All combinations were assessed for synergy. The degree of synergy was further analyzed by dividing the isolates into groups based on their susceptibility and resistance to the individual agents in the combination. The overall effectiveness of the various aminoglycoside-antipseudomonal penicillin combinations was assessed regarding their ability to kill the isolates either as single agents or through synergy. Piperacillin was the most active antipseudomonal penicillin, and tobramycin and amikacin were the most active aminoglycosides when used as single agents. When tested against isolates at a higher inoculum concentration, ticarcillin was significantly more active than the other beta-lactams. The highest degree of overall synergy was noted with gentamicin-ticarcillin (78.2% of strains) and amikacin-piperacillin (77% of strains). When assessed for overall effectiveness, all combinations containing amikacin were the most active. The combination of amikacin-piperacillin was the most effective, with activity against 96% of all isolates.     

Antibacterial activity of ticarcillin, tobramycin and gentamicin alone and in combination against Pseudomonas aeruginosa in vitro.

Using the biophotometer with ticarcillin no persistent bactericidal effect was found against Pseudomonas aeruginosa NCTC 10490. After addition of 1.2 microgram/ml gentamicin an increase of multiplication of bacteria was observed, but not after 1.2 microgram/ml tobramycin. With 6.2 microgram/ml tobramycin bactericidal effects lasted more than 24 h. In tube dilution test with Isotonic Sensi-test Broth out of 109 examined strains 51% were resistant to gentamicin, 16% to tobramycin and 4.5% to ticarcillin. If MIC values of gentamicin and tobramycin were calculated for magnesium-free media the resistance rate would be 10% for gentamicin and 3% for tobramycin. Combining subinhibitory doses of gentamicin or tobramycin with ticarcillin, most of the strains resistant to gentamicin and tobramycin became susceptible. The rate of inactivation of tobramycin by ticarcillin depends on the fluid into which they are placed. In combination therapy both antibiotics should be applied separately and immediately one after the other.     

In vitro activity of sisomicin and netilmicin alone and in combination with nafcillin, oxacillin and methicillin against enterococci

The in vitro activity of sisomicin and netilmicin alone and in combination with nafcillin, oxacillin and methicillin against 30 strains of enterococci was investigated. Sisomicin and netilmicin alone were not very effective against enterococci. There was enhanced killing of some strains of enterococci by the combination of sisomicin or netilmicin with one of the three penicillinase-resistant penicillins. Nafcillin was the most effective penicillinase-resistant penicillin in the combination. Sisomicin appeared to be slightly more effective than netilmicin in combination with one of the penicillinase-resistant penicillins against enterococci.     

In vitro activity of ceftriaxone alone and in combination with gentamicin, tobramycin, and amikacin against Pseudomonas aeruginosa.

The in vitro activity of ceftriaxone alone and in combination with gentamicin, tobramycin, and amikacin against 50 Pseudomonas aeruginosa strains was studied by the broth dilution method and the time-kill curve method. The majority of the P. aeruginosa strains tested were resistant to ceftriaxone. Combining ceftriaxone with the aminoglycosides resulted in synergism, antagonism, or indifference.     

Effect of calcium, magnesium, and zinc on ticarcillin and tobramycin alone and in combination against Pseudomonas aeruginosa.

Correlation between in vitro and in vivo test results for synergy between carboxypenicillins and aminoglycosides against Pseudomonas aeruginosa is poor. Although the divalent cation content of culture media is known to affect aminoglycoside susceptibility testing for P. aeruginosa, this effect of divalent cations has not been examined for synergy testing of carboxypenicillin-aminoglycoside interaction against P. aeruginosa. The minimal inhibitory concentrations (MICs) of tobramycin and ticarcillin and the interaction of these drugs in combination were studied by a microtitration method for 36 strains of P. aeruginosa in Mueller-Hinton broth with varying supplements of calcium, magnesium, and zinc. The supplementation of Mueller-Hinton broth to 50 or 100 mg of calcium per liter had a significant effect in increasing the tobramycin MIC (P less than 0.01), as well as decreasing the degree of synergy between ticarcillin and tobramycin (P less than 0.01). Supplementation to 20 mg of magnesium per liter, 1.0 mg of zinc per liter, or both did not significantly affect tobramycin MIC or the interaction of tobramycin and ticarcillin. Supplementation to 50 or 100 mg of calcium per liter rendered any additional effect of magnesium and zinc on aminoglycoside MIC and aminoglycoside-carboxypenicillin interaction negligible. If these results for ticarcillin and tobramycin are confirmed for other carboxypenicillins and aminoglycosides, then the Mueller-Hinton broth used for P. aeruginosa aminoglycoside susceptibility and synergy testing may need to be supplemented only with calcium at a concentration of 50 mg/liter.     

In vitro activities of tritrpticin alone and in combination with other antimicrobial agents against Pseudomonas aeruginosa

The in vitro activity of the cathelicidin tritrpticin was investigated against multidrug-resistant Pseudomonas aeruginosa. The isolates were susceptible to the peptide at concentrations of 0.50 to 8 mg/liter. Tritrpticin completely inhibits lipopolysaccharide procoagulant activity at a 10 microM concentration. Fractionary inhibitory concentration indexes (0.385, 0.312, and 0.458) demonstrated synergy between the peptide and beta-lactams.     

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.     

Influence of cation supplements on activity of netilmicin against Pseudomonas aeruginosa in vitro and in vivo.

In vitro studies were performed with 74 Pseudomonas aeruginosa isolates which were collected during a multicenter trial. The isolates were obtained from 70 patients who had been treated with netilmicin as the only antipseudomonal antibiotic. Clinically, 83% of the patients were cured or improved, and 64% of the Pseudomonas isolates were eliminated by chemotherapy. The 74 clinical isolates and 38 additional isolates with known mechanisms of aminoglycoside resistance were tested in three separate laboratories by disk diffusion methods and by microdilution tests with three broth media (Mueller-Hinton broth with full, half, and no cation supplements). Isolates that responded to netilmicin therapy and those that failed to respond were all susceptible by the disk test, and most were susceptible by microdilution tests with unsupplemented broth. However, over half of the clinical isolates appeared to be resistant when cations were added to the broth medium. Strains capable of producing enzymes that inactivate netilmicin were resistant by all methods tested. Broth dilution and agar dilution results were most comparable when half of the recommended cation supplements was added to Mueller-Hinton broth. Further consideration should be given to reducing the concentration of cations that are added to Mueller-Hinton broth when netilmicin susceptibility tests are being performed. However, additional studies with other aminoglycosides are needed before appropriate testing conditions can be standardized.     

Bactericidal and synergistic activity of moxalactam alone and in combination with gentamicin against Pseudomonas aeruginosa.

The bactericidal activity of moxalactam, alone and in combination with gentamicin, was studied with macrobroth two-dimensional checkerboard and killing curve techniques against gentamicin-resistant and -susceptible strains of Pseudomonas aeruginosa. Moxalactam was bactericidal at concentrations equal to or at least two to four times its inhibitory concentrations. Synergy at clinically applicable concentrations of moxalactam and gentamicin occurred with 6 of 14 gentamicin-resistant strains and 4 of 4 gentamicin-susceptible strains by the checkerboard technique and with 7 of 14 gentamicin-resistant strains by the killing curve technique. Synergy between moxalactam and gentamicin against gentamicin-resistant strains of P. aeruginosa is unpredictable and strain- and method-dependent.     

Latamoxef in combination with aminoglycosides against Pseudomonas aeruginosa: similarity with ticarcillin

The in vitro effect of latamoxef against 50 clinical strains of Pseudomonas aeruginosa was compared to that of ticarcillin, both alone and in combination with the aminoglycosides gentamicin, tobramycin and amikacin. Alone, the MIC90 of latamoxef was consistently one-half the MIC90 of ticarcillin. The two antibiotics appeared similar in regard to inoculum effect and bacterial killing. Adding of one-quarter the minimum inhibitory concentration of the aminoglycoside antibiotic to the beta lactam caused reduction in MIC90 of the latter (either ticarcillin or latamoxef) by one-half and decreased the MIC50 by almost one-quarter the concentration required by the beta lactams singly. Therefore, latamoxef singly or in combination with aminoglycosides behaved similarly but was more active than ticarcillin. Using combinations of antibiotics likely to be achieved in the serum of patients, a beneficial in vitro effect (either additive, partially synergistic or synergistic) generally occurred for the beta lactam-aminoglycoside combination if the strain was relatively sensitive to the aminoglycoside used in this combination. It occurred much less frequently for the highly aminoglycoside-resistant isolates.     

In vitro activity of ceftriaxone and amikacin against Pseudomonas aeruginosa

Ceftriaxone and amikacin were combined at ratios of 1:1, 5:1, and 10:1 and tested in vitro against Pseudomonas aeruginosa. The activity was determined using the Steers-Foltz replicator and the agar dilution technique with Mueller-Hinton agar. Under all conditions tested, including those simulating severe infection (10(5) to 10(6) colony-forming units per spot), the organisms were more susceptible to the combination than to the single agents. With a conventional inoculum of 10(4) colony-forming units per spot, the combinations gave 97-100% coverage against P. aeruginosa. The increased activity of the combinations resulted in MIC90 values which were below the expected serum/plasma levels for significantly longer time periods than the MIC90 values observed with the individual agents.     

Comparison of the in vitro activity of BL-P1654 with gentamicin and carbenicillin against Pseudomonas aeruginosa

The in vitro activity of 6-[d-alpha-(3 guanylureido)-phenylacetamido]-penicillanic acid (BL-P1654) was evaluated against 117 clinical isolates of Pseudomonas aeruginosa, many of which were known to be resistant to both gentamicin and carbenicillin. BL-P1654 was two to eight times more active than carbenicillin against P. aeruginosa. However, all 28 highly carbenicillin-resistant isolates (minimal inhibitory concentration [MIC] > 500 mug/ml) were also highly resistant to BL-P1654. When an MIC of 32 mug/ml or less and a zone of inhibition of 12 mm or more by a 10-mug disk were used as criteria indicating susceptibility to BL-P1654, the false-resistance rate by the disk test was 10.6% and the false-susceptibility rate was 4.2%. The combination of BL-P1654 and gentamicin was synergistic against 45 of 70 isolates of P. aeruginosa tested, but synergism was demonstrated against only 4 of 24 highly gentamicin-resistant (MIC > 63 mug/ml), 1 of 12 highly BL-P1654-resistant (MIC > 250 mug/ml) isolates, and none of nine isolates highly resistant to both of these antibiotics.     

In vitro activity of antibiotics alone and in combination against Actinobacillus actinomycetemcomitans.

The MICs for 90% of the organisms tested (MIC90S) of 11 antibiotics against 24 clinical isolates of Actinobacillus actinomycetemcomitans were determined by the MIC 2000 system. The lowest MIC90S (16 micrograms/ml) were observed with ceftriaxone and rifampin. The next lowest MIC90S were found with cephapirin, tetracycline, and chloramphenicol (3.12 micrograms/ml). The MIC90S of penicillin, ampicillin, ticarcillin, piperacillin, and amikacin were each greater than or equal to 12.5 micrograms/ml. Antibiotic synergy was studied by the killing curve method and was defined as a greater than or equal to 2 log10 reduction in CFU when two antibiotics were used in combination at one-fourth the MBC for each compared with the effect of each antibiotic alone at one-half the MBC. Synergism between rifampin and penicillin, cephapirin, or ceftriaxone was tested for with 12 A. actinomycetemcomitans strains. In 7 of 37 instances, synergism was demonstrated for the combinations rifampin plus ceftriaxone (n = 3) or rifampin plus penicillin (n = 4); in 9 instances, an additive effect was noted, and impaired killing with drug combinations compared with the effect of a single antibiotic was suggested in 4 strains. The majority of strains were indifferent to the combinations. Similarly, variable results were observed when the combination of trimethoprim and cephapirin was tested against eight A. actinomycetemcomitans strains. Our data suggest that rifampin and cephapirin are the most active of the 11 antibiotics studied against A. actinomycetemcomitans. In addition, in vitro synergism between rifampin and other antibiotics or between trimethoprim and cephapirin was not consistently demonstrable.     

In vitro activity of ciprofloxacin in combination with ceftazidime, aztreonam, and azlocillin against multiresistant isolates of Pseudomonas aeruginosa.

The combinations of ciprofloxacin plus ceftazidime, ciprofloxacin plus aztreonam, and ciprofloxacin plus azlocillin were evaluated for the presence of synergy against multiresistant isolates of Pseudomonas aeruginosa. The frequency of synergy was dependent on antibiotic susceptibilities. If the organism was resistant to ciprofloxacin, synergy was observed in more than 50% of the isolates; however, if the organism was resistant to the beta-lactam (with the exception of ceftazidime), synergy was generally observed in less than 10% of the isolates. Antagonism was not observed with any of the combinations. These results may be helpful in making clinical decisions in treating P. aeruginosa infections.     

In vitro activity of fosfomycin, alone and in combination, against methicillin-resistant Staphylococcus aureus.

We tested 148 strains of clinical isolates of methicillin-resistant Staphylococcus aureus against fosfomycin alone and in combination with methicillin, cefamandole, gentamicin, trimethoprim, and vancomycin. Fosfomycin inhibited 90% of the 148 methicillin-resistant S. aureus strains at a concentration of 4 micrograms/ml. Synergism was observed in 97 strains (66%) with fosfomycin-cefamandole and in 69 strains (46%) with fosfomycin-methicillin. The combinations of fosfomycin with vancomycin, gentamicin, and trimethoprim were indifferent in most strains.     

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.     

Effect of clavulanic acid on the activity of ticarcillin against Pseudomonas aeruginosa.

We studied the ability of clavulanic acid (CA) to induce beta-lactamase in Pseudomonas aeruginosa isolates and what effect this might have on the susceptibilities to beta-lactam agents. We first used a disk approximation method to test 4 laboratory and 16 clinical P. aeruginosa isolates against antipseudomonal beta-lactam agents for truncation by CA and found this to be very common. All antimicrobial compounds except imipenem demonstrated truncation in the vicinity of CA. We also evaluated the extent to which chromosomal beta-lactamase is induced by CA and found this to occur to some degree in most isolates and to be dependent on the concentration of CA. Finally, we performed time kill curves on these isolates to compare bacterial growth in ticarcillin alone with growth in ticarcillin-CA (the CA at 2 or 4 micrograms/ml). We found that CA at this concentration has neither an antagonistic nor a synergistic antibacterial effect in combination with ticarcillin.