In vitro studies of activities of some antifungal agents against Candida albicans ATCC 10231 by the turbidimetric method.

Different criteria (the drug concentration which inhibited 50% of growth [IC1/2], the lowest drug concentration at which growth was just less than 30% of that in a positive control well [IC30], the visual inhibitory concentration [ICv], and the minimum fungicidal concentration [MFC]) were applied to study the effects of some antifungal agents against Candida albicans. Amphotericin B, flucytosine, and bifonazole produced total growth inhibition. Clotrimazole, itraconazole, ketoconazole, and miconazole produced partial growth inhibition. The values of IC1/2 and IC30 were similar for all agents and avoided the problems of partial inhibition; the values of ICv and MFC were higher than those of IC1/2 and IC30.     

In vitro biofilm characterization and activity of antifungal agents alone and in combination against sessile and planktonic clinical Candida albicans isolates

Thirty clinical isolates of Candida albicans were collected from blood or other sterile site infections. Biofilm dry weight and metabolic activity were measured for each isolate. Planktonic and sessile antifungal susceptibilities of each isolate were determined for amphotericin B deoxycholate, caspofungin, and voriconazole. Sessile susceptibilities were determined for the combination of caspofungin/voriconazole. No significant differences in biofilm dry weight or metabolic activity were found between bloodstream and other invasive isolates. Planktonic MIC90 values and sessile MIC90 (SMIC90) values were 0.25 and 2, 0.06 and >256, and 0.5 and 2 microg/mL for amphotericin, voriconazole, and caspofungin, respectively. The SMIC90 of the combination of caspofungin/voriconazole against sessile isolates was 0.5/2 microg/mL. Therefore, the source of invasive C. albicans clinical isolates did not affect in vitro biofilm formation. Susceptibility to antifungal agents decreased when C. albicans was associated with biofilm, and the combination of caspofungin/voriconazole did not appear to provide enhanced activity compared with caspofungin alone.     

In vitro activities of SCH27899 alone and in combination with 17 other antimicrobial agents

SCH27899, an everninomicin antibiotic, was tested for its in vitro activity against 718 bacterial isolates representing 27 species. The Enterobacteriaceae and nonenteric gram-negative bacilli were resistant to > or = 8.0 microg/ml, but all others were inhibited by < or = 1.0 microg/ml. When tested in combination with 17 other antimicrobial agents against 110 strains, SCH27899 demonstrated no significant antagonism or synergy. Consequently, combination therapy is not contraindicated.     

In vitro activities of amphotericin B in combination with four antifungal agents and rifampin against Aspergillus spp.

Strains of Aspergillus fumigatus, Aspergillus flavus, and Aspergillus niger were tested for in vitro susceptibility with a microtiter plate system in buffered yeast-nitrogen base and in buffered minimal essential medium. Isolates were tested against amphotericin B, flucytosine, rifampin, ketoconazole, ICI 153,066, and Bay n 7133 and against combinations of amphotericin B with each of the other five drugs. Combinations of amphotericin B and rifampin were the most active against all three species of Aspergillus. Flucytosine combined with amphotericin B produced little or no reduction of the MICs at which 90% of the strains were inhibited compared with amphotericin B alone. With one exception, the addition of ketoconazole, ICI 153,066, or Bay n 7133 to amphotericin B did not consistently alter the MICs. The addition of ICI 153,066 markedly increased the MICs of amphotericin B against the A. flavus isolates in both media. When the azoles were tested alone, Bay n 7133 was the most active against A. fumigatus, but was two- to fivefold less active against A. flavus. Ketoconazole was the most active azole against A. flavus.     

In vitro activities of voriconazole in combination with three other antifungal agents against Candida glabrata

Candida glabrata has recently emerged as a significant pathogen involved in both superficial and deep-seated infections. In the present study, a checkerboard broth microdilution method was performed to investigate the in vitro activities of voriconazole (VOR) in combination with terbinafine (TRB), amphotericin B (AMB), and flucytosine (5FC) against 20 clinical isolates of C. glabrata. Synergy, defined as a fractional inhibitory concentration (FIC) index of < or = 0.50, was observed in 75% of VOR-TRB, 10% of VOR-AMB, and 5% of VOR-5FC interactions. None of these combinations yielded antagonistic interactions (FIC index > 4). When synergy was not achieved, there was still a decrease in the MIC of one or both drugs used in the combination. In particular, the MICs were reduced to < or = 1.0 microg/ml as a result of the combination for all isolates for which the AMB MIC at the baseline was > or = 2.0 microg/ml. By a disk diffusion assay, the halo diameters produced by antifungal agents in combination were greater that those produced by each drug alone. Finally, killing curves showed that VOR-AMB exhibited synergistic interactions, while VOR-5FC sustained fungicidal activities against C. glabrata. These studies demonstrate that the in vitro activity of VOR against this important yeast pathogen can be enhanced upon combination with other drugs that have different modes of action or that target a different step in the ergosterol pathway. Further studies are warranted to elucidate the potential beneficial effects of such combination regimens in vivo.     

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 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 activity of teichomycin and vancomycin alone and in combination with rifampin.

The antibacterial activity of teichomycin, a glycopeptide antibiotic similar to vancomycin, has been evaluated in vitro and compared with that of vancomycin. Test strains included 130 staphylococci and 132 streptococci, with representatives of the major currently recognized species or groups, and lesser numbers of clostridia, propionibacteria, and group JK bacteria. Teichomycin was found to be more active than vancomycin. Its minimum inhibitory concentration (MIC) was two- to fourfold lower than that of vancomycin with staphylococci and anaerobic bacteria, and two- to eightfold lower with streptococci. No significant differences were observed with group JK bacteria. For most strains tested, minimum bactericidal concentrations (MBCs) of both teichomycin and vancomycin either equalled or exceeded by twofold the respective MICs. Higher MBC-to-MIC ratios were obtained for enterococci and pneumococci with both antibiotics. Both teichomycin and vancomycin showed similar in vitro interactions with rifampin in combination tests. Neither antagonism nor (with very few exceptions) synergism occurred.     

In vitro evaluation of the activities of azithromycin alone and combined with pyrimethamine against Toxoplasma gondii.

By using an in vitro microassay to assess drug interaction, azithromycin combined to pyrimethamine was found more active than pyrimethamine alone against Toxoplasma gondii, and additivity between those drugs was demonstrated. Our results show that the combination of azithromycin and pyrimethamine may lead to a more rapid control of T. gondii.     

In vitro activities of membrane-active peptides alone and in combination with clinically used antimicrobial agents against Stenotrophomonas maltophilia

The in vitro activities of buforin II, cecropin P1, and magainin II, alone and in combination with six clinically used antimicrobial agents, against 12 clinical isolates of Stenotrophomonas maltophilia were investigated. Antimicrobial activities were measured by MIC and time-kill studies. The isolates were susceptible to the peptides at concentrations in the range of 0.50 to 16 microg/ml. Synergy was observed when the peptides were combined with polymyxin E, meropenem, ceftazidime, piperacillin, and clarithromycin.     

In vitro activities of various antimicrobials alone and in combination with tigecycline against carbapenem-intermediate or -resistant Acinetobacter baumannii

The activities of tigecycline alone and in combination with other antimicrobials are not well defined for carbapenem-intermediate or -resistant Acinetobacter baumannii (CIRA). Pharmacodynamic activity is even less well defined when clinically achievable serum concentrations are considered. Antimicrobial susceptibility testing of clinical CIRA isolates from 2001 to 2005 was performed by broth or agar dilution, as appropriate. Tigecycline concentrations were serially increased in time-kill studies with a representative of the most prevalent carbapenem-resistant clone (strain AA557; imipenem MIC, 64 mg/liter). The in vitro susceptibility of the strain was tested by time-kill studies in duplicate against the average free serum steady-state concentrations of tigecycline alone and in combination with various antimicrobials. Ninety-three CIRA isolates were tested and were found to have the following antimicrobial susceptibility profiles: tigecycline, MIC(50) of 1 mg/liter and MIC(90) of 2 mg/liter; minocycline, MIC(50) of 0.5 mg/liter and MIC(90) of 8 mg/liter; doxycycline, MIC(50) of 2 mg/liter and MIC(90) of > or =32 mg/liter; ampicillin-sulbactam, MIC(50) of 48 mg/liter and MIC(90) of 96 mg/liter; ciprofloxacin, MIC(50) of > or =16 mg/liter and MIC(90) of > or =16 mg/liter; rifampin, MIC(50) of 4 mg/liter and MIC(90) of 8 mg/liter; polymyxin B, MIC(50) of 1 mg/liter and MIC(90) of 1 mg/liter; amikacin, MIC(50) of 32 mg/liter and MIC(90) of > or =32 mg/liter; meropenem, MIC(50) of 16 mg/liter and MIC(90) of > or =128 mg/liter; and imipenem, MIC(50) of 4 mg/liter and MIC(90) of 64 mg/liter. Among the tetracyclines, the isolates were more susceptible to tigecycline than minocycline and doxycycline, according to FDA breakpoints (95%, 88%, and 71% of the isolates were susceptible to tigecycline, minocycline, and doxycycline, respectively). Concentration escalation studies with tigecycline revealed a maximal killing effect near the MIC, with no additional extent or rate of killing at concentrations 2x to 4x the MIC for tigecycline. Time-kill studies demonstrated indifference for tigecycline in combination with the antimicrobials tested. Polymyxin B, minocycline, and tigecycline are the most active antimicrobials in vitro against CIRA. Concentration escalation studies demonstrate that tigecycline may need to approach concentrations higher than those currently achieved in the bloodstream to adequately treat CIRA bloodstream infections. Future studies should evaluate these findings in vivo.     

In vitro activity of citropin 1.1 alone and in combination with clinically used antimicrobial agents against Rhodococcus equi

OBJECTIVES: The aim of this study was to investigate the in vitro activity of citropin 1.1, an antimicrobial peptide derived from the Australian tree frog Litoria citropa, alone and in combination with ampicillin, ceftriaxone, doxycycline, netilmicin, ciprofloxacin, rifampicin, linezolid, vancomycin, clarithromycin and imipenem against 12 nosocomial isolates of Rhodococcus equi. METHODS: Antimicrobial activity of citropin 1.1 was measured by MIC, MBC, time-kill studies and chequerboard titration method. RESULTS: All isolates were inhibited at concentrations of citropin 1.1 between 2 and 8 mg/L. Combination studies demonstrated synergy only when the peptide was combined with clarithromycin, doxycycline and rifampicin. CONCLUSIONS: Our findings show that citropin 1.1 is active against R. equi and that its activity could be enhanced when it is combined with hydrophobic antibiotics.     

In vitro activities of quinupristin-dalfopristin and cefepime,alone and in combination with various antimicrobials,against multidrug-resistant staphylococci and enterococci in an in vitro pharmacodynamic model

Use of combinations of antimicrobials that together achieve synergistic activities against targeted microorganisms is one potential strategy for overcoming bacterial resistance. As the incidence of infections caused by multidrug-resistant staphylococci and enterococci increases, the importance of devising additional synergistic drug combinations for these bacteria is magnified. We evaluated a number of antimicrobial combinations, with a focus on quinupristin-dalfopristin (Q-D), cefepime, and linezolid, using a previously described in vitro pharmacodynamic model. The combination of Q-D with either linezolid or vancomycin, as well as the combination of cefepime-vancomycin, resulted in enhanced killing (> or =2-log(10) increase in killing versus the most-active single agent) against methicillin-resistant Staphylococcus aureus (MRSA) 494. An improved effect (<2 log(10) kill increase in kill) against MRSA 494 was noted for cefepime plus either Q-D or linezolid, as well as linezolid-vancomycin. Similar relationships were observed for a methicillin-susceptible S. aureus isolate (isolate 1199). Against methicillin-resistant S. epidermidis R444, enhanced killing was achieved with the combination of cefepime-linezolid, while improvement was noted for vancomycin with either cefepime or linezolid. The combination of cefepime and vancomycin also achieved enhanced killing against a glycopeptide-intermediate-susceptible S. aureus isolate (isolate 992). The combination of linezolid and doxycycline achieved an enhanced effect against vancomycin-resistant Enterococcus faecalis (VREFc) and E. faecium. Q-D plus ampicillin or linezolid resulted in similar enhancement of activity against the VREFc isolate. The results of this study suggest a number of novel antimicrobial combinations that may be useful against staphylococci and enterococci. Combination regimens including cefepime, Q-D, and/or linezolid warrant further investigation for the treatment of refractive infections due to multidrug-resistant gram-positive pathogens.     

In vitro antistaphylococcal activity of pefloxacin alone and in combination with other antistaphylococcal drugs.

MICs of pefloxacin and nine antistaphylococcal drugs were determined for 200 isolates of Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus haemolyticus, Staphylococcus hominis, and Staphylococcus saprophyticus. All the strains were susceptible to pefloxacin, vancomycin, and rifampin. Oxacillin-resistant strains were uniformly resistant to cephalothin and were more likely to be resistant to gentamicin, erythromycin, clindamycin, doxycycline, and trimethoprim-sulfamethoxazole than were oxacillin-susceptible strains. Time-kill studies with 23 strains of S. aureus, S. epidermidis, and S. haemolyticus indicated that the relative order of bactericidal activities was gentamicin greater than or equal to pefloxacin greater than oxacillin greater than vancomycin greater than rifampin. Pefloxacin combined with oxacillin or vancomycin killed staphylococci more rapidly than oxacillin or vancomycin alone but less rapidly than pefloxacin alone. Gentamicin combined with oxacillin, vancomycin, or pefloxacin resulted in the most rapid killing of gentamicin-susceptible strains. Rifampin combined with oxacillin, vancomycin, or pefloxacin reduced the bactericidal activities of those drugs, but rifampin resistance was not observed as it was with rifampin alone. Pefloxacin is a potentially useful antistaphylococcal agent.     

In vitro activity of systemic antifungal agents against Malassezia furfur.

The activity of four antifungal agents against 15 systemic (blood and vascular catheter) and 10 superficial (skin) Malassezia furfur isolates was evaluated. MIC ranges were similar for the two groups of organisms: amphotericin B, 0.3 to 2.5 micrograms/ml; flucytosine, greater than 100 micrograms/ml; miconazole, 0.4 to 1.5 micrograms/ml; and ketoconazole, 0.025 to 0.4 micrograms/ml.     

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 activities of ureidopenicillins alone and in combination with amikacin and three cephalosporin antibiotics.

The MIC and MBC activity of mezlocillin alone and in combination with two concentrations of ceftizoxime, moxalactam, and amikacin and a single concentration of cefoxitin was studied in a broth microdilution partial checkerboard against 472 strains of aerobic gram-negative and gram-positive bacteria. Azlocillin was tested alone and in the same combinations against Pseudomonas aeruginosa. Of the gram-negative bacilli tested, 38% were gentamicin resistant. Antagonism (less than or equal to a fourfold ureidopenicillin MIC increase) was observed frequently with combinations of ureidopenicillins plus cefoxitin and sporadically with ureidopenicillins plus ceftizoxime or moxalactam. Partial synergism (less than or equal to a fourfold ureidopenicillin MIC decrease) was evident with both combinations of ureidopenicillins plus amikacin and ureidopenicillins plus ceftizoxime or moxalactam, the percentage being dependent upon the individual species and combinations.     

In vitro activity of moxalactam alone and in combination with penicillin against common meningeal pathogens.

Moxalactam demonstrated marked activity against beta-lactamase-positive and -negative Haemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitidis by both standard minimal inhibitory concentration testing and growth curve studies. Moxalactam was ineffective against S. pneumoniae partially susceptible to penicillin G. Moxalactam (5 micrograms/ml) and penicillin (1 microgram/ml) in combination were indifferent to each other's antibacterial activity, exerting neither synergism nor antagonism against these organisms.