Activity and Diffusion of LY333328 in Experimental Endocarditis Due to Vancomycin-Resistant Enterococcus faecalis

The activity of LY333328 against Enterococcus faecalis JH2-2, which is susceptible to glycopeptides, and against its transconjugants E. faecalis BM4281 and BM4316, with VanB and VanA phenotypes, respectively, was investigated. LY333328 was active in vitro against the three strains, for which MICs were 2 μg/ml on agar and 0.25 μg/ml in broth. LY333328 was bactericidal in broth against E. faecalis JH2-2 and BM4281 at a concentration of 8 μg/ml and against BM4316 at a concentration of 30 μg/ml. The protein binding of LY333328 to rabbit serum was >99%, and the bactericidal activity of LY333328 in broth was reduced when it was tested in the presence of 90% rabbit serum. Autoradiographic studies performed in rabbits with enterococcal endocarditis showed that ¹⁴[C]LY333328 was distributed heterogeneously throughout cardiac vegetations. In rabbits with aortic endocarditis, a regimen of 20 mg of LY333328 per kg of body weight administered intramuscularly twice a day for 5 days after a loading dose of 40 mg/kg was active against the three strains in vivo (P < 0.01), whereas vancomycin was not active against the VanB-type strain and teicoplanin was not active against the VanA-type strain. We conclude that the activity of LY333328 is not significantly modified by acquired resistance to glycopeptides in E. faecalis either in vitro or in experimental endocarditis.     

Inhibitory and bactericidal effects of telithromycin (HMR 3647, RU 56647) and five comparative antibiotics, used singly and in combination, against vancomycin-resistant and vancomycin-susceptible enterococci

The inhibitory and bactericidal effects of telithromycin (HMR 3647, RU 66647) were compared with those of gentamicin, ampicillin, erythromycin, azithromycin and vancomycin against 74 strains of enterococci (34 Enterococcus faecalis and 40 Enterococcus faecium) by agar dilution, broth dilution, time kill assays and postantibiotic effect (PAE). The telithromycin MIC(90) for vancomycin-sensitive (VSE) E. faecalis strains tested using the agar dilution method was 8 microg/ml. For a different group of VSE E. faecalis strains tested using the broth dilution method it was 0.06 microg/ml The telithromycin MIC(90)s for vancomycin-resistant (VRE) and VSE E. faecium strains, determined using the agar dilution method, were 4 and 8 microg/ml, respectively, while for a different set of VRE and VSE E. faecium strains tested using the broth macrodilution method, they were 32 and 16 microg/ml, respectively. Telithromycin MBC(90)s for E. faecalis were 4-6 tubes higher and for E. faecium 3-5 tubes higher, respectively, than the MIC(90)s. In time kill assays, telithromycin had bactericidal activity against only 1 of 7 E. faecium strains; for all other E. faecium and E. faecalis strains, only inhibitory activity was demonstrated. Neither synergy nor drug interference was observed when telithromycin was used in combination with ampicillin, vancomycin or gentamicin. At 10 times the MIC, the PAE of telithromycin against E. faecalis was 2.8 h, while for E. faecium it was 1.6 h. Telithromycin should be evaluated for therapy of enterococcal infections, including those caused by VRE organisms. However, because of the strain-to-strain variability in susceptibility to telithromycin, MIC determinations are important, especially for erythromycin-resistant strains.     

In Vitro Activities of 15 Antimicrobial Agents against Clinical Isolates of South African Enterococci

The activities of a panel of currently available antibiotics and the investigational agents LY 333328, linezolid, CL 331,002, CL 329,998, moxifloxacin (BAY 12-8039), trovafloxacin, and quinupristin-dalfopristin against 274 clinical isolates of enterococci were determined. No vancomycin resistance or β-lactamase production was observed. Except for 12 isolates (all non-Enterococcus faecalis) showing reduced susceptibility to quinupristin-dalfopristin (MIC, ≥4 μg/ml), the new agents exhibited promising in vitro antienterococcal activity.     

Activities and Time-Kill Studies of Selected Penicillins, β-Lactamase Inhibitor Combinations, and Glycopeptides against Enterococcus faecalis

The activities of piperacillin, piperacillin-tazobactam, ticarcillin, ticarcillin-clavulanate, ampicillin, ampicillin-sulbactam, vancomycin, and teicoplanin were tested against 212 Enterococcus faecalis strains (9 β-lactamase producers) by standard agar dilution MIC testing (10⁴ CFU/spot). The MICs at which 50 and 90% of the isolates were inhibited (MIC₅₀s and MIC₉₀s, respectively) were as follows (μg/ml): piperacillin, 4 and 8; piperacillin-tazobactam, 4 and 8; ticarcillin, 64 and 128; ticarcillin-clavulanate, 64 and 128; ampicillin, 2 and 2; ampicillin-sulbactam, 1 and 2; vancomycin, 1 and 4; and teicoplanin, 0.5 and 1. Agar dilution MIC testing of the nine β-lactamase-positive strains with an inoculum of 10⁶ CFU/spot revealed higher β-lactam MICs (piperacillin, 64 to >256 μg/ml; ticarcillin, 128 to >256 μg/ml; and ampicillin, 16 to 128 μg/ml); however, MICs with the addition of inhibitors were similar to those obtained with the lower inoculum. Time-kill studies of 15 strains showed that piperacillin-tazobactam was bactericidal (99.9% killing) for 14 strains after 24 h at four times the MIC, with 90% killing of all 15 strains at two times the MIC. After 12 and 6 h, 90% killing of 14 and 13 strains, respectively, was found at two times the MIC. Ampicillin gave 99.9% killing of 14 β-lactamase-negative strains after 24 h at eight times the MIC, with 90% killing of all 15 strains at two times the MIC. After 12 and 6 h, 90% killing of 14 and 13 strains, respectively, was found at two times the MIC. Killing by ticarcillin-clavulanate was slower than that observed for piperacillin-tazobactam, relative to the MIC. For the one β-lactamase-producing strain tested by time-kill analysis with a higher inoculum, addition of the three inhibitors (including sulbactam) to each of the β-lactams resulted in bactericidal activity at 24 h at two times the MIC. For an enzyme-negative strain, addition of inhibitors did not influence kinetics. Kinetics of vancomycin and teicoplanin were significantly slower than those of the β-lactams, with bactericidal activity against 6 strains after 24 h at eight times the MIC, with 90% killing of 12 and 14 strains, respectively, at four times the MIC. Slower-kill kinetics by both glycopeptides were observed at earlier periods.     

In vitro activity and spectrum of LY333328, a novel glycopeptide derivative.

Reference methods were used to determine the potency of LY333328, a semisynthetic glycopeptide derivative with a key N-alkylation substitution, against 833 strains (393 gram-positive strains and representative gram-negative bacilli) with various defined resistance mechanisms. The MICs at which 90% of the isolates are inhibited (MIC90S) (in micrograms per milliliter) of LY333328 and the percentages of strains at < or = 8 micrograms/ml were as follows: for oxacillin-susceptible Staphylococcus aureus, 2 and 100%, and for oxacillin-resistant Staphylococcus aureus, 4 and 100%; for oxacillin-susceptible Staphylococcus epidermis, 4 and 100%, and for oxacillin-resistant Staphylococcus aureus, 8 and 96%; for Streptococcus serogroups A, B, C, and G, 0.25 to 1 and 100%; for Streptococcus pneumoniae < or = 0.015 to 0.06 and 100%; for Enterococcus faecalis, 2 and 100%; and for vancomycin-susceptible Enterococcus faecium, 0.25 and 100%, and for vancomycin-resistant Enterococcus faecium, 4 and 100%. LY333328 was not active (MIC50, > or = 16 micrograms/ml) against more than 400 representative strains of Enterobacteriaceae, pseudomonads, Acinetobacter spp., Stenotrophomonas maltophilia, Haemophilus influenzae, Moraxella catarrhalis, pathogenic Neisseria spp., and anaerobic gram-negative bacilli. Gram-positive anaerobes were LY333328 susceptible (MICs, < or = 2 micrograms/ml). Test methods and conditions may have affected MICs of LY333328, with most (species variation) agar dilution MICs being greater than the broth microdilution MICs.     

Activities of Three Quinolones, Alone and in Combination with Extended-Spectrum Cephalosporins or Gentamicin, against Stenotrophomonas maltophilia

The present study examined the activities of trovafloxacin, levofloxacin, and ciprofloxacin, alone and in combination with cefoperazone, ceftazidime, cefpirome, and gentamicin, against 100 strains of Stenotrophomonas maltophilia by the MIC determination method and by synergy testing of the combinations by the time-kill and checkerboard titration methods for 20 strains. The respective MICs at which 50% and 90% of isolates were inhibited for the drugs used alone were as follows: trovafloxacin, 0.5 and 2.0 μg/ml; levofloxacin, 2.0 and 4.0 μg/ml; ciprofloxacin, 4.0 and 16.0 μg/ml; cefoperazone, >128.0 and >128.0 μg/ml; ceftazidime, 32.0 and >128.0 μg/ml; cefpirome, >128.0 and >128.0 μg/ml; and gentamicin, 128.0 and >128.0 μg/ml. Synergistic fractional inhibitory concentration indices (≤0.5) were found for ≥50% of strains for trovafloxacin-cefoperazone, trovafloxacin-ceftazidime, levofloxacin-cefoperazone, levofloxacin-ceftazidime, ciprofloxacin-cefoperazone, and ciprofloxacin-ceftazidime, with other combinations affecting fewer strains. For 20 strains tested by the checkerboard titration and time-kill methods, synergy (≥100-fold drop in count compared to the count achieved with the more active compound) was more pronounced after 12 h due to regrowth after 24 h. At 12 h, trovafloxacin at 0.004 to 0.5 μg/ml showed synergy with cefoperazone for 90% of strains, with ceftazidime for 95% of strains with cefpirome for 95% of strains, and with gentamicin for 65% of strains. Levofloxacin at 0.03 to 0.5 μg/ml and ciprofloxacin at 0.5 to 2.0 μg/ml showed synergy with cefoperazone for 80% of strains, with ceftazidime for 90 and 85% of strains, respectively, with cefpirome for 85 and 75% of strains, respectively, and with gentamicin for 65 and 75% of strains, respectively. Time-kill assays were more discriminatory than checkerboard titration assays in demonstrating synergy for all combinations.     

In Vitro Activities of Quinupristin-Dalfopristin and the Streptogramin RPR 106972 against Mycoplasma pneumoniae

The in vitro activities of quinupristin-dalfopristin and streptogramin RPR 106972 were determined with 44 strains of Mycoplasma pneumoniae and compared to those of macrolides, minocycline, and quinolones. All isolates tested were highly susceptible to macrolides and to quinupristin-dalfopristin (MIC at which 90% of the isolates are inhibited [MIC₉₀], 0.0625 μg/ml), followed by RPR 106972 (MIC₉₀, 0.5 μg/ml), quinolones, and minocycline.     

Synergy of daptomycin with oxacillin and other beta-lactams against methicillin-resistant Staphylococcus aureus

We previously observed marked synergy between daptomycin and both rifampin and ampicillin against vancomycin-resistant enterococci (VRE). Because the synergy between daptomycin and ampicillin was observed for 100% of VRE strains with high-level ampicillin resistance (ampicillin MIC of > or =128 microg/ml), we looked for synergy between daptomycin and other beta-lactams against 18 strains of methicillin-resistant Staphylococcus aureus (MRSA) by employing a time-kill method using Mueller-Hinton broth supplemented to 50 mg of Ca2+/liter. All strains were resistant to oxacillin (16 of 18 strains were resistant at drug concentrations of > or =256 microg/ml), and all strains were susceptible to daptomycin (the MIC at which 90% of the tested isolates were inhibited was 1 microg/ml). Daptomycin was tested at concentrations of 2, 1, 0.5, 0.25, 0.125, and 0.0625 microg/ml alone or in combination with oxacillin at a fixed concentration of 32 microg/ml. Synergy was found for all 18 strains with daptomycin at one-half the MIC in combination with 32 microg of oxacillin/ml, and synergy was found for 11 of 18 strains (61%) with daptomycin at one-fourth the MIC or less in combination with oxacillin. At 24 h, the daptomycin-oxacillin combination with daptomycin at one-half the MIC showed bactericidal activity against all 18 strains, and the combination with one-fourth the daptomycin MIC showed bactericidal activity against 9 of 18 strains. We also used a novel screening method to look for synergy between daptomycin and other beta-lactams. In this approach, daptomycin was incorporated into Ca(2+)-supplemented Mueller-Hinton agar at subinhibitory concentrations, and synergy was screened by comparing test antibiotic Kirby-Bauer disks on agar with and without daptomycin. By this method, daptomycin with ampicillin-sulbactam, ticarcillin-clavulanate, or piperacillin-tazobactam showed synergy comparable to or greater than daptomycin with oxacillin. For seven of the eight strains tested, time-kill studies confirmed synergy between daptomycin and ampicillin-sulbactam with ampicillin in the range of 2 to 8 microg/ml. The combination of daptomycin and beta-lactams may be useful for the treatment of MRSA infection, but further studies are needed to elucidate the mechanisms and to determine the in vivo efficacy of the combination.     

Susceptibility of Anaerobic Bacteria to Sulfamethoxazole/Trimethoprim and Routine Susceptibility Testing

The minimal inhibitory concentrations (MICs) of sulfamethoxazole and trimethoprim against 144 strains of obligately anaerobic bacteria were determined on Diagnostic Sensitivity Test agar (Oxoid) or in prereduced Diagnostic Sensitivity Test broth, both supplemented with sodium pyruvate (1 mg/ml), hemin (5 μg/ml), and vitamin K₁ (1 μg/ml). Fifty-eight percent of the strains were susceptible to sulfamethoxazole alone (MIC ≤ 16 μg/ml), only 12% were susceptible to trimethoprim alone (MIC ≤ 1 μg/ml), and 85% were susceptible to the combination of sulfamethoxazole plus trimethoprim (MIC ≤ 16 μg/ml) at a ratio of 19:1. All 45 strains of the Bacteroides fragilis group were susceptible to the combination. Synergy of the combination was often observed by a checkerboard MIC determination of 123 strains, usually most markedly when the ratio of the two components was near 1:1. However, there was also synergism at the ratio of sulfamethoxazole to trimethoprim of 16:1 in 61 (53.5%) of the 114 strains that could be evaluated for synergistic activity. When the strains were tested by the broth-disk test proposed by Wilkins and Thiel, modified by using prereduced Diagnostic Sensitivity Test broth instead of brain heart infusion broth and by using a smaller inoculum, there was over 90% correlation with the MICs. Poor results were found when the broth-disk tests were performed in brain heart infusion broth. There was very poor correlation between inhibition zone diameters by an agar diffusion method and MICs.     

Antistaphylococcal activity of dalbavancin, an experimental glycopeptide

Dalbavancin, tested against 146 staphylococci, was more potent than other drugs tested, with an MIC at which 50% of staphylococci were inhibited of 0.03 μg/ml and an MIC at which 90% of staphylococci were inhibited of 0.06 μg/ml by microdilution. For all strains, MICs of vancomycin, linezolid, ranbezolid, oritavancin, daptomycin, and quinupristin-dalfopristin were ≤4.0 μg/ml. Dalbavancin was bactericidal at four times the MIC against all six strains tested.     

Susceptibilities of Penicillin- and Erythromycin-Susceptible and -Resistant Pneumococci to HMR 3647 (RU 66647), a New Ketolide, Compared with Susceptibilities to 17 Other Agents

Susceptibility of 230 penicillin- and erythromycin-susceptible and -resistant pneumococci to HMR 3647 (RU 66647), a new ketolide, was tested by agar dilution, and results were compared with those of erythromycin, azithromycin, clarithromycin, roxithromycin, rokitamycin, clindamycin, pristinamycin, ciprofloxacin, sparfloxacin, trimethoprim-sulfamethoxazole, doxycycline, chloramphenicol, cefuroxime, ceftriaxone, imipenem, and vancomycin. HMR 3647 was very active against all strains tested, with MICs at which 90% of the strains were inhibited (MIC₉₀s) of 0.03 μg/ml for erythromycin-susceptible strains (MICs, ≤0.25 μg/ml) and 0.25 μg/ml for erythromycin-resistant strains (MICs, ≥1.0 μg/ml). All other macrolides yielded MIC₉₀s of 0.03 to 0.25 and >64.0 μg/ml for erythromycin-susceptible and -resistant strains, respectively. The MICs of clindamycin for 51 of 100 (51%) erythromycin-resistant strains were ≤0.125 μg/ml. The MICs of pristinamycin for all strains were ≤1.0 μg/ml. The MIC₉₀s of ciprofloxacin and sparfloxacin were 4.0 and 0.5 μg/ml, respectively, and were unaffected by penicillin or erythromycin susceptibility. Vancomycin and imipenem inhibited all strains at ≤1.0 μg/ml. The MICs of cefuroxime and cefotaxime rose with those of penicillin G. The MICs of trimethoprim-sulfamethoxazole, doxycycline, and chloramphenicol were variable but were generally higher in penicillin- and erythromycin-resistant strains. HMR 3647 had the best kill kinetics of all macrolides tested against 11 erythromycin-susceptible and -resistant strains, with uniform bactericidal activity (99.9% killing) after 24 h at two times the MIC and 99% killing of all strains at two times the MIC after 12 h for all strains. Pristinamycin showed more rapid killing at 2 to 6 h, with 99.9% killing of 10 of 11 strains after 24 h at two times the MIC. Other macrolides showed significant activity, relative to the MIC, against erythromycin-susceptible strains only.     

In Vitro Activity of the New Glycopeptide LY333328 against Multiply Resistant Gram-Positive Clinical Isolates

The in vitro activity of LY333328 was compared with those of vancomycin and teicoplanin against 425 gram-positive clinical isolates, including a variety of multiply resistant strains. LY333328 at ≤4 μg/ml inhibited all microorganisms tested, including methicillin- and teicoplanin-resistant staphylococci, glycopeptide-resistant enterococci, penicillin- and multiply resistant pneumococci, and viridans and beta-hemolytic streptococci.     

Influence of Human Serum on Pharmacodynamic Properties of an Investigational Glycopeptide, LY333328, and Comparator Agents against Staphylococcus aureus

The MICs and MBCs of 15 antibiotics for two strains of Staphylococcus aureus were determined in Mueller-Hinton broth (MHB) and 90% serum–10% MHB. Subsequent experiments established that highly protein-bound antibiotics (≥80%), such as LY333328, demonstrated higher MICs and MBCs, less killing over an 8-h interval, and shorter postantibiotic effects in 90% serum–10% MHB than in MHB alone. Albumin was demonstrated to be almost solely responsible for changes in the aforementioned pharmacodynamic parameters of LY333328.     

In Vitro Activities of Cefminox against Anaerobic Bacteria Compared with Those of Nine Other Compounds

The agar dilution MIC method was used to test the activity of cefminox, a β-lactamase-stable cephamycin, compared with those of cefoxitin, cefotetan, moxalactam, ceftizoxime, cefotiam, cefamandole, cefoperazone, clindamycin, and metronidazole against 357 anaerobes. Overall, cefminox was the most active β-lactam, with an MIC at which 50% of isolates are inhibited (MIC₅₀) of 1.0 μg/ml and an MIC₉₀ of 16.0 μg/ml. Other β-lactams were less active, with respective MIC₅₀s and MIC₉₀s of 2.0 and 64.0 μg/ml for cefoxitin, 2.0 and 128.0 μg/ml for cefotetan, 2.0 and 64.0 μg/ml for moxalactam, 4.0 and >128.0 μg/ml for ceftizoxime, 16.0 and >128.0 μg/ml for cefotiam, 8.0 and >128.0 μg/ml for cefamandole, and 4.0 and 128.0 μg/ml for cefoperazone. The clindamycin MIC₅₀ and MIC₉₀ were 0.5 and 8.0 μg/ml, respectively, and the metronidazole MIC₅₀ and MIC₉₀ were 1.0 and 4.0 μg/ml, respectively. Cefminox was especially active against Bacteroides fragilis (MIC₉₀, 2.0 μg/ml), Bacteroides thetaiotaomicron (MIC₉₀, 4.0 μg/ml), fusobacteria (MIC₉₀, 1.0 μg/ml), peptostreptococci (MIC₉₀, 2.0 μg/ml), and clostridia, including Clostridium difficile (MIC₉₀, 2.0 μg/ml). Time-kill studies performed with six representative anaerobic species revealed that at the MIC all compounds except ceftizoxime were bactericidal (99.9% killing) against all strains after 48 h. At 24 h, only cefminox and cefoxitin at 4× the MIC and cefoperazone at 8× the MIC were bactericidal against all strains. After 12 h, at the MIC all compounds except moxalactam, ceftizoxime, cefotiam, cefamandole, clindamycin, and metronidazole gave 90% killing of all strains. After 3 h, cefminox at 2× the MIC produced the most rapid effect, with 90% killing of all strains.     

Characterization of Vancomycin-Resistant Enterococcus faecium Isolates from the United States and Their Susceptibility In Vitro to Dalfopristin-Quinupristin

In the course of clinical studies with the investigational streptogramin antimicrobial dalfopristin-quinupristin, isolates of vancomycin-resistant Enterococcus faecium were referred to our laboratory from across the United States. Seventy-two percent of the strains were of the VanA type, phenotypically and genotypically, while 28% were of the VanB type. High-level resistance to streptomycin or gentamicin was observed in 86 and 81%, respectively, of the VanA strains but in only 69 and 66%, respectively, of the VanB strains. These enterococci were resistant to ampicillin (MIC for 50% of the isolates tested [MIC₅₀] and MIC₉₀, 128 and 256 μg/ml, respectively) and to the other approved agents tested, with the exception of chloramphenicol (MIC₉₀, 8 μg/ml) and novobiocin (MIC₉₀, 1 μg/ml). Considering all of the isolates submitted, dalfopristin-quinupristin inhibited 86.4% of them at concentrations of ≤1 μg/ml and 95.1% of them at ≤2 μg/ml. However, for the data set comprised of only the first isolate submitted for each patient, 94.3% of the strains were inhibited at concentrations of ≤1 μg/ml and 98.9% were inhibited at concentrations of ≤2 μg/ml. Multiple drug resistance was very common among these isolates of vancomycin-resistant E. faecium, while dalfopristin-quinupristin inhibited the majority at concentrations that are likely to be clinically relevant.     

Bacteriostatic and Bactericidal In Vitro Activities of Clarithromycin and Erythromycin against Periodontopathic Actinobacillus actinomycetemcomitans

The susceptibilities of 87 periodontitis-associated strains of Actinobacillus actinomycetemcomitans to clarithromycin and erythromycin were determined by standard methodology recommended for Haemophilus influenzae. For clarithromycin the MIC at which 90% of the isolates were inhibited was ≤2.0 μg/ml and the minimal bactericidal concentration at which 90% of the strains were killed was ≤4.0 μg/ml, suggesting that it would be a candidate for therapeutic trials in patients with periodontitis.     

Emergence of Extensively Drug-Resistant Haemophilus parainfluenzae in Switzerland

Two homosexual men were colonized in the urethra with Haemophilus parainfluenzae nonsusceptible to ampicillin (MIC, 8 μg/ml), amoxicillin-clavulanate (MIC, 4 μg/ml), cefotaxime (MIC, 1.5 μg/ml), cefepime (MIC, 3 μg/ml), meropenem (MIC, 0.5 μg/ml), cefuroxime, azithromycin, ciprofloxacin, tetracycline, and chloramphenicol (all MICs, ≥32 μg/ml). Repetitive extragenic palindromic PCR (rep-PCR) showed that the strains were indistinguishable. The isolates had amino acid substitutions in PBP3, L4, GyrA, and ParC and possessed Mef(A), Tet(M), and CatS resistance mechanisms. This is the first report of extensively drug-resistant (XDR) H. parainfluenzae.     

Treatment of Vancomycin-Resistant Enterococcus faecium with RP 59500 (Quinupristin-Dalfopristin) Administered by Intermittent or Continuous Infusion, Alone or in Combination with Doxycycline, in an In Vitro Pharmacodynamic Infection Model with Simulated Endocardial Vegetations

Quinupristin-dalfopristin is a streptogramin antibiotic combination with activity against vancomycin-resistant Enterococcus faecium (VREF), but emergence of resistance has been recently reported. We studied the activity of quinupristin-dalfopristin against two clinical strains of VREF (12311 and 12366) in an in vitro pharmacodynamic model with simulated endocardial vegetations (SEVs) to determine the potential for resistance selection and possible strategies for prevention. Baseline MICs/minimal bactericidal concentrations (μg/ml) for quinupristin-dalfopristin, quinupristin, dalfopristin, and doxycycline were 0.25/2, 64/>512, 4/512, and 0.125/8 for VREF 12311 and 0.25/32, 128/>512, 2/128, and 0.25/16 for VREF 12366, respectively. Quinupristin-dalfopristin regimens had significantly less activity against VREF 12366 than VREF 12311. An 8-μg/ml simulated continuous infusion was the only bactericidal regimen with time to 99.9% killing = 90 hours. The combination of quinupristin-dalfopristin every 8 h with doxycycline resulted in more killing compared to either drug alone. Quinupristin-dalfopristin-resistant mutants (MICs, 4 μg/ml; resistance proportion, ~4 × 10⁻⁴) emerged during the quinupristin-dalfopristin monotherapies for both VREF strains. Resistance was unstable in VREF 12311 and stable in VREF 12366. The 8-μg/ml continuous infusion or addition of doxycycline to quinupristin-dalfopristin prevented the emergence of resistance for both strains over the 96-h test period. These findings replicated the development of resistance reported in humans and emphasized bacterial factors (drug susceptibility, high inoculum, organism growth phase) and infectious conditions (penetration barriers) which could increase chances for clinical resistance. The combination of quinupristin-dalfopristin with doxycycline and the administration of quinupristin-dalfopristin as a high-dose continuous infusion warrant further study to determine their potential clinical utility.     

In Vitro Susceptibilities of Bordetella pertussis and Bordetella parapertussis to Two Ketolides (HMR 3004 and HMR 3647), Four Macrolides (Azithromycin, Clarithromycin, Erythromycin A, and Roxithromycin), and Two Ansamycins (Rifampin and Rifapentine)

When tested by agar dilution on Mueller-Hinton agar supplemented with 5% horse blood, the ketolides HMR 3004 and HMR 3647 were slightly more active (MIC at which 90% of the isolates were inhibited [MIC₉₀], 0.03 μg/ml) against Bordetella pertussis than azithromycin, clarithromycin, erythromycin A, and roxithromycin. Azithromycin (MIC₉₀, 0.06 μg/ml) was the most active compound against B. parapertussis. Rifampin and rifapentine were considerably less active.     

Daptomycin synergy with rifampicin and ampicillin against vancomycin-resistant enterococci

We used a novel screening method to look for synergy between daptomycin and 18 other antibiotics against 19 strains of high-level vancomycin-resistant enterococci (VRE) (vancomycin MIC > or = 256 mg/L). In this approach, daptomycin was incorporated into Ca(2+)-supplemented Mueller-Hinton agar at subinhibitory concentrations, and synergy was screened by comparing test antibiotic Etest MICs on agar with and without daptomycin. A striking reduction in the rifampicin MIC was seen in 11/15 (73.3%) VRE that were resistant to rifampicin, from > or =12 mg/L to a mean +/- s.d. of 0.22 +/- 0.21 mg/L at daptomycin 0.25 x MIC and 0.85 +/- 0.90 mg/L at daptomycin 0.125 x MIC. Synergy was also observed for 13/19 (68%) isolates with ampicillin (MIC > or = 128 mg/L). There was no significant synergy between daptomycin and any other antibiotic by this screening method. If confirmed by further studies, daptomycin with either rifampicin or ampicillin may be useful in the management of infections caused by VRE.