In vitro activity of an oral streptogramin antimicrobial, XRP2868, against gram-positive bacteria.

The comparative in vitro potency of XRP2868, a new oral semisynthetic streptogramin antibiotic, was evaluated against gram-positive bacteria. XRP2868 inhibited all staphylococci at < or = 1 microg/ml and all non-pneumococcal streptococci at < or = 0.25 microg/ml and was fourfold more potent than quinupristin-dalfopristin against Staphylococcus aureus and Enterococcus faecium.

In vitro activity of the new oxazolidinone AZD2563 against Enterococci.

The activity of a new oxazolidinone antimicrobial, AZD2563, was assessed against >500 clinical isolates of enterococci representing six species. All isolates, including those resistant to other antibiotic classes, were inhibited by AZD2563 at concentrations

Prevalence of the fsr locus in Enterococcus faecalis infections.

The fsr locus of Enterococcus faecalis confers virulence in animal models. A retrospective analysis of fsr prevalence in diverse E. faecalis clinical isolates demonstrated fsr in all endocarditis isolates versus 53% of stool isolates (P = 0.005). This supports a role for fsr-mediated virulence in the pathogenesis of enterococcal infections in humans.

Antimicrobial activity of quinupristin-dalfopristin combined with other antibiotics against vancomycin-resistant enterococci.

Interactions between quinupristin-dalfopristin and six other antimicrobials were examined by checkerboard arrays against 50 clinical isolates of vancomycin-resistant Enterococcus faecium selected to represent a range of susceptibilities to individual agents. Unequivocal synergistic or antagonistic interactions at clinically relevant concentrations were infrequently encountered when the streptogramin was combined with chloramphenicol, ampicillin, imipenem, vancomycin, or teicoplanin. Combinations with doxycycline resulted in synergistic inhibition in 36% of checkerboards. Against 10 strains of Enterococcus faecalis, synergistic interactions were found when quinupristin-dalfopristin was combined with doxycycline (four strains), either glycopeptide (three strains), or ampicillin (two strains). Combination with quinupristin-dalfopristin increased the ampicillin MIC from 1 to 4 microg/ml for one strain. For 10 strains of E. faecium, interactions were also assessed by time-kill methods using concentrations of the agents attainable in human serum. Most of these antimicrobials augmented killing by quinupristin-dalfopristin to a minor degree. Against 2 of the 12 strains in this collection that were not highly resistant to gentamicin, the combination of quinupristin-dalfopristin (2 microg/ml) plus gentamicin (5 microg/ml) resulted in killing approaching 3 log(10) CFU/ml. With the exception of doxycycline, inhibitory interactions between quinupristin-dalfopristin and other agents tested against vancomycin-resistant strains of E. faecium were uncommon at clinically relevant concentrations.

Linezolid resistance in a clinical isolate of Staphylococcus aureus.

The new oxazolidinone antimicrobial, linezolid, has been approved for the treatment of infections caused by various gram-positive bacteria, including meticillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). Although instances of linezolid resistance in VRE have been reported, resistance has not been encountered among clinical isolates of S aureus. We have characterised an MRSA isolate resistant to linezolid that was recovered from a patient treated with this agent for dialysis-associated peritonitis.

Restoration of vancomycin susceptibility in Enterococcus faecalis by antiresistance determinant gene transfer.

We assessed the ability of gene transfer to reverse vancomycin resistance in class A (VanA) glycopeptide-resistant Enterococcus faecalis. Recombinant shuttle vectors containing a vanH promoter-vanA antisense gene cassette fully restored vancomycin susceptibility through a combined transcriptional activator binding domain decoy and inducible vanA antisense RNA effect.

Diversity of domain V of 23S rRNA gene sequence in different Enterococcus species.

The highly conserved central loop of domain V of 23S RNA (nucleotides 2042 to 2628; Escherichia coli numbering) is implicated in peptidyltransferase activity and represents one of the target sites for macrolide, lincosamide, and streptogramin B antibiotics. DNA encoding domain V (590 bp) of several species of Enterococcus was amplified by PCR. Twenty enterococcal isolates were tested, including Enterococcus faecium (six isolates), Enterococcus faecalis, Enterococcus avium, Enterococcus durans, Enterococcus gallinarum, Enterococcus casseliflavus (two isolates of each), and Enterococcus raffinosus, Enterococcus mundtii, Enterococcus malodoratus, and Enterococcus hirae (one isolate of each). For all isolates, species identification by biochemical testing was corroborated by 16S rRNA gene sequencing. The sequence of domain V of the 23S rRNA gene from E. faecium and E. faecalis differed from those of all other enterococci. The domain V sequences of E. durans and E. hirae were identical. This was also true for E. gallinarum and E. casseliflavus. E. avium differed from E. casseliflavus by 23 bases, from E. durans by 16 bases, and from E. malodoratus by 2 bases. E. avium differed from E. raffinosus by one base. Despite the fact that domain V is considered to be highly conserved, substantial differences were identified between several enterococcal species.

In vitro activities of the glycylcycline GAR-936 against gram-positive bacteria.

The in vitro activities of GAR-936, the 9-t-butylglycylamido derivative of minocycline, were compared with those of doxycycline, minocycline, and tetracycline against 527 gram-positive clinical isolates. GAR-936 inhibited all strains, including those resistant to other tetracyclines, at concentrations of

Activities of taurolidine in vitro and in experimental enterococcal endocarditis.

In vitro, the antimicrobial agent taurolidine inhibited virtually all of the bacteria tested, including vancomycin-resistant enterococci, oxacillin-resistant staphylococci, and Stenotrophomonas maltophilia, at concentrations between 250 and 2,000 microg/ml. Taurolidine was not effective in experimental endocarditis. While it appears unlikely that this antimicrobial would be useful for systemic therapy, its bactericidal activity and the resistance rates found (<10(-9)) are favorable indicators for its possible development for topical use.

In-vitro activity of the new ketolide antibiotic HMR 3647 against gram-positive bacteria.

The comparative in-vitro activity of HMR 3647, a new ketolide antibiotic, was investigated against 492 clinical isolates of gram-positive bacteria, including multiply resistant strains, by an agar-dilution technique. All streptococci tested were inhibited by the new ketolide at concentrations < or = 0.5 mg/L. HMR 3647 was more potent than erythromycin against staphylococci. For enterococci the new compound yielded an MIC90 of 8 mg/L. Erysipelothrix spp., Pediococcus spp., Leuconostoc spp., Lactobacillus spp., JK diphtheroids and Listeria monocytogenes were also susceptible to the new ketolide.

In vitro activity of BAY 12-8039, a new fluoroquinolone, against species representative of respiratory tract pathogens.

The in vitro antibacterial activity of BAY 12-8039, a novel 8-methoxy-quinolone, was compared with those of other quinolones, amoxicillin/clavulanate, cefuroxime and erythromycin against species commonly implicated in respiratory tract infections as well as viridans group streptococci. The new compound was highly active against methicillin-susceptible staphylococci (MIC90 0.125 microgram/ml), penicillin-susceptible and penicillin-resistant pneumococci (MIC90 0.5 and MIC50 0.25 microgram/ml, respectively), penicillin-susceptible and penicillin-resistant viridans group streptococci (MIC90 0.5 and 0.25 microgram/ml, respectively), group A streptococci (MIC90 0.25 microgram/ml), M. catarrhalis (MIC90 0.125 microgram/ml) and H. influenzae (MIC90 0.063 microgram/ml), irrespective of beta-lactamase production. It was, however, less active against methicillin-resistant staphylococci (MIC50 and MIC90, 2 and 4 micrograms/ml, respectively). The new compound demonstrated bactericidal activity at concentrations 2, 4, 8 times the MIC against representative isolates of the above collection. At a concentration of eight times the MIC, the frequency of spontaneous resistance ranged from 2.5 x 10(-7) to < 4 x 10(-8). These results suggested that BAY 12-8039 would be a promising agent for the eradication of respiratory tract pathogens and that clinical trials assessing its efficacy for the management of infections caused by these organisms are warranted.

Susceptibilities of Legionella spp. to newer antimicrobials in vitro.

The in vitro activities of 13 antimicrobial agents against 30 strains of Legionella spp. were determined. Rifapentine, rifampin, and clarithromycin were the most potent agents (MICs at which 90% of isolates are inhibited [MIC90s], < or = 0.008 microgram/ml). The ketolide HMR 3647 and the fluoroquinolones levofloxacin and BAY 12-8039 (MIC90s, 0.03 to 0.06 microgram/ml) were more active than erythromycin A or roxithromycin. The MIC90s of dalfopristin-quinupristin and linezolid were 0.5 and 8 micrograms/ml, respectively. Based on class characteristics and in vitro activities, several of these agents may have potential roles in the treatment of Legionella infections.

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 microg/ml, respectively) and to the other approved agents tested, with the exception of chloramphenicol (MIC90, 8 microg/ml) and novobiocin (MIC90, 1 microg/ml). Considering all of the isolates submitted, dalfopristin-quinupristin inhibited 86.4% of them at concentrations of < or = 1 microg/ml and 95.1% of them at < or = 2 microg/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 < or = 1 microg/ml and 98.9% were inhibited at concentrations of < or = 2 microg/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.

In vitro activity of trimethoprim alone compared with trimethoprim-sulfamethoxazole and other antimicrobials against bacterial species associated with upper respiratory tract infections.

Trimethoprim-sulfamethoxazole has been used to treat various respiratory tract infections. Nevertheless, for many patients, intolerance of the sulfonamide component precludes use of this combination. This study examined the activity of trimethoprim alone in comparison to that of trimethoprim-sulfamethoxazole and other antimicrobials against bacterial species implicated in respiratory tract infections. For Haemophilus influenzae, minimal inhibitory concentrations of trimethoprim were equal to or one dilution greater than those of trimethoprim-sulfamethoxazole, with 56 of 58 strains inhibited by the former at < or = 0.25 microgram/ml. All oxacillin-susceptible Staphylococcus aureus and 96.7% of Streptococcus pyogenes were inhibited by trimethoprim < or = 2 micrograms/ml. In contrast, only 50% of Streptococcus pneumoniae were inhibited by this concentration of trimethoprim, whereas 93.3% were susceptible to the combination at < or = 2/38 micrograms/ml. All oxacillin-resistant S. aureus and all Moraxella catarrhalis were resistant to trimethoprim, although many of the former and all of the latter were susceptible to trimethoprim-sulfamethoxazole.

In vitro activity of RU 64004, a new ketolide antibiotic, against gram-positive bacteria.

The comparative in vitro activity of RU 64004 (also known as HMR 3004), a new ketolide antibiotic, was tested by agar dilution against approximately 500 gram-positive organisms, including multiply resistant enterococci, streptococci, and staphylococci. All streptococci were inhibited by < or = 1 microg of RU 64004 per ml. The ketolide was more potent than other macrolides against erythromycin A-susceptible staphylococci and was generally more potent than clindamycin against erythromycin A-resistant strains susceptible to this agent. Clindamycin-resistant staphylococci (MIC, > 128 microg/ml) proved resistant to the ketolide, but some erythromycin A- and clindamycin-resistant enterococci remained susceptible to RU 64004.

Influence of erythromycin resistance, inoculum growth phase, and incubation time on assessment of the bactericidal activity of RP 59500 (quinupristin-dalfopristin) against vancomycin-resistant Enterococcus faecium.

RP 59500, a mixture of two semisynthetic streptogramin antibiotics (quinupristin and dalfopristin), is one of a few investigational agents currently in clinical trials with inhibitory activity against multiple-drug-resistant strains of Enterococcus faecium. We evaluated the bactericidal activity of this antimicrobial against 30 recent clinical isolates of vancomycin-resistant E. faecium, including 23 erythromycin-resistant (MIC, >256 microg/ml) and 7 erythromycin-intermediate (MIC, 2 to 4 microg/ml) strains. All isolates were inhibited by RP 59500 at 0.25 to 1.0 microg/ml. The bactericidal activity of RP 59500 was markedly influenced by the erythromycin susceptibility of the strains and by several technical factors, such as inoculum growth phase and time of incubation of counting plates. As determined by time-kill methods, RP 59500 at a concentration of 2 or 8 microg/ml failed to kill erythromycin-resistant organisms under any conditions. Bactericidal activity was observed against all seven erythromycin-intermediate isolates when log-phase inocula were used and the cells were counted after 48 h of incubation (mean reductions in viable bacteria for RP 59500 at concentrations of 2 and 8 microg/ml, 3.45 and 3.50 log10 CFU/ml, respectively), but killing was diminished when the plates were examined at 72 h (mean killing, 3.06 and 2.95 log10, CFU/ml, respectively). No bactericidal activity was observed when stationary-phase cultures were used. On the basis of these data, we expect that bactericidal activity of RP 59500 against the multiple-drug-resistant E. faecium strains currently encountered would be distinctly uncommon.

In vitro activities in new oxazolidinone antimicrobial agents against enterococci.

The comparative in vitro activities of two new oxazolidinone antimicrobial agents, U-100592 and U-100766, against 180 isolates of enterococci representing several resistance profiles were examined by using an agar dilution technique. The two oxazolidinones inhibited all isolates, including strains resistant to vancomycin, ampicillin, and minocycline, at concentrations between 1 and 4 micrograms/ml.

Comparative in vitro activity of levofloxacin and ofloxacin against gram-positive bacteria.

The in vitro activity of levofloxacin against 506 Gram-positive bacteria was compared with those of D(-)-ofloxacin, ofloxacin, ciprofloxacin, and sparfloxacin. Levofloxacin was generally twice as active as ofloxacin against these organisms (range, 0-3 twofold dilutions). Sparfloxacin appeared to have the greatest activity overall, but for several groups of organisms minimum inhibitory concentrations (MIC90s) of this compound were within one twofold dilution of those of levofloxacin. Resistance to levofloxacin (MIC > or = 8 micrograms/ml) was not encountered among streptococcal species, was rare in methicillin-susceptible staphylococci (1.7%), and was infrequent in vancomycin-susceptible Enterococcus faecalis and Enterococcus faecium (8.7%). Resistance was more common among vancomycin-resistant enterococci and methicillin-resistant staphylococci.

In vitro activity of WY-49605, a penem antimicrobial.

The in vitro activity of the penem antimicrobial WY-49605 was compared with those of other agents available for oral administration. Based on concentrations inhibiting 90% of isolates (MIC(90)s), the penem inhibited methicillin-susceptible staphylococci (MIC(90) = 0.25 microg/ml), penicillin-susceptible streptococci (MIC(90) < or = 0.12 microg/ml) and several other Gram-positive genera at concentrations comparable or superior to the most active comparison agents. WY-49605 and cefpodoxime were the most active agents against members of the family Enterobacteriaceae. Most strains of Enterococcus faecalis and Bacteroides fragilis were susceptible to the new agent at concentrations < or =4microg/ml, while Pseudomonas aeruginosa, Enterococcus faecium, and methicillin-resistant Staphylococcus aureus were resistant to all agents tested.

In vitro activity of A-86719.1, a novel 2-pyridone antimicrobial agent.

This study evaluated the in vitro activity of A-86719.1, a novel 2-pyridone antimicrobial agent. The drug inhibited all tested members of the family Enterobacteriaceae at < or = 0.5 microgram/ml and all tested Pseudomonas aeruginosa, Burkholderia (Pseudomonas) cepacia, and Xanthomonas maltophilia strains at < or = 2 micrograms/ml. All but two strains of gram-positive bacteria were inhibited by < or = 1 microgram of the new drug per ml, including isolates highly resistant to ciprofloxacin.