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1.  Efficacies of piperacillin-tazobactam and cefepime in rats with experimental intra-abdominal abscesses due to an extended-spectrum beta-lactamase-producing strain of Klebsiella pneumoniae. 
The in vivo activities of piperacillin-tazobactam and cefepime were compared with those of ticarcillin-clavulanate, ceftazidime, cefotaxime, and imipenem in a rat model of intra-abdominal abscess with a strain of Klebsiella pneumoniae elaborating an extended-spectrum beta-lactamase (TEM-26). With the exception of ceftazidime, all of the antimicrobial agents significantly reduced bacterial counts within abscesses at the end of therapy compared with those in untreated controls. Residual viable cell counts (mean +/- standard deviation in log10 CFU/gram) were as follows: control, 8.76 +/- 0.97; ceftazidime, 8.00 +/- 0.76; piperacillin-tazobactam, 3.87 +/- 1.72; ticarcillin-clavulanate, 3.74 +/- 1.34; cefepime, 3.15 +/- 1.19; cefotaxime, 2.61 +/- 0.77; imipenem, 2.41 +/- 0.93. Imipenem was more effective than either of the inhibitor combinations (P < 0.05). Cefotaxime was unexpectedly effective given its poor in vivo activity against this organism in our earlier studies, which used a different dose and total duration of therapy (L. B. Rice, J. D. C. Yao, K. Klimm, G. M. Eliopoulos, and R. C. Moellering, Jr., Antimicrob. Agents Chemother. 35:1243-1244, 1991). These observations suggest that the effectiveness of cephalosporins in the treatment of experimental infections caused by extended-spectrum beta-lactamase-producing K. pneumoniae may be highly dependent on dosing regimens, even for a specific organism and site of infection.
PMCID: PMC163849  PMID: 9145868
2.  Activity of ampicillin-sulbactam and oxacillin in experimental endocarditis caused by beta-lactamase-hyperproducing Staphylococcus aureus. 
Using a rat model of aortic valve infective endocarditis, we previously found that oxacillin was equally effective against an oxacillin-susceptible strain of Staphylococcus aureus and a beta-lactamase-hyperproducing borderline oxacillin-susceptible strain of S. aureus; also, ampicillin-sulbactam was less effective than oxacillin against both isolates and at low doses was less effective against the borderline-susceptible strain than against the fully oxacillin-susceptible strain (C. Thauvin-Eliopoulos, L. B. Rice, G. M. Eliopoulos, and R. C. Moellering, Jr., Antimicrob. Agents Chemother. 34:728-732, 1990). In the present study, we extended this work, using alternative treatment schedules and additional bacterial strains. Extending treatment with low doses of ampicillin-sulbactam (500 and 250 mg/kg of body weight per day, respectively) to 6.5 days resulted in equalization of effectiveness against the previously studied strains BOSSA-1 and OSSA-1 (3.75 +/- 1.61 log10 and 4.71 +/- 1.79 log10 CFU of residual viable bacteria per g, respectively). Against the borderline oxacillin-susceptible strain BOSSA-1, increasing the sulbactam dosage from 500 to 2,000 mg/kg/day while maintaining a fixed dose of ampicillin (1,000 mg/kg/day) by continuous infusion resulted in lower bacterial counts (4.93 +/- 1.84 log10 versus 3.65 +/- 1.26 log10 CFU of residual viable bacteria per g, respectively), but this difference was of only borderline significance; differences in efficacy between the low-dose and high-dose sulbactam regimens were exaggerated when intermittent intravenous administration was used (6.19 +/- 1.90 log10 versus 3.37 +/- 1.41 log10 CFU/g, respectively; P < 0.001). However, for any individual sulbactam dosage, the model of administration (continuous versus intermittent infusion) did not affect the activity of the regimen. When additional strains were used in the model, oxacillin and ampicillin-sulbactam (1,000 plus 2,000 mg/kg/day) were equally effective against both oxacillin-susceptible and borderline oxacillin-resistant strains of S. aureus. These results support the predictions that oxacillin would be clinically effective in the treatment of infections caused by borderline oxacillin-susceptible strains of S. aureus and that, except at very low doses, ampicillin-sulbactam would also be as effective against borderline-susceptible strains as against fully oxacillin-susceptible strains of S. aureus.
PMCID: PMC187700  PMID: 8460919
3.  In vitro activity and mechanism of action of A21978C1, a novel cyclic lipopeptide antibiotic. 
The in vitro activity of A21978C1, a novel cyclic polypeptide antibiotic, was compared with those of vancomycin, teichomycin, and several beta-lactam antibiotics against gram-positive bacteria. The new drug was at least as active as vancomycin against all species of streptococci and staphylococci tested, including methicillin-resistant Staphylococcus aureus and penicillin-resistant pneumococci. Activity of the drug was found to be strongly correlated with the calcium concentration in test media. Against enterococci, A21978C1 was bactericidal at concentrations near the MIC (MIC for 100% of the strains, 2 micrograms/ml), but combining that drug with gentamicin resulted in bactericidal synergism by time-kill methods. Studies were undertaken to examine the mechanism of action of the drug. A21978C1 did not interact with penicillin-binding proteins of bacterial cell membranes. No direct effect of the drug on the synthesis of DNA, RNA, or protein by a susceptible strain of Streptococcus faecalis could be demonstrated. However, A21978C1 inhibited peptidoglycan synthesis in early-log-phase cultures of both Streptococcus faecalis and Staphylococcus aureus.
PMCID: PMC176277  PMID: 3994349
4.  Failure of trimethoprim-sulfamethoxazole therapy in experimental enterococcal endocarditis. 
To assess the potential efficacy of trimethoprim-sulfamethoxazole (TMP-SMX) against serious enterococcal infections, we used a rat enterococcal endocarditis model comparing TMP-SMX therapy (500 mg of TMP plus 2,500 mg of SMX per kg of body weight per day given every 8 h by intragastric gavage) with intravenous ampicillin therapy (1,000 mg/kg per day). Despite concentrations of active drug in serum well in excess of the MIC and MBC, the mean residual vegetation bacterial titer in TMP-SMX-treated rats was similar to that in untreated controls (8.4 +/- 1.1 versus 8.6 +/- 1.3 log10 CFU/g) and significantly higher than that in the ampicillin-treated group (3.6 +/- 1.5 log10 CFU/g; P less than or equal to 0.001). This demonstrates discordance between in vitro activity and in vivo efficacy of TMP-SMX in serious enterococcal infection.
PMCID: PMC171927  PMID: 2126691
5.  Efficacy of teicoplanin in two dosage regimens for experimental endocarditis caused by a beta-lactamase-producing strain of Enterococcus faecalis with high-level resistance to gentamicin. 
Optimal therapy for the treatment of infections caused by strains of enterococci demonstrating high-level resistance to gentamicin and other aminoglycosides has not been established. The present study examined the efficacy of teicoplanin, a glycopeptide antibiotic active against gram-positive bacterial infections in various animal models, in the treatment of experimental endocarditis due to a beta-lactamase-producing strain of Enterococcus faecalis with high-level resistance to gentamicin. Vancomycin was used as a comparative antibiotic. In the first set of experiments, both antimicrobial agents were administered by continuous intravenous infusion for 5 days at dosages which yielded comparable mean levels in serum (plus or minus the standard deviation) of 14.6 +/- 4.3 micrograms/ml for teicoplanin and 14.3 +/- 2.2 micrograms/ml for vancomycin. These regimens proved similarly effective in sterilizing cardiac vegetations (38 versus 50% of treated animals, respectively; P greater than 0.05). Mean (plus or minus the standard deviation) residual bacterial titers within vegetations were reduced to 3.2 +/- 1.2 log10 CFU/g and 3.4 +/- 1.7 log10 CFU/g, respectively. In separate experiments, the potential of teicoplanin to cure endocarditis was assessed, using two dosage regimens: (i) 30 mg/kg per day (mean level in serum, 13 micrograms/ml) for 10 days or (ii) 150 mg/kg per day (mean level in serum, 84 micrograms/ml) for 5 days. Surviving animals were sacrificed 10 days after the discontinuation of therapy. Both teicoplanin regimens were more effective than the comparative vancomycin (150 mg/kg per day) regimen: 92 versus 43% cured (P =0.025) in the standard-dose group, and 82 versus 37% cured (P = 0.015) in the high-dose group. Results in this rat model of enterococcal endocarditis show that teicoplanin may prove useful in the treatment of serious infections due to high level-gentamicin-resistant enterococci in humans.
PMCID: PMC171700  PMID: 2141778
6.  In vitro activity of SK&F 104662, a new glycopeptide antibiotic. 
The in vitro activity of SK&F 104662, a new glycopeptide antibiotic, against gram-positive bacteria was evaluated. Activity was comparable to those of teicoplanin and vancomycin against most organisms. SK&F 104662 inhibited diphtheroids at concentrations of less than or equal to 0.5 microgram/ml. Addition of human serum to the test medium lowered the inhibitory activity of this glycopeptide against some organisms by as much as eightfold.
PMCID: PMC284265  PMID: 2548444
7.  Mechanism of action and in vitro and in vivo activities of S-6123, a new oxazolidinone compound. 
The in vitro activity of S-6123, a synthetic antimicrobial compound of the new oxazolidinone series, was compared with those of other orally administered agents against 328 clinical isolates. The compound was moderately active (MICs, 16 to 64 micrograms/ml) against 90% of staphylococci, nonenterococcal streptococci, and Haemophilus influenzae, including strains resistant to beta-lactam antibiotics. S-6123 was minimally active against enterococci and facultative gram-negative bacilli. Nevertheless, the compound had significant activity in a lethal rat Escherichia coli peritonitis model at serum concentrations of one-tenth the MIC against the infecting organism. The drug demonstrated only bacteriostatic activity against susceptible organisms. Studies to define the mechanism of antibacterial action revealed that S-6123 inhibited ribosomal protein synthesis without inhibiting DNA or RNA synthesis. This compound represents a new series of antibacterial agents not related to any other antibacterial compound of natural or synthetic origin.
PMCID: PMC175864  PMID: 3058018
8.  In vitro activity of BMY-28100, a new oral cephalosporin. 
The activity of BMY-28100, a new orally administered cephalosporin, was compared with those of cephalexin and cefaclor. BMY-28100 was the most active drug against Staphylococcus aureus (MIC for 90% of strains tested [MIC90], 1.0 microgram/ml), streptococci (MIC90S, less than or equal to 0.125 microgram/ml), and Klebsiella pneumoniae (MIC90, 2 micrograms/ml). The drug was active against Haemophilus influenzae and gonococci but not against other organisms generally resistant to cephem antibiotics.
PMCID: PMC174801  PMID: 3496846
9.  Continuous-infusion ampicillin therapy of enterococcal endocarditis in rats. 
Intermittent administration of ampicillin alone has resulted in high failure rates in previously described animal models of enterococcal endocarditis. We developed a rat model of enterococcal endocarditis which permits comparison of continuous intravenous infusion of ampicillin with intramuscular therapy. Continuous low-dose ampicillin infusion (450 mg/kg [body weight] per day) was compared with the same dose given intramuscularly in three divided doses and with high-dose infusion (4.5 g/kg per day) of the drug. For the infecting strain of Streptococcus faecalis, the MIC and MBC were 1 microgram/ml. Mean ampicillin levels in serum were 53.9 +/- 4.8 (peak) and less than 1 (trough), 8.7 +/- 1.4, and 244 +/- 29 micrograms/ml for intramuscular, low-dose, and high-dose regimens, respectively. Ampicillin infusion therapy significantly increased the survival rate and sterilization of blood cultures. Continuous infusions were superior to intermittent therapy in eradicating bacteremia. After 5 days of treatment, low-dose ampicillin infusion was more effective than intermittent therapy in sterilizing cardiac vegetations (P less than 0.01). Continuous-infusion therapy at either dose was significantly more effective than intramuscular injection in reducing bacterial titers in cardiac vegetations (5.4 +/- 1.0 log10 CFU/g [low dose], 4.8 +/- 0.3 log10 CFU/g [high dose], and 7.7 +/- 0.3 log10 CFU/g [intramuscular]). However, no statistically significant advantage was found for high-dose compared with low-dose ampicillin infusion in lowering bacterial titers in vegetations (P greater than 0.3).
PMCID: PMC174678  PMID: 3105445
10.  In vitro activities of the quinolone antimicrobial agents A-56619 and A-56620. 
The in vitro activities of two new quinolone antimicrobial agents, A-56619 and A-56620, were compared with those of norfloxacin, ciprofloxacin, and other antimicrobial agents. The activity of A-56620 was comparable to that of ciprofloxacin against Escherichia coli, Enterobacter cloacae, and Aeromonas hydrophila (MICs for 90% of the strains were less than or equal to 0.06 micrograms/ml); Acinetobacter anitratus and Staphylococcus aureus (MIC for 90% of the strains was 0.5 micrograms/ml); and most streptococci. Against other gram-negative strains, A-56620 demonstrated activity comparable to that of norfloxacin, but the new drug was two to eight times more active than norfloxacin against gram-positive isolates. A-56620 was more active than A-56619 against most gram-negative organisms tested. Of the members of the family Enterobacteriaceae examined, 88% were inhibited by A-56619 and 99% by A-56620 at concentrations of less than or equal to 1.0 microgram/ml. By time-kill methods, the new quinolones were bactericidal against gram-negative bacilli during the first 6 h of incubation, but against S. aureus and enterococci the drugs were primarily bacteriostatic during this period. The frequency of spontaneous resistance to 10 micrograms of these drugs per ml was less than 10(-8) for all species tested except E. cloacae, but by serial passage through incremental concentrations of the antimicrobial agents, colonies many-fold more resistant than the initial isolate could be selected. However, resistance to concentrations of the drug greater than 100 micrograms/ml remained stable after passage on antibiotic-free media in only 1 of 35 strains tested.
PMCID: PMC180295  PMID: 3935046
11.  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. 
Antimicrobial Agents and Chemotherapy  1997;41(12):2749-2753.
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.
PMCID: PMC164201  PMID: 9420051
12.  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.
PMCID: PMC163882  PMID: 9145901
13.  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.
PMCID: PMC163410  PMID: 8807077
14.  Comparative in vitro activities of L-695,256, a novel carbapenem, against gram-positive bacteria. 
The in vitro activity of a prototype 2-aryl carbapenem, L-695,256, against gram-positive bacteria was examined. All streptococci and oxacillin-susceptible and -resistant staphylococci were inhibited at concentrations of < or = 0.125, < or = 0.125, and 4 micrograms/ml, respectively. The activity of L-695,256 was superior to that of imipenem against other organisms intrinsically resistant to beta-lactams.
PMCID: PMC162669  PMID: 7786011
15.  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.
PMCID: PMC162641  PMID: 7785983
16.  In vitro activity of biapenem against clinical isolates of gram-positive and gram-negative bacteria. 
The in vitro activity of biapenem, a new carbapenem previously designated L-627, was compared with those of imipenem and several other antimicrobial agents against 771 clinical bacterial isolates. Against gram-positive organisms, biapenem was found to be approximately as active as imipenem, inhibiting 90% of isolates of most species at concentrations within one dilution of the MIC of imipenem for 90% of the isolates. Against gram-negative organisms and Bacteroides fragilis, biapenem was at least as active as and often more active than imipenem, with MICs for 90% of the isolates two- to eightfold lower than those of imipenem.
PMCID: PMC188112  PMID: 8239623
17.  Absence of synergistic activity between ampicillin and vancomycin against highly vancomycin-resistant enterococci. 
Antimicrobial Agents and Chemotherapy  1992;36(10):2201-2203.
The emergence of clinical enterococcal isolates resistant to both ampicillin and vancomycin is a cause of great concern, as there are few therapeutic alternatives for treatment of infections caused by such organisms. We evaluated the effects of the combination of ampicillin with vancomycin against vancomycin-resistant clinical enterococcal isolates. Using both the checkerboard technique and time-kill curves, we examined 28 strains of enterococci (17 Enterococcus faecalis and 11 Enterococcus faecium strains) with different levels of resistance to vancomycin. Of these, 15 strains were also highly gentamicin resistant, and 9 demonstrated resistance to ampicillin. Only seven strains of E. faecalis were inhibited synergistically by the combination of vancomycin with ampicillin, and even then, the concentrations of vancomycin at which synergism was demonstrated were above levels achievable in serum. None of the ampicillin-resistant isolates (all E. faecium) were inhibited synergistically at any concentration of the drugs. In no instance was bactericidal synergism observed, and in most cases the combination resulted in less killing than with ampicillin alone. Antagonism was not observed at clinically relevant concentrations. The results of this study suggest that the combination of vancomycin with ampicillin has little to offer against these emerging pathogens.
PMCID: PMC245476  PMID: 1444300
18.  Antimicrobial susceptibilities of a Corynebacterium CDC group I1 strain isolated from a patient with endocarditis. 
We encountered a case of native valve endocarditis due to Corynebacterium CDC group I1 which was successfully controlled with antimicrobial agents alone. This organism and three other isolates of this group were susceptible to penicillin, vancomycin, and gentamicin. The combination of penicillin with subinhibitory concentrations of gentamicin resulted in a 1,000-fold decrease in CFU per milliliter at 24 h compared with penicillin alone. Augmentation of killing was noted also with vancomycin plus gentamicin but to a degree that did not meet strict criteria for bactericidal synergism.
PMCID: PMC190341  PMID: 1416833
19.  Antimicrobial susceptibility changes in Enterococcus faecalis following various penicillin exposure regimens. 
Penicillin-"virgin" strains of Enterococcus faecalis collected from a population of individuals with no previous antibiotic exposure were subjected in vitro to penicillin delivered as repeated pulses, stepwise increasing concentrations, or sustained levels of a single concentration. Changes in resistance to penicillin were assessed by determination of MICs, and time-kill studies were performed to evaluate changes in tolerance to the bactericidal effects of penicillin. Isogenic clones, derived from various exposure regimens, which exhibited changes in either resistance or tolerance were further examined for changes in penicillin-binding proteins. Exposure to repeated pulses of penicillin resulted in the development of tolerance to penicillin without changes in the level of resistance. Clones derived from a regimen of stepwise increases in the penicillin concentration acquired both increased penicillin resistance and tolerance. Clones selected after prolonged continuous exposure to a fixed concentration of penicillin displayed minimally increased resistance to penicillin, but they retained the lytic, nontolerant response to the bactericidal effect of penicillin. Clones which acquired tolerance to the bactericidal effect of penicillin without changes in penicillin resistance exhibited a penicillin-binding protein pattern identical to that of the parental strain. Increased labeling of several penicillin-binding proteins accompanied the development of increased penicillin resistance in both penicillin-tolerant and nontolerant strains. Exposure of E. faecalis to penicillin in repeated pulses of brief duration, for prolonged periods at a constant concentration, or in stepwise graded concentrations can result in the selection of clones with increased resistance to the inhibitory or bactericidal effects of penicillin, or both. These observations may be relevant to the selection of dosing regimes for penicillin in the treatment of enterococcal infections, when bactericidal synergism cannot be achieved with penicillin-aminoglycoside combinations.
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PMCID: PMC189238  PMID: 1590676
20.  Increasing resistance to beta-lactam antibiotics among clinical isolates of Enterococcus faecium: a 22-year review at one institution. 
Antimicrobial Agents and Chemotherapy  1991;35(11):2180-2184.
To identify any change in the antibiotic resistance of Enterococcus faecium, we examined the antibiotic susceptibilities of clinical strains (n = 84) isolated at one institution during the 22 years since 1968. A significant increase in resistance to penicillin was observed during the study period: the MICs of penicillin for 50 and 90% of isolates tested were 16 and 64 micrograms/ml, respectively, from 1969 to 1988 (n = 48; geometric mean MIC, 14 micrograms/ml) , whereas they were 256 and 512 micrograms/ml, respectively, from 1989 to 1990 (n = 36; geometric mean MIC, 123 micrograms/ml) (P less than 0.001). A comparable increase in resistance to ampicillin was also noted (P less than 0.001). No strains produced detectable beta-lactamase. In contrast, susceptibilities to vancomycin, teicoplanin, and ciprofloxacin remained stable. High-level resistance to gentamicin was observed in none of 48 isolates from 1969 to 1988, but was present in 22 of 36 strains (61%) from 1989 to 1990 (P less than 0.001) and was significantly associated with resistance (MIC, greater than or equal to 128 micrograms/ml) to penicillin (P less than 0.001). To assess the potential evolution of antibiotic resistance in this species, clinical isolates (n = 24) were compared with strains isolated in 1968 from a human population in the Solomon Islands that was never exposed to antibiotics. Solomon Island isolates were significantly more susceptible than all clinical strains to penicillin, ampicillin, and vancomycin (P less than 0.001 for each), but they exhibited no differences in susceptibility to teicoplanin or ciprofloxacin. The penicillin-binding affinity of penicillin-binding protein 5 (PBP 5) in penicillin-resistant clinical strains (MIC, 512 micrograms/ml) was notably lower than that in strains with more typical susceptibilities, suggesting an alteration in this PBP as a possible mechanism for increased penicillin resistance. Solomon Island strains most susceptible to penicillin demonstrated a prominent PBP 5* and the absence of PBP 5. These changes in the antibiotic resistance of E. faecium emphasize the importance of identifying this species in patients with serious enterococcal infections and the necessity of assessing its susceptibility to both beta-lactams and aminoglycosides if effective therapy is to be identified.
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PMCID: PMC245356  PMID: 1803989
21.  Outbreak of ceftazidime resistance caused by extended-spectrum beta-lactamases at a Massachusetts chronic-care facility. 
Antimicrobial Agents and Chemotherapy  1990;34(11):2193-2199.
During a 4-month period in late 1988, we isolated ceftazidime-resistant strains of Klebsiella pneumoniae and other members of the family Enterobacteriaceae from 29 patients at a chronic-care facility in Massachusetts. Ceftazidime resistance resulted from two distinct extended-spectrum beta-lactamases of the TEM type which efficiently hydrolyzed the cephalosporin: YOU-1 with a pI of 5.57 and YOU-2 with a pI of 5.2. Genes encoding these enzymes were present on different but closely related high-molecular-weight, multiple antibiotic resistance plasmids of the H12 incompatibility group and were transferable by conjugation in vitro. Agarose gel electrophoresis of extracts from clinical isolates indicated that this outbreak arose from plasmid transmission among different strains of the family Enterobacteriaceae rather than from dissemination of a single resistant isolate. Isolation rates of ceftazidime-resistant organisms transiently decreased after use of this drug was restricted, but resistant isolates continued to be recovered 7 months after empiric use of ceftazidime ceased.
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PMCID: PMC172022  PMID: 2073110
22.  In vitro activity of BAY v 3522, a new cephalosporin for oral administration. 
The activity of BAY v 3522 was tested against over 500 clinical bacterial isolates and compared with the activities of ampicillin, amoxicillin-clavulanate, cefaclor, cefixime, cefuroxime, cephalexin, and/or ciprofloxacin, erythromycin, and metronidazole. BAY v 3522 activity against staphylococci and streptococci equaled or exceeded those of the other agents. BAY v 3522 exhibited no significant advantage over cefaclor, cefuroxime, or cephalexin against gram-negative bacilli.
PMCID: PMC171947  PMID: 2285305
23.  Efficacy of oxacillin and ampicillin-sulbactam combination in experimental endocarditis caused by beta-lactamase-hyperproducing Staphylococcus aureus. 
Optimal therapy of infections caused by borderline oxacillin-susceptible, beta-lactamase-hyperproducing Staphylococcus aureus has not been established. We used a rat model of aortic valve endocarditis to examine efficacies of antibiotic regimens against a borderline oxacillin-susceptible strain as compared with a fully susceptible S. aureus strain. Animals were treated with oxacillin alone or in combination with sulbactam or with ampicillin-sulbactam combinations at two dose levels. Infections caused by the borderline susceptible and fully susceptible strains responded equally well to oxacillin alone, with residual bacterial titers in vegetations falling to 4.8 +/- 1.6 and 4.4 +/- 1.7 (mean +/- standard deviation) log10 CFU/g, respectively. Addition of sulbactam to oxacillin (1:2) did not enhance the efficacy of oxacillin against either strain in the animal model. A high-dose regimen of ampicillin-sulbactam (2:1) yielding mean (+/- standard deviation) levels in serum of 16.8 +/- 7.4 and 9.5 +/- 1.1 micrograms/ml, respectively, proved equally effective against both strains (bacterial titers, 6.6 log10 CFU/g). However, at lower doses (8.3 +/- 2.6 and 5.9 +/- 2.4 micrograms/ml, the combination showed greater efficacy against the fully susceptible strain, with residual titers of 7.1 +/- 2.0 versus 9.0 +/- 1.6 log10 CFU/g (P less than 0.05). In vitro studies revealed that the beta-lactamase inhibitor sulbactam was also a potent inducer of staphylococcal beta-lactamase at clinically relevant concentrations. Based on this short-term in vivo therapy study, oxacillin would be predicted to be clinically effective in the therapy of infections caused by borderline oxacillin-susceptible strains of S. aureus, while the combination of ampicillin with sulbactam appears to be inferior to oxacillin alone against such infections.
PMCID: PMC171681  PMID: 2360813
24.  Comparative in vitro activity of SM7338, a new carbapenem antimicrobial agent. 
The comparative in vitro activity of SM7338 was tested against 670 routine clinical isolates and 130 cefoperazone-resistant isolates of bacteria by agar dilution methods. SM7338 was at least as active as imipenem against gram-negative organisms but was slightly less active against gram-positive organisms. SM7338 was particularly active against members of the family Enterobacteriaceae, with MICs for 90% of strains of less than or equal to 0.125 micrograms/ml for all species tested. Differences in activity between SM7338 and imipenem were particularly striking against Proteus vulgaris, Proteus mirabilis, and Morganella morganii, against which MICs of SM7338 and imipenem for 90% of strains were 0.125 and 4 micrograms/ml, respectively. The presence of unique plasmid-mediated beta-lactamases in Pseudomonas aeruginosa PU21 transconjugants did not affect activity substantially, except in the case of OXA-2 (eightfold-increased MIC) and OXA-3 (fourfold-increased MIC). SM7338 was also active against a laboratory-derived strain of P. aeruginosa which hyperproduced chromosomal beta-lactamase, inhibiting both the wild type and the mutant at a concentration of 1.0 micrograms/ml.
PMCID: PMC172631  PMID: 2508543
25.  Treatment of experimental endocarditis caused by a beta-lactamase-producing strain of Enterococcus faecalis with high-level resistance to gentamicin. 
Several antimicrobial regimens were evaluated in the treatment of experimental enterococcal endocarditis due to a beta-lactamase-producing, highly gentamicin-resistant strain of Enterococcus faecalis. Ampicillin alone cleared bacteremia in the majority of rats and reduced titers of bacteria within vegetations (6.84 versus 8.80 log10 CFU/g in controls) but did not sterilize valves. Ampicillin-sulbactam combinations, vancomycin, daptomycin, and imipenem each reduced residual bacterial titers within vegetations to 4.01 log10 CFU/g or less; in 26 to 43% of animals receiving 5 days of therapy, titers of bacteria were reduced to undetectable levels. In a separate experiment, rats received ampicillin-sulbactam, daptomycin, or vancomycin for 10 days and were then observed for 10 days after termination of therapy for evidence of relapse. In surviving rats, valves remained sterile in four of five rats treated with ampicillin-sulbactam, in five of seven treated with daptomycin, but in only one of eight receiving vancomycin.
PMCID: PMC176055  PMID: 2506803

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