In Vitro Activity of β-Lactams in Combination with β-Lactamase Inhibitors against Multidrug-Resistant Mycobacterium tuberculosis Isolates

The combination of β-lactams and β-lactamase inhibitors has been shown to have potent in vitro activity against multidrug-resistant tuberculosis (MDR-TB) isolates. In order to identify the most potent β-lactam–β-lactamase inhibitor combination against MDR-TB, we selected nine β-lactams and three β-lactamase inhibitors, which belong to different subgroups. A total of 121 MDR-TB strains were included in this study. Out of the β-lactams used herein, biapenem was the most effective against MDR-TB and had an MIC₅₀ value of 8 μg/ml. However, after the addition of clavulanate or sulbactam, meropenem exhibited the most potent anti-MDR-TB activity with an MIC₅₀ value of 4 μg/ml. For meropenem, 76 (62.8%), 41 (33.9%), and 22 (18.2%) of the 121 MDR-TB strains were subjected to a synergistic effect when the drug was combined with sulbactam, tazobactam, or clavulanate, respectively. Further statistical analysis revealed that significantly more strains experienced a synergistic effect when exposed to the combination of meropenem with sulbactam than when exposed to meropenem in combination with tazobactam or clavulanate, respectively (P < 0.01). In addition, a total of 10.7% (13/121) of isolates harbored mutations in the blaC gene, with two different nucleotide substitutions: AGT333AGG and ATC786ATT. For the strains with a Ser111Arg substitution in BlaC, a better synergistic effect was observed in the meropenem-clavulanate and in the amoxicillin-clavulanate combinations than that in a synonymous single nucleotide polymorphism (SNP) group. In conclusion, our findings demonstrate that the combination of meropenem and sulbactam shows the most potent activity against MDR-TB isolates. In addition, the Ser111Arg substitution of BlaC may be associated with an increased susceptibility of MDR-TB isolates to meropenem and amoxicillin in the presence of clavulanate.     

Comparative In Vitro Killing Activities of Meropenem, Imipenem, Ceftriaxone, and Ceftriaxone plus Vancomycin at Clinically Achievable Cerebrospinal Fluid Concentrations against Penicillin-Resistant Streptococcus pneumoniae Isolates from Children with Meningitis

The activities of meropenem, imipenem, ceftriaxone, and vancomycin were evaluated against 80 penicillin-susceptible and -resistant Streptococcus pneumoniae strains. Meropenem, imipenem, ceftriaxone, and vancomycin MICs at which 90% of the isolates are inhibited were 0.5, 0.25, 1, and 0.25 μg/ml, respectively. Against penicillin-resistant strains, the best killing activity at cerebrospinal fluid concentrations was obtained with imipenem and ceftriaxone-vancomycin. However, while the killing activity of imipenem was significantly greater than that of meropenem, no significant difference was observed between the activities of meropenem and ceftriaxone-vancomycin.     

Novel Approach To Optimize Synergistic Carbapenem-Aminoglycoside Combinations against Carbapenem-Resistant Acinetobacter baumannii

Acinetobacter baumannii is among the most dangerous pathogens and emergence of resistance is highly problematic. Our objective was to identify and rationally optimize β-lactam-plus-aminoglycoside combinations via novel mechanism-based modeling that synergistically kill and prevent resistance of carbapenem-resistant A. baumannii. We studied combinations of 10 β-lactams and three aminoglycosides against four A. baumannii strains, including two imipenem-intermediate (MIC, 4 mg/liter) and one imipenem-resistant (MIC, 32 mg/liter) clinical isolate, using high-inoculum static-concentration time-kill studies. We present the first application of mechanism-based modeling for killing and resistance of A. baumannii using Monte Carlo simulations of human pharmacokinetics to rationally optimize combination dosage regimens for immunocompromised, critically ill patients. All monotherapies achieved limited killing (≤2.3 log₁₀) of A. baumannii ATCC 19606 followed by extensive regrowth for aminoglycosides. Against this strain, imipenem-plus-aminoglycoside combinations yielded more rapid and extensive killing than other β-lactam-plus-aminoglycoside combinations. Imipenem at 8 mg/liter combined with an aminoglycoside yielded synergistic killing (>5 log₁₀) and prevented regrowth of all four strains. Modeling demonstrated that imipenem likely killed the aminoglycoside-resistant population and vice versa and that aminoglycosides enhanced the target site penetration of imipenem. Against carbapenem-resistant A. baumannii (MIC, 32 mg/liter), optimized combination regimens (imipenem at 4 g/day as a continuous infusion plus tobramycin at 7 mg/kg of body weight every 24 h) were predicted to achieve >5 log₁₀ killing without regrowth in 98.2% of patients. Bacterial killing and suppression of regrowth were best achieved for combination regimens with unbound imipenem steady-state concentrations of at least 8 mg/liter. Imipenem-plus-aminoglycoside combination regimens are highly promising and warrant further evaluation.     

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 of Biapenem plus RPX7009, a Carbapenem Combined with a Serine β-Lactamase Inhibitor,against Anaerobic Bacteria

Biapenem is a carbapenem being developed in combination with RPX7009, a new inhibitor of serine β-lactamases. Biapenem was tested alone and in combination with fixed concentrations of RPX7009 by agar dilution against 377 recent isolates of anaerobes. A separate panel of 27 isolates of Bacteroides spp. with decreased susceptibility or resistance to imipenem was also tested. Comparator drugs included meropenem, piperacillin-tazobactam, ampicillin-sulbactam, cefoxitin, ceftazidime, metronidazole, clindamycin, and tigecycline plus imipenem, doripenem, and ertapenem for the 27 selected strains. For recent consecutive strains of Bacteroides species, the MIC₉₀ for biapenem-RPX7009 was 1 μg/ml, with a MIC₉₀ of 4 μg/ml for meropenem. Other Bacteroides fragilis group species showed a MIC₉₀ of 0.5 μg/ml for both agents. The MIC₉₀s for biapenem-RPX7009 were 0.25 μg/ml for Prevotella spp., 0.125 μg/ml for Fusobacterium nucleatum and Fusobacterium necrophorum, 2 μg/ml for Fusobacterium mortiferum, 0.5 μg/ml for Fusobacterium varium, ≤0.5 μg/ml for Gram-positive cocci and rods, and 0.03 to 8 μg/ml for clostridia. Against 5 B. fragilis strains harboring a known metallo-beta-lactamase, biapenem-RPX7009 MICs were comparable to those of other carbapenems (≥32 μg/ml). Against Bacteroides strains with an imipenem MIC of 2 μg/ml, biapenem-RPX7009 had MICs of 0.5 to 2 μg/ml, with MICs of 0.5 to 32 μg/ml for meropenem, doripenem, and ertapenem. For strains with an imipenem MIC of 4 μg/ml, the MICs for biapenem-RPX7009 were 4 to 16 μg/ml, with MICs of 8 to >32 μg/ml for meropenem, doripenem, and ertapenem. The inhibitor RPX7009 had no antimicrobial activity when tested alone, and it showed little or no potentiation of biapenem versus anaerobes. Biapenem-RPX7009 showed activity comparable to that of imipenem and was superior to meropenem, doripenem, and ertapenem against imipenem-nonsusceptible Bacteroides spp.     

In vitro activity of doripenem, a carbapenem for the treatment of challenging infections caused by gram-negative bacteria, against recent clinical isolates from the United States

Doripenem, a 1β-methylcarbapenem, is a broad-spectrum antibiotic approved for the treatment of complicated urinary tract and complicated intra-abdominal infections. An indication for hospital-acquired pneumonia including ventilator-associated pneumonia is pending. The current study examined the activity of doripenem against recent clinical isolates for the purposes of its ongoing clinical development and future longitudinal analysis. Doripenem and comparators were tested against 12,581 U.S. clinical isolates collected between 2005 and 2006 including isolates of Staphylococcus aureus, coagulase-negative staphylococci, Streptococcus pneumoniae, Enterobacteriaceae, Pseudomonas aeruginosa, and Acinetobacter spp. MICs (μg/ml) were established by broth microdilution. By MIC₉₀, doripenem was comparable to imipenem and meropenem in activity against S. aureus (methicillin susceptible, 0.06; resistant, 8) and S. pneumoniae (penicillin susceptible, ≤0.015; resistant, 1). Against ceftazidime-susceptible Enterobacteriaceae, the MIC₉₀ of doripenem (0.12) was comparable to that of meropenem (0.12) and superior to that of imipenem (2), though susceptibility of isolates exceeded 99% for all evaluated carbapenems. The activity of doripenem was not notably altered against ceftazidime-nonsusceptible or extended-spectrum β-lactamase screen-positive Enterobacteriaceae. Doripenem was the most potent carbapenem tested against P. aeruginosa (MIC₉₀/% susceptibility [%S]: ceftazidime susceptible = 2/92%S, nonsusceptible = 16/61%S; imipenem susceptible = 1/98.5%S, nonsusceptible = 8/56%S). Against imipenem-susceptible Acinetobacter spp., doripenem (MIC₉₀ = 2, 89.1%S) was twice as active by MIC₉₀ as were imipenem and meropenem. Overall, doripenem potency was comparable to those of meropenem and imipenem against gram-positive cocci and doripenem was equal or superior in activity to meropenem and imipenem against Enterobacteriaceae, including β-lactam-nonsusceptible isolates. Doripenem was the most active carbapenem tested against P. aeruginosa regardless of β-lactam resistance.     

Bactericidal activity of DU-6859a compared to activities of three quinolones, three beta-lactams, clindamycin, and metronidazole against anaerobes as determined by time-kill methodology.

The activities of DU-6859a, ciprofloxacin, levofloxacin, sparfloxacin, piperacillin, piperacillin-tazobactam, imipenem, clindamycin, and metronidazole against 11 anaerobes were tested by the broth microdilution and time-kill methods. DU-6859a was the most active drug tested (broth microdilution MICs, 0.06 to 0.5 microg/ml), followed by imipenem (MICs, 0.002 to 4.0 microg/ml). Broth macrodilution MICs were within 3 (but usually 1) dilutions of the broth microdilution MICs. All compounds were bactericidal at the MIC after 48 h; after 24 h, 90% killing was shown for all strains when the compounds were used at four times the MIC. DU-6859a at < or = 0.5 microg/ml was bactericidal after 48 h.     

In vitro activity of BMS-181139, a new carbapenem with potent antipseudomonal activity.

The in vitro activities of the carbapenem BMS-181139 were determined in comparison with those of imipenem, meropenem, ciprofloxacin, ceftriaxone, and vancomycin. BMS-181139 was the most active against species of Pseudomonas and related genera Alteromonas and Burkholderia, with MICs for 147 of 149 isolates of < 4 micrograms/ml. Of 22 imipenem-resistant (MIC > 8 micrograms/ml) P. aeruginosa strains, only 1 required an MIC of BMS-181139 of > 4 micrograms/ml, compared with 14 requiring the same meropenem MIC. BMS-181139 was the most active carbapenem against the majority of other gram-negative species except members of the tribe Proteeae, against which meropenem was more active. Although imipenem was more active against gram-positive species, BMS-18139 MICs at which 90% of strain tested were inhibited were < 1 microgram/ml for these species. BMS-181139 was generally active against isolates resistant to ciprofloxacin or broad-spectrum cephalosporins, including those containing plasmid-encoded beta-lactamases or high levels of chromosome-encoded beta-lactamases, as well as anaerobes except Clostridium difficile. Inoculum effects were noted for all three carbapenems against Klebsiella pneumoniae, Enterobacter cloacae, and Serratia marcescens but not Escherichia coli, Pseudomonas aeruginosa, or Staphylococcus aureus. BMS-181139's inoculum effect tended to be more marked. BMS-181139 exhibited bactericidal activity at the MIC for some strains and up to four to eight times the MIC for others. The postantibiotic effect of BMS-181139 was equal to or less than that of imipenem and, like meropenem, exhibited intraspecies variability. BMS-181139 was 30-fold more stable than imipenem and 7-fold more stable than meropenem to hydrolysis by hog kidney dehydropeptidase.     

In vitro pharmacodynamics of piperacillin, piperacillin-tazobactam, and ciprofloxacin alone and in combination against Staphylococcus aureus, Klebsiella pneumoniae, Enterobacter cloacae, and Pseudomonas aeruginosa.

The time-kill curve methodology was used to determine the pharmacodynamics of piperacillin, ciprofloxacin, piperacillin-tazobactam and the combinations piperacillin-ciprofloxacin and ciprofloxacin-piperacillin-tazobactam. Kill curve studies were performed for piperacillin, ciprofloxacin, and piperacillin-tazobactam at concentrations of 0.25 to 50 times the MICs for 13 strains of bacteria: four Pseudomonas aeruginosa, three Enterobacter cloacae, three Klebsiella pneumoniae, and three Staphylococcus aureus isolates (tazobactam concentrations of 0.5, 4, and 12 micrograms/ml). By using a sigmoid Emax model and nonlinear least squares regression, the 50% lethal concentrations and the maximum lethal rates of each agent were determined for each bacterial strain. For piperacillin-ciprofloxacin and ciprofloxacin-piperacillin-tazobactam, kill curve studies were performed with concentrations obtained by the fractional maximal effect method (R. C. Li, J. J. Schentag, and D. E. Nix, Antimicrob. Agents Chemother. 37:523-531, 1993) and from individual 50% lethal concentrations and maximum lethal rates. Ciprofloxacin-piperacillin-tazobactam was evaluated only against the four P. aeruginosa strains. Interactions between piperacillin and ciprofloxacin were generally additive. At physiologically relevant concentrations of piperacillin and ciprofloxacin, ciprofloxacin had the highest rates of killing against K. pneumoniae. Piperacillin-tazobactam (12 micrograms/ml) had the highest rate of killing against E. cloacae. Piperacillin-ciprofloxacin with relatively higher ciprofloxacin concentrations had the greatest killing rates against S. aureus. This combination had significantly higher killing rates than piperacillin (P < 0.002). For all the bacterial strains tested, killing rates by ciprofloxacin were significantly higher than those by piperacillin-tazobactam (4 and 12 micrograms/ml had significantly higher killing rates than piperacillin alone (P < 0.02 and P < 0.004, respectively). The effect of the combination of piperacillin-ciprofloxacin, in which piperacillin concentrations were relatively higher, was not statistically different from that of piperacillin alone (p > or = 0.71). The combination of ciprofloxacin-piperacillin-tazobactam achieved greater killing than other combinations or monotherapies against P. aeruginosa. The reduction in the initial inoculum was 1 to 4 logs greater with ciprofloxacin-piperacillin-tazobactam at 4 and 12 micrograms/ml than with any other agent or combination of agents. On the basis of the additive effects prevalently demonstrated in the in vitro study, the combinations of piperacillin-ciprofloxacin and piperacillin-tazobactam are rational therapeutic options. Greater killing of P. aeruginosa was demonstrated with ciprofloxacin-piperacillin--tazobactam. Since treatment failure of P. aeruginosa pneumonia is a significant problem, clinical studies are warranted.     

Antimicrobial susceptibilities of clinical isolates of Acinetobacter baumannii from Singapore.

The in vitro activities of 17 antimicrobial agents alone or in combination against 70 clinical isolates of Acinetobacter baumannii from Singapore were determined by broth microdilution. The MICs of amoxicillin, ampicillin, ceftazidime, ceftriaxone, gentamicin, and piperacillin for 90% of the strains were > or = 128 micrograms/ml. Addition of sulbactam to ampicillin produced improved activity, whereas adding tazobactam to piperacillin did not. The MICs of amikacin, ciprofloxacin, and imipenem for 90% of the strains were 32, 32, and 16 micrograms/ml, respectively.     

β-Lactams Enhance Daptomycin Activity against Vancomycin-Resistant Enterococcus faecalis and Enterococcus faecium in In Vitro Pharmacokinetic/Pharmacodynamic Models

Enterococcus faecalis and Enterococcus faecium are frequently resistant to vancomycin and β-lactams. In enterococcal infections with reduced glycopeptide susceptibility, combination therapy is often administered. Our objective was to conduct pharmacokinetic/pharmacodynamic (PK/PD) models to evaluate β-lactam synergy with daptomycin (DAP) against resistant enterococci. One E. faecalis strain (R6981) and two E. faecium strains (R6370 and 8019) were evaluated. DAP MICs were obtained. All strains were evaluated for response to LL37, an antimicrobial peptide, in the presence and absence of ceftaroline (CPT), ertapenem (ERT), and ampicillin (AMP). After 96 h, in vitro models were run simulating 10 mg DAP/kg body weight/day, 600 mg CPT every 8 h (q8h), 2 g AMP q4h, and 1 g ERT q24h, both alone and in combination against all strains. DAP MICs were 2, 4, and 4 μg/ml for strains R6981, R6370, and 8019, respectively. PK/PD models demonstrated bactericidal activity with DAP-CPT, DAP-AMP, and DAP-ERT combinations against strain 8019 (P < 0.001 and log₁₀ CFU/ml reduction of >2 compared to any single agent). Against strains R6981 and R6370, the DAP-AMP combination demonstrated enhancement against R6370 but not R6981, while the combinations of DAP-CPT and DAP-ERT were bactericidal, demonstrated enhancement, and were statistically superior to all other regimens at 96 h (P < 0.001) against both strains. CPT, ERT, and AMP similarly augmented LL37 killing against strain 8019. In strains R6981 and R6370, CPT and ERT aided LL37 more than AMP (P < 0.001). Compared to DAP alone, combination regimens provide better killing and prevent resistance. Clinical research involving DAP combinations is warranted.     

Comparative Activities of Clinafloxacin against Gram-Positive and -Negative Bacteria

Activities of clinafloxacin, ciprofloxacin, levofloxacin, sparfloxacin, trovafloxacin, piperacillin, piperacillin-tazobactam, trimethoprim-sulfamethoxazole, ceftazidime, and imipenem against 354 ciprofloxacin-susceptible and -intermediate-resistant organisms were tested by agar dilution. Clinafloxacin yielded the lowest quinolone MICs (≤0.5 μg/ml against ciprofloxacin-susceptible organisms and ≤16.0 μg/ml against ciprofloxacin-intermediate-resistant organisms) compared to those of levofloxacin, trovafloxacin, and sparfloxacin. Ceftazidime, piperacillin alone or combined with tazobactam, trimethoprim-sulfamethoxazole, and imipenem usually yielded higher MICs against ciprofloxacin-resistant strains.     

MYSTIC Europe 2007: activity of meropenem and other broad-spectrum agents against nosocomial isolates

The Meropenem Yearly Susceptibility Test Information Collection (MYSTIC) Program is a longitudinal antimicrobial surveillance study that has been in existence since 1997 in centers that are actively prescribing meropenem. This report examines the results from the study in Europe in 2007. A total of 5208 isolates were examined for activity (MIC) of meropenem and other broad-spectrum antibacterial comparators. Cumulative susceptibility rates using Clinical and Laboratory Standards Institute criteria against all methicillin-susceptible staphylococci were imipenem (97.7%) > meropenem (97.3%) > piperacillin/tazobactam (96.2%) > tobramycin (94.2%) > gentamicin (92.0%) > ciprofloxacin (84.0%) > ceftazidime (39.8%). Against all species of Enterobacteriaceae, the rates were meropenem (99.4%) > imipenem (98.3%) > tobramycin (92.0%) > gentamicin (89.5%) > ceftazidime (86.2%) > piperacillin/tazobactam (85.5%) > ciprofloxacin (84.2%). Meropenem was most effective against the nonfermenters, although multidrug-resistant Acinetobacter spp. and imipenem-resistant Pseudomonas aeruginosa strains were reported. The continued need for surveillance studies such as MYSTIC is exemplified, and results from these types of surveillance can, hopefully, help in the correct choice of empiric therapy.     

Evaluation of Several Dosing Regimens of Cefepime, with Various Simulations of Renal Function, against Clinical Isolates of Pseudomonas aeruginosa in a Pharmacodynamic Infection Model

The objectives of this study were as follows: (i) to examine the killing activity of 2-g doses of cefepime against two clinical isolates (mucoid and nonmucoid) of Pseudomonas aeruginosa in a pharmacodynamic in vitro infection model, (ii) to compare the percentage of time above the MIC (T > MIC) for each of the regimens against P. aeruginosa, and (iii) to evaluate the area under the bactericidal curve for each regimen. Cefepime was administered at intervals of 8, 12, and 24 h with and without tobramycin, and two different levels of renal function were simulated: normal (creatinine clearance [CL_CR] = 90 ml/min) and decreased (CR_CL = 60 ml/min). Also, the killing activity of cefepime with and without tobramycin was compared to the killing activity of ceftazidime (2 g every 8 h) with and without tobramycin. The T > MIC was 100% in the central chamber except for the regimen in which cefepime was administered every 12 h and the CL_CR was 90 ml/min, which provided concentrations above the MIC for 92% of the dosing interval against the C31 (mucoid; MIC of cefepime, 4 μg/ml) isolate and for 75% of the interval against the C34 (nonmucoid; MIC of cefepime, 8 μg/ml) isolate. All cefepime and ceftazidime monotherapy simulations resulted in 99.9% killing of the nonmucoid isolate within 4 to 8 h and within 4 to 6 h, respectively. Against the mucoid isolate, 99.9% killing was achieved only with combination therapy. The results of this study indicate that cefepime dosed at 2 g every 12 h under conditions of normal renal function and every 24 h with decreased creatinine clearance (60 ml/min) is effective both as monotherapy and in combination therapy against a nonmucoid strain of P. aeruginosa. With cefepime MICs of 4 and 8 μg/ml, the single-agent regimens provided T > MIC values in the central chamber for 92 and ≥75% of the dosing interval against the mucoid and nonmucoid isolates, respectively. Cefepime dosed at 2 g every 12 h, with a creatinine clearance of 90 ml/min, and every 24 h, with a creatinine clearance of 60 ml/min, resulted in killing activity equivalent to that of ceftazidime dosed at 2 g every 8 h. None of the monotherapies provided adequate killing of the mucoid strain of P. aeruginosa despite drug concentrations being above the MIC for ≥92% of all dosing intervals. Finally, combination therapy with tobramycin and either cefepime or ceftazidime enhanced the killing of both the mucoid and nonmucoid P. aeruginosa isolates.     

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.     

Outer Membrane Profiles of Clonally Related Klebsiella pneumoniae Isolates from Clinical Samples and Activities of Cephalosporins and Carbapenems

Fifteen isolates of Klebsiella pneumoniae producing extended-spectrum β-lactamases (ESBLs) isolated during a nosocomial outbreak were studied. The strains belonged to the same clonal type, as shown by pulsed-field gel electrophoretic analysis of chromosomal DNA. All the isolates were resistant to extended-spectrum cephalosporins, aztreonam, gentamicin, and fluoroquinolones and were susceptible to carbapenems, tobramycin, netilmicin, and amikacin. None of the isolates expressed the OmpK36 porin. Eight isolates, for which the MICs of cefoxitin were ≥64 μg/ml, showed a diminished level or no expression of a 35-kDa porin. The MICs of meropenem, cefotaxime, and cefpirome were three to eight times higher for porin-deficient isolates than for isolates expressing the 35-kDa porin, but the MICs of imipenem increased two times for porin-deficient isolates compared to those for isolates expressing the porin. This MIC increase reverted to a level similar to that for the parental strain when porin-deficient isolates were transformed with the gene coding for the K. pneumoniae porin OmpK36. It is concluded that the high level of resistance to cefoxitin and the increase in the MICs of meropenem, cefotaxime, and cefpirome for the ESBL-producing K. pneumoniae isolates studied are associated with porin deficiency.     

Comparative In Vitro Appraisal of Piperacillin, Including Its Activity Against Salmonella typhi

Piperacillin was evaluated in vitro against 711 clinical isolates of aerobic and anerobic gram-positive and gram-negative bacteria, including 76 isolates of Salmonella typhi. Piperacillin minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) were compared with those of a range of β-lactam, aminoglycoside, and other antimicrobial agents, and inoculum size effects were considered. The relationship between dilution and disk diffusion tests was studied by regression analysis. In addition, piperacillin was assessed in combination with aminoglycoside and other β-lactam drugs. This investigation has confirmed the activity of piperacillin against a broad range of bacteria, including Pseudomonas, Enterobacteriaceae, Neisseria, β-lactamase-negative Haemophilus influenzae, and Staphylococcus aureus as well as enterococci, Bacteroides fragilis, and other anaerobes. All strains of Pseudomonas aeruginosa were inhibited by ≤32 μg/ml or less, demonstrating again the potential usefulness of piperacillin in the treatment of pseudomonal infections. S. typhi proved susceptible to piperacillin, all isolates being inhibited by 1 μg/ml. Inoculum size experiments showed that inocula of 10⁸ CFU resulted in MICs and MBCs appreciably higher than those resulting from inocula of 10⁶ CFU, and inocula of 10² CFU resulted in MICs and MBCs appreciably lower than those resulting from inocula of 10⁴ CFU. Piperacillin was active against all gentamicin-resistant pseudomonads tested, but not against gentamicin-resistant klebsiellas and enterobacters. Combinations of piperacillin with tobramycin and amikacin were consistently synergistic against Pseudomonas and Serratia isolates. Less consistent results were shown when piperacillin was combined with aminoglycosides or cephalothin against Klebsiella and indole-positive Proteus isolates, although synergy was observed in most cases. Occasional antagonistic reactions were encountered with piperacillin-cephalothin or piperacillin-tobramycin combinations against the latter isolates.     

Antimicrobial Susceptibility Testing of 59 Strains of Campylobacter fetus subsp. fetus

The susceptibilities of 59 Campylobacter fetus subsp. fetus isolates to eight antibiotics were studied by the agar dilution, E-test, and disk diffusion methods. None of the isolates were β-lactamase producers. All were susceptible to ampicillin, gentamicin, imipenem, and meropenem as determined by the three methods, with MICs at which 90% of the isolates are inhibited (MIC₉₀s) (determined by agar dilution) of 2, 1, ≤0.06, and 0.12 μg/ml, respectively. Twenty-seven percent of the isolates were resistant to tetracycline, with complete agreement between the agar dilution and disk diffusion results. The MIC₉₀s determined by agar dilution were 2 μg/ml for erythromycin, 1 μg/ml for ciprofloxacin, and 8 μg/ml for cefotaxime.     

Susceptibilities of non-Pseudomonas aeruginosa gram-negative nonfermentative rods to ciprofloxacin, ofloxacin, levofloxacin, D-ofloxacin, sparfloxacin, ceftazidime, piperacillin, piperacillin-tazobactam, trimethoprim-sulfamethoxazole, and imipenem.

Agar dilution MICs of 10 agents against 410 non-Pseudomonas aeruginosa gram-negative nonfermentative rods were determined. MICs at which 50 and 90% of the isolates were inhibited, respectively, were as follows (in micrograms per milliliter): sparfloxacin, 0.5 and 8.0; levofloxacin, 1.0 and 8.0; ciprofloxacin, 2.0 and 32.0; ofloxacin, 2.0 and 32.0; D-ofloxacin, 32.0 and > 64.0; ceftazidime, 8.0 and 64.0; piperacillin with or without tazobactam, 16.0 and > 64.0; trimethoprim-sulfamethoxazole, 0.5 and > 64.0; imipenem, 2.0 and > 64.0. With the exception of those for Stenotrophomonas maltophilia, Burkholderia cepacia, and Alcaligenes faecalis-A. odorans, agar dilution MICs for all strains tested were within 1 dilution of inhibitory (bacteriostatic) levels as determined by time-kill methodology.