Optimal Piperacillin-Tazobactam Dosing Strategies against Extended-Spectrum-ß-Lactamase-Producing Enterobacteriaceae.

Piperacillin-tazobactam has been proposed as an alternative to carbapenems for the treatment of infections caused by extended-spectrum-ß-lactamase (ESBL)-producing Enterobacteriaceae However, limited understanding of optimal dosing strategies for this combination may curtail its utility. In this study, we correlated various exposures of piperacillin-tazobactam to efficacy, using a modified pharmacokinetic/pharmacodynamic index. Using a clinical Klebsiella pneumoniae isolate expressing CTX-M-15, piperacillin MIC values were determined with increasing tazobactam concentrations and fitted to a sigmoid inhibitory maximum effect (Emax) model. A hollow-fiber infection model (HFIM) was used to evaluate the efficacy of escalating tazobactam dosing with a fixed piperacillin exposure. Simulated drug concentrations from the HFIM were incorporated in the Emax model to determine the percentage of free time above instantaneous MIC (%fT>MICi) associated with each experimental exposure. The target %fT>MICi associated with growth suppression was prospectively validated using an SHV-12-producing isolate of Escherichia coli and 2 other CTX-M-15-producing K. pneumoniae isolates. Based on our reference isolate, piperacillin-tazobactam exposures of %fT>MICi of ≥55.1% were associated with growth suppression. Despite underlying differences, these findings were consistent with prospective observations in 3 other clinical isolates. Our modeling approach can be applied relatively easily in the clinical setting, and it appeared to be robust in predicting the effectiveness of various piperacillin-tazobactam exposures. This modified pharmacokinetic/pharmacodynamic index could be used to characterize response to other ß-lactam/ß-lactamase inhibitor combinations.

Pharmacokinetics and Pharmacodynamics of Minocycline against Acinetobacter baumannii in a Neutropenic Murine Pneumonia Model.

Multidrug-resistant (MDR) Acinetobacter baumannii is increasingly more prevalent in nosocomial infections. Although in vitro susceptibility of A. baumannii to minocycline is promising, the in vivo efficacy of minocycline has not been well established. In this study, the in vivo activity of minocycline was evaluated in a neutropenic murine pneumonia model. Specifically, we investigated the relationship between minocycline exposure and bactericidal activity using five A. baumannii isolates with a broad range of susceptibility (MIC ranged from 0.25 mg/liter to 16 mg/liter). The pharmacokinetics of minocycline (single dose of 25 mg/kg of body weight, 50 mg/kg, 100 mg/kg, and a humanized regimen, given intraperitoneally) in serum and epithelial lining fluid (ELF) were characterized. Dose linearity was observed for doses up to 50 mg/kg and pulmonary penetration ratios (area under the concentration-time curve in ELF from 0 to 24 h [AUCELF,0-24]/area under the concentration time curve in serum from 0 to 24 h [AUCserum,0-24]) ranged from 2.5 to 2.8. Pharmacokinetic-pharmacodynamics (PK-PD) index values in ELF for various dose regimens against different A. baumannii isolates were calculated. The maximum efficacy at 24 h was approximately 1.5-log-unit reduction of pulmonary bacterial burdens from baseline. The AUC/MIC ratio was the PK-PD index most closely correlating to the bacterial burden (r2 = 0.81). The required AUCELF,0-24/MIC for maintaining stasis and achieving 1-log-unit reduction were 140 and 410, respectively. These findings could guide the treatment of infections caused by A. baumannii using minocycline in the future. Additional studies to examine resistance development during therapy are warranted.

Determining ß-lactam exposure threshold to suppress resistance development in Gram-negative bacteria.

Objectives: ß-Lactams are commonly used for nosocomial infections and resistance to these agents among Gram-negative bacteria is increasing rapidly. Optimized dosing is expected to reduce the likelihood of resistance development during antimicrobial therapy, but the target for clinical dose adjustment is not well established. We examined the likelihood that various dosing exposures would suppress resistance development in an in vitro hollow-fibre infection model. Methods: Two strains of Klebsiella pneumoniae and two strains of Pseudomonas aeruginosa (baseline inocula of ∼10 8  cfu/mL) were examined. Various dosing exposures of cefepime, ceftazidime and meropenem were simulated in the hollow-fibre infection model. Serial samples were obtained to ascertain the pharmacokinetic simulations and viable bacterial burden for up to 120 h. Drug concentrations were determined by a validated LC-MS/MS assay and the simulated exposures were expressed as C min /MIC ratios. Resistance development was detected by quantitative culture on drug-supplemented media plates (at 3× the corresponding baseline MIC). The C min /MIC breakpoint threshold to prevent bacterial regrowth was identified by classification and regression tree (CART) analysis. Results: For all strains, the bacterial burden declined initially with the simulated exposures, but regrowth was observed in 9 out of 31 experiments. CART analysis revealed that a C min /MIC ratio ≥3.8 was significantly associated with regrowth prevention (100% versus 44%, P = 0.001). Conclusions: The development of ß-lactam resistance during therapy could be suppressed by an optimized dosing exposure. Validation of the proposed target in a well-designed clinical study is warranted.

Analytical and functional determination of polymyxin B protein binding in serum.

To enhance our understanding of the pharmacological properties of polymyxin B, serum protein binding for polymyxin B1, B2, and B3 and for isoleucine-polymyxin B1 was evaluated. Using equilibrium dialysis and ultrafiltration, comparable protein binding was found in all 4 components of polymyxin B (92% to 99%). Protein binding in human serum was further assessed using a functional assay, the results of which were in general agreement with previous findings (approximately 90%).

Uptake of polymyxin B into renal cells.

Polymyxin B is increasingly used as a treatment of last resort against multidrug-resistant Gram-negative infections. Using a mammalian kidney cell line, we demonstrated that polymyxin B uptake into proximal tubular epithelial cells was saturable and occurred primarily through the apical membrane, suggesting the involvement of transporters in the renal uptake of polymyxin B. Megalin might play a role in the uptake and accumulation of polymyxin B into renal cells.

An evaluation of multiple phenotypic screening methods for Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae.

Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae may display MICs to carbapenems within susceptible or intermediate ranges, prompting confirmatory testing. Four phenotypic methods to detect KPC producers were evaluated against a collection of clinical Enterobacteriaceae isolates. Meropenem-phenylboronic acid double disk synergy testing demonstrated the best performance with 100% sensitivity and specificity.

Assessment of antimicrobial combinations for Klebsiella pneumoniae carbapenemase-producing K. pneumoniae.

BACKGROUND: The prevalence of bla(KPC) among gram-negative bacteria continues to increase worldwide. Limited treatment options exist for this multidrug-resistant phenotype, often necessitating combination therapy. We investigated the in vitro and in vivo efficacy of multiple antimicrobial combinations. METHODS: Two clinical strains of Klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae were studied. The killing activities of six 2-agent combinations of amikacin, doripenem, levofloxacin, and rifampin were quantitatively assessed using a validated mathematical model. Combination time-kill studies were conducted using clinically relevant concentrations; observed bacterial burdens were modeled using 3-dimensional response surfaces. Selected combinations were further validated in a neutropenic murine pneumonia model, using human-like dosing exposures. RESULTS: The most enhanced killing effect in time-kill studies was seen with amikacin plus doripenem. Compared with placebo controls, this combination resulted in significant reduction of the bacterial burden in tissue at 24 hours, along with prolonged animal survival. In contrast, amikacin plus levofloxacin was found to be antagonistic in time-kill studies, showing inferior animal survival, as predicted. CONCLUSIONS: Our modeling approach appeared to be robust in assessing the effectiveness of various combinations for KPC-producing isolates. Amikacin plus doripenem was the most effective combination in both in vitro and in vivo infection models. Empirical selection of combinations against KPCs may result in antagonism and should be avoided.

In vitro pharmacodynamics of AZD5206 against Staphylococcus aureus.

AZD5206 is a novel antimicrobial agent with potent in vitro activity against Staphylococcus aureus. We evaluated the in vitro pharmacodynamics of AZD5206 against a standard wild-type methicillin-susceptible strain (ATCC 29213) and a clinical strain of methicillin-resistant S. aureus (SA62). Overall, bacterial killing against a low baseline inoculum was more remarkable. Low dosing exposures and/or high baseline inoculum resulted in early reduction in bacterial burden, followed by regrowth and selective amplification of the resistant population.

Novel modeling framework to guide design of optimal dosing strategies for ß-lactamase inhibitors.

The scarcity of new antibiotics against drug-resistant bacteria has led to the development of inhibitors targeting specific resistance mechanisms, which aim to restore the effectiveness of existing agents. However, there are few guidelines for the optimal dosing of inhibitors. Extending the utility of mathematical modeling, which has been used as a decision support tool for antibiotic dosing regimen design, we developed a novel mathematical modeling framework to guide optimal dosing strategies for a beta-lactamase inhibitor. To illustrate our approach, MK-7655 was used in combination with imipenem against a clinical isolate of Klebsiella pneumoniae known to produce KPC-2. A theoretical concept capturing fluctuating susceptibility over time was used to define a novel pharmacodynamic index (time above instantaneous MIC [T>MIC(i)]). The MK-7655 concentration-dependent MIC reduction was characterized by using a modified sigmoid maximum effect (E(max))-type model. Various dosing regimens of MK-7655 were simulated to achieve escalating T>MIC(i) values in the presence of a clinical dose of imipenem (500 mg every 6 h). The effectiveness of these dosing exposures was subsequently validated by using a hollow-fiber infection model (HFIM). An apparent trend in the bacterial response was observed in the HFIM with increasing T>MIC(i) values. In addition, different dosing regimens of MK-7655 achieving a similar T>MIC(i) (69%) resulted in comparable bacterial killing over 48 h. The proposed framework was reasonable in predicting the in vitro activity of a novel beta-lactamase inhibitor, and its utility warrants further investigations.

In vitro activity of MK-7655, a novel ß-lactamase inhibitor, in combination with imipenem against carbapenem-resistant Gram-negative bacteria.

Carbapenem-resistant bacteria represent a significant treatment challenge due to the lack of active antimicrobials available. MK-7655 is a novel ß-lactamase inhibitor under clinical development. We investigated the combined killing activity of imipenem and MK-7655 against four imipenem-resistant bacterial strains, using a mathematical model previously evaluated in our laboratory. Time-kill studies (TKS) were conducted with imipenem and MK-7655 against a KPC-2-producing Klebsiella pneumoniae isolate (KP6339) as well as 3 Pseudomonas aeruginosa isolates (PA24226, PA24227, and PA24228) with OprD porin deletions and overexpression of AmpC. TKS were performed using 25 clinically achievable concentration combinations in a 5-by-5 array. Bacterial burden at 24 h was determined in triplicate by quantitative culture and mathematically modeled using a three-dimensional response surface. Mathematical model assessments were evaluated experimentally using clinically relevant dosing regimens of imipenem, with or without MK-7655, in a hollow-fiber infection model (HFIM). The combination of imipenem and MK-7655 was synergistic for all strains. Interaction indices were as follows: for KP6339, 0.50 (95% confidence interval [CI], 0.42 to 0.58); for PA24226, 0.60 (95% CI, 0.58 to 0.62); for PA24227, 0.70 (95% CI, 0.66 to 0.74); and for PA24228, 0.55 (95% CI, 0.49 to 0.61). In the HFIM, imipenem plus MK-7655 considerably reduced the bacterial burden at 24 h, while failure with imipenem alone was seen against all isolates. Sustained suppression of bacterial growth at 72 h was achieved with simulated doses of 500 mg imipenem plus 500 mg MK-7655 in 2 (KP6339 and PA24227) strains, and it was achieved in an additional strain (PA24228) when the imipenem dose was increased to 1,000 mg. Additional studies are being conducted to determine the optimal dose and combinations to be used in clinical investigations.

Quantitative impact of neutrophils on bacterial clearance in a murine pneumonia model.

The rapid increase in the prevalence of antibiotic-resistant pathogens is a global problem that has challenged our ability to treat serious infections. Currently, clinical decisions on treatment are often based on in vitro susceptibility data. The role of the immune system in combating bacterial infections is unequivocal, but it is not well captured quantitatively. In this study, the impact of neutrophils on bacterial clearance was quantitatively assessed in a murine pneumonia model. In vitro time-growth studies were performed to determine the growth rate constants of Acinetobacter baumannii ATCC BAA 747 and Pseudomonas aeruginosa PAO1. The absolute neutrophil count in mice resulting from different cyclophosphamide preparatory regimens was determined. The dynamic change of bacterial (A. baumannii BAA 747) burden in mice with graded immunosuppression over 24 h was captured by a mathematical model. The fit to the data was satisfactory (r(2) = 0.945). The best-fit maximal kill rate (K(k)) of the bacterial population by neutrophils was 1.743 h(-1), the number of neutrophils necessary for 50% maximal killing was 190.8/µl, and the maximal population size was 1.8 × 10(9) CFU/g, respectively. Using these model parameter estimates, the model predictions were subsequently validated by the bacterial burden change of P. aeruginosa PAO1 at 24 h. A simple mathematical model was proposed to quantify the contribution of neutrophils to bacterial clearance and predict the bacterial growth/suppression in animals. Our results provide a novel framework to link in vitro and in vivo information and may be used to improve clinical treatment of bacterial infections.

Modelling biphasic killing of fluoroquinolones: guiding optimal dosing regimen design.

OBJECTIVES: Fluoroquinolones are commonly believed to exhibit concentration-dependent killing, but time-kill studies have revealed that fluoroquinolone activity could be a complex combination of concentration-dependent and -independent killing. We had previously developed a mathematical modelling framework to describe the dynamics of bacterial populations under the effect of antimicrobials, which could facilitate the design of optimal dosing regimens. Our objective was to extend the framework to describe the effect of fluoroquinolones on heterogeneous populations of Escherichia coli and Staphylococcus aureus. METHODS: A mathematical model was fitted to time-kill data of moxifloxacin (0-128× MIC; MIC = 0.0625 mg/L) against E. coli MG1655 and levofloxacin (0-64× MIC; MIC = 0.25 mg/L) against S. aureus ATCC 29213 over 24 h. Based on the best-fit model parameters, the likelihood of resistance development associated with various dosing regimens was predicted. Subsequently, in vitro studies with a hollow-fibre infection model were selectively performed to validate the mathematical model predictions, using simulated human half-lives (moxifloxacin = 12 h; levofloxacin = 5-7 h). RESULTS: Bacterial regrowth and resistance development were observed with suboptimal dosing regimens. Parallel time-growth studies substantiated the modelling assumption that there was no significant biofitness cost in resistant mutants. The mechanism of fluoroquinolone resistance was confirmed by PCR. CONCLUSIONS: Our model was found to be reasonable in characterizing biphasic killing of fluoroquinolones and predicting dosing regimens to suppress resistance development. Our work demonstrated improvements resulting from using the proposed mathematical modelling as a decision support tool for guiding the design of dosing regimens.

Impact of recA on levofloxacin exposure-related resistance development.

Genetic mutations are one of the major mechanisms by which bacteria acquire drug resistance. One of the known mechanisms for inducing mutations is the SOS response system. We investigated the effect of disrupting recA, an inducer of the SOS response, on resistance development using an in vitro hollow-fiber infection model. A clinical Staphylococcus aureus isolate and a laboratory wild-type strain of Escherichia coli were compared to their respective recA-deleted isogenic daughter isolates. Approximately 2 × 10(5) CFU/ml of bacteria were subjected to escalating levofloxacin exposures for up to 120 h. Serial samples were obtained to ascertain simulated drug exposures and total and resistant bacterial burdens. Quinolone resistance determining regions of gyrA and grlA (parC for E. coli) in levofloxacin-resistant isolates were sequenced to confirm the mechanism of resistance. The preexposure MICs of the recA-deleted isolates were 4-fold lower than those of their respective parents. In S. aureus, a lower area under the concentration-time curve over 24 h at steady state divided by the MIC (AUC/MIC) was required to suppress resistance development in the recA-deleted mutant (an AUC/MIC of >23 versus an AUC/MIC of >32 was necessary in the mutant versus the parent isolate, respectively), and a prominent difference in the total bacterial burden was observed at 72 h. Using an AUC/MIC of approximately 30, E. coli resistance emergence was delayed by 24 h in the recA-deleted mutant. Diverse mutations in gyrA were found in levofloxacin-resistant isolates recovered. Disruption of recA provided additional benefits apart from MIC reduction, attesting to its potential role for pharmacologic intervention. The clinical relevance of our findings warrants further investigations.

Impact of multidrug-resistant Pseudomonas aeruginosa bacteremia on patient outcomes.

Trends of rising rates of resistance in Pseudomonas aeruginosa make selection of appropriate empirical therapy increasingly difficult, but whether multidrug-resistant (MDR) P. aeruginosa is associated with worse clinical outcomes is not well established. The objective of this study was to determine the impact of MDR (resistance to three or more classes of antipseudomonal agents) P. aeruginosa bacteremia on patient outcomes. We performed a retrospective cohort study of adult patients with P. aeruginosa bacteremia from 2005 to 2008. Patients were identified by the microbiology laboratory database, and pertinent clinical data were collected. Logistic regression was used to explore independent risk factors for 30-day mortality. Classification and regression tree analysis was used to determine threshold breakpoints for continuous variables. Kaplan-Meier survival analysis was used to compare time to mortality, after normalization of the patients' underlying risks by propensity scoring. A total of 109 bacteremia episodes were identified; 25 episodes (22.9%) were caused by MDR P. aeruginosa. Patients with MDR P. aeruginosa bacteremia were more likely to receive inappropriate empirical therapy (44.0% and 6.0%, respectively; P < 0.001) and had longer prior hospital stays (32.6 +/- 37.3 and 14.4 +/- 43.6 days, respectively; P = 0.046). Multivariate regression revealed that 30-day mortality was associated with multidrug resistance (odds ratio [OR], 6.8; 95% confidence interval [CI], 1.9 to 24.0), immunosuppression (OR, 5.0; 95% CI, 1.4 to 17.5), and an APACHE II score of > or = 22 (OR, 29.0; 95% CI, 5.0 to 168.2). Time to mortality was also shorter in the MDR cohort (P = 0.011). Multidrug resistance is a significant risk factor for 30-day mortality in patients with P. aeruginosa bacteremia; efforts to curb the spread of MDR P. aeruginosa could be beneficial.

Mathematical modeling to characterize the inoculum effect.

Killing by beta-lactams is well known to be reduced against a dense bacterial population, commonly known as the inoculum effect. However, the underlying mechanism of this phenomenon is not well understood. We proposed a semi-mechanistic mathematical model to account for the reduced in vitro killing observed. Time-kill studies were performed with 4 baseline inocula (ranging from approximately 1 × 10(5) to 1 × 10(8) CFU/ml) of Escherichia coli ATCC 25922 (MIC, 2 mg/liter). Constant but escalating piperacillin concentrations used ranged from 0.25× to 256× MIC. Serial samples were taken over 24 h to quantify viable bacterial burden, and all the killing profiles were mathematically modeled. The inoculum effect was attributed to a reduction of effective drug concentration available for bacterial killing, which was expressed as a function of the baseline inoculum. Biomasses associated with different inocula were examined using a colorimetric method. Despite identical drug-pathogen combinations, the baseline inoculum had a significant impact on bacterial killing. Our proposed mathematical model was unbiased and reasonable in capturing all 28 killing profiles collectively (r(2) = 0.88). Biomass was found to be significantly more after 24 h with a baseline inoculum of 1 × 10(8) CFU/ml, compared to one where the initial inoculum was 1 × 10(5) CFU/ml (P = 0.002). Our results corroborated previous observations that in vitro killing by piperacillin was significantly reduced against a dense bacterial inoculum. This phenomenon can be reasonably captured by our proposed mathematical model, and it may improve prediction of bacterial response to various drug exposures in future investigations.

Prevalence, resistance mechanisms, and susceptibility of multidrug-resistant bloodstream isolates of Pseudomonas aeruginosa.

Pseudomonas aeruginosa is an important pathogen commonly implicated in nosocomial infections. The occurrence of multidrug-resistant (MDR) P. aeruginosa strains is increasing worldwide and limiting our therapeutic options. The MDR phenotype can be mediated by a variety of resistance mechanisms, and the corresponding relative biofitness is not well established. We examined the prevalence, resistance mechanisms, and susceptibility of MDR P. aeruginosa isolates (resistant to > or =3 classes of antipseudomonal agents [penicillins/cephalosporins, carbapenems, quinolones, and aminoglycosides]) obtained from a large, university-affiliated hospital. Among 235 nonrepeat bloodstream isolates screened between 2005 and 2007, 33 isolates (from 20 unique patients) were found to be MDR (crude prevalence rate, 14%). All isolates were resistant to carbapenems and quinolones, 91% were resistant to penicillins/cephalosporins, and 21% were resistant to the aminoglycosides. By using the first available isolate for each bacteremia episode (n = 18), 13 distinct clones were revealed by repetitive-element-based PCR. Western blotting revealed eight isolates (44%) to have MexB overexpression. Production of a carbapenemase (VIM-2) was found in one isolate, and mutations in gyrA (T83I) and parC (S87L) were commonly found. Growth rates of most MDR isolates were similar to that of the wild type, and two isolates (11%) were found to be hypermutable. All available isolates were susceptible to polymyxin B, and only one isolate was nonsusceptible to colistin (MIC, 3 mg/liter), but all isolates were nonsusceptible to doripenem (MIC, >2 mg/liter). Understanding and continuous monitoring of the prevalence and resistance mechanisms of MDR P. aeruginosa would enable us to formulate rational treatment strategies to combat nosocomial infections.

Pharmacodynamics of moxifloxacin against a high inoculum of Escherichia coli in an in vitro infection model.

OBJECTIVES: Escherichia coli is the leading bacterial species implicated in intra-abdominal infections. In these infections a high bacterial burden with pre-existing resistant mutants are likely to be encountered and resistance could be amplified with suboptimal dosing. Our objective was to investigate the pharmacodynamics of moxifloxacin against a high inoculum of E. coli using an in vitro hollow fibre infection model (HFIM). METHODS: Three wild-type strains of E. coli (ATCC 25922, MG1655 and EC28044) were studied by exposing approximately 2 x 10(8) cfu/mL (20 mL) to escalating dosing regimens of moxifloxacin (ranging from 30 to 400 mg, once daily). Serial samples were obtained from HFIM over 120 h to enumerate the total and resistant subpopulation. Quinolone resistance-determining regions of gyrA and parC of resistant isolates were sequenced to confirm the mechanism of resistance. RESULTS: The pre-exposure MIC of the three wild-type strains was 0.0625 mg/L. Simulated moxifloxacin concentration profiles in HFIM were satisfactory (r(2) >or= 0.94). Placebo experiments revealed natural mutants, but no resistance amplification. Regrowth and resistance amplification was observed between 30 mg/day (AUC/MIC = 47) and 80 mg/day dose (AUC/MIC = 117). Sustained bacterial suppression was achieved at >or=120 mg/day dose (AUC/MIC = 180). Point mutations in gyrA (D87G or S83L) were detected in resistant isolates. CONCLUSIONS: Our results suggest that suboptimal dosing may facilitate resistance amplification in a high inoculum of E. coli. The clinical dose of moxifloxacin (400 mg/day) was adequate to suppress resistance development in three wild-type strains. Clinical relevance of these findings warrants further in vivo investigation.

Quantitative assessment of combination antimicrobial therapy against multidrug-resistant Acinetobacter baumannii.

Treatment of multidrug-resistant bacterial infections poses a therapeutic challenge to clinicians; combination therapy is often the only viable option for multidrug-resistant infections. A quantitative method was developed to assess the combined killing abilities of antimicrobial agents. Time-kill studies (TKS) were performed using a multidrug-resistant clinical isolate of Acinetobacter baumannii with escalating concentrations of cefepime (0 to 512 mg/liter), amikacin (0 to 256 mg/liter), and levofloxacin (0 to 64 mg/liter). The bacterial burden data in single and combined (two of the three agents with clinically achievable concentrations in serum) TKS at 24 h were mathematically modeled to provide an objective basis for comparing various antimicrobial agent combinations. Synergy and antagonism were defined as interaction indices of <1 and >1, respectively. A hollow-fiber infection model (HFIM) simulating various clinical (fluctuating concentrations over time) dosing exposures was used to selectively validate our quantitative assessment of the combined killing effect. Model fits in all single-agent TKS were satisfactory (r(2) > 0.97). An enhanced combined overall killing effect was seen in the cefepime-amikacin combination (interactive index, 0.698; 95% confidence interval [CI], 0.675 to 0.722) and the cefepime-levofloxacin combination (interactive index, 0.929; 95% CI, 0.903 to 0.956), but no significant difference in the combined overall killing effect for the levofloxacin-amikacin combination was observed (interactive index, 0.994; 95% CI, 0.982 to 1.005). These assessments were consistent with observations in HFIM validation studies. Our method could be used to objectively rank the combined killing activities of two antimicrobial agents when used together against a multidrug-resistant A. baumannii isolate. It may offer better insights into the effectiveness of various antimicrobial combinations and warrants further investigations.

Prevalence of extended-spectrum beta-lactamase and carbapenemase-producing bloodstream isolates of Klebsiella pneumoniae in a tertiary care hospital.

To improve prescribing of empiric therapy, the local molecular epidemiology of extended-spectrum beta-lactamases (ESBLs) and Klebsiella pneumoniae carbapenemases (KPCs) in bloodstream isolates of K. pneumoniae were evaluated. Isolates resistant to third generation cephalosporins were screened phenotypically for ESBLs and carbapenemases, and subsequently confirmed by PCR for the presence of ESBL (blaTEM, blaSHV and blaCTX-M) and carbapenemase (blaKPC, blaVIM, blaNDM and blaOXA-48) genes. Hydrolytic activity (functional gene expression) was quantified using a nitrocefin degradation assay and correlated to ceftazidime or meropenem MIC. Clonality was assessed by repetitive element-based PCR. Beta-lactamases were functionally expressed in 13 isolates (15.5%); 7 (53.8%) harboured blaCTX-M-15 and 6 (46.2%) carried the blaKPC-2 gene. Correlation of hydrolytic activity to MIC yielded a coefficient of 98% for isolates expressing ESBLs alone and 56% for carbapenemase producers. Four unique ESBL-expressing clones and five carbapenem-resistant clones were identified. All 13 resistant isolates were susceptible to ceftazidime/avibactam (MIC ≤ 8/4 mg/L).

Prevalence, mechanisms, and risk factors of carbapenem resistance in bloodstream isolates of Pseudomonas aeruginosa.

We examined the prevalence of various carbapenem resistance mechanisms in Pseudomonas aeruginosa bloodstream isolates from a university-affiliated hospital. Isolates obtained in 2003 and 2004 were screened for meropenem/imipenem resistance, and clonality was assessed by repetitive-element-based polymerase chain reaction. The presence of carbapenemase and AmpC overexpression was ascertained by spectrophotometric assays. Outer membrane protein profiles were examined by sodium dodecyl sulfate polyacrylamide gel electrophoresis, and efflux pump overexpression was confirmed by Western blotting. We examined 129 nonrepeat isolates; 21 isolates (from 13 distinct clones) were resistant to meropenem or imipenem (prevalence rate = 16.3%). Nineteen (90.5%) carbapenem-resistant isolates had reduced OprD expression, and 6 (28.6%) isolates had overexpression of MexB. Increased length of hospital stay was identified as a significant risk factor for bacteremia due to carbapenem-resistant P. aeruginosa. Understanding the prevalence and mechanism of carbapenem resistance in P. aeruginosa may guide empiric therapy for nosocomial infections in our hospital.