Breakpoints: current practice and future perspectives

Evolution of the proportion of colistin-resistant isolates in animal clinical Escherichia coli over time - A hierarchical mixture model approach

Colistin resistance has been the subject of much attention since mcr genes encoding plasmid-mediated colistin resistance description in 2015. To date, surveillance data about resistance levels encountered in food-producing animals are scarce. In France, the Resapath dataset, consisting in a large collection of disk diffusion antibiogram results transmitted by a network of laboratories. It offers a unique opportunity to study the evolution of resistance towards colistin over the past 15 years in Escherichia coli isolated from diseased food-producing animals. This study used a Bayesian hierarchical Gaussian mixture model to estimate the resistant proportions from those data. This non-classical approach deals with the colistin-specific problem of overlapping distributions of diameters measured for susceptible and resistant isolates that makes the definition of epidemiological cut-off very hard. This model also considers the variability observed between the measurements performed by different laboratories. Proportion of resistant isolates has been calculated for several food-producing animals and most encountered diseases. From those estimations, a marked evolution of the proportions of resistant isolates is noticeable, for swine suffering from digestive disorders. In this group, an increase over the 2006-2011 period from 0.1% [ 0.0%, 1.2%] in 2006-28.6% [25.1%, 32.3%] in 2011 was followed by a decrease to reach 3.6% [2.3%;5.3%] in 2018. For isolates related to digestive disorders in calves, percentages increased and reached 7% in 2009 then decreased as for swine. In contrast, for poultry productions, estimated proportions and credibility intervals were constantly very close to zero.

Prudent Use of Tylosin for Treatment of Mycoplasma gallisepticum Based on Its Clinical Breakpoint and Lung Microbiota Shift

The aim of this study was to explore the prudent use of tylosin for the treatment of chronic respiratory infectious diseases in chickens caused by Mycoplasma gallisepticum (MG) based on its clinical breakpoint (CBP) and its effect on lung microbiota. The CBP was established based on the wild-type/epidemiological cutoff value (CO_WT/ECV), pharmacokinetics-pharmacodynamics (PK-PD) cutoff value (CO_PD), and clinical cutoff value (CO_CL) of tylosin against MG. The minimum inhibitory concentration (MIC) of tylosin against 111 MG isolates was analyzed and the CO_WT was 2 μg/ml. M17 with MIC of 2 μg/ml was selected as a representative strain for the PK-PD study. The CO_PD of tylosin against MG was 1 μg/ml. The dosage regimen formulated by the PK-PD study was 3 days administration of tylosin at a dose of 45.88 mg/kg b.w. with a 24-h interval. Five different MIC MGs were selected for clinical trial, and the CO_CL of tylosin against MG was 0.5 μg/ml. According to the CLSI decision tree, the CBP of tylosin against MG was set up as 2 μg/ml. The effect of tylosin on lung microbiota of MG-infected chickens was analyzed by 16S rRNA gene sequencing. Significant change of the lung microbiota was observed in the infection group and treatment group based on the principal coordinate analysis and the Venn diagrams of the core and unique OTU. The phyla Firmicutes and Proteobacteria showed difference after MG infection and treatment. This study established the CBP of tylosin against MG. It also provided scientific data for the prudent use of tylosin based on the evaluation of MG infection and tylosin treatment on the lung microbiota.

A Bayesian latent class mixture model with censoring for correlation analysis in antimicrobial resistance across populations

Background

The emergence of antimicrobial resistance across populations is a global threat to public health. Surveillance programs often monitor human and animal populations to evaluate trends of emergence in these populations. Many national level antibiotic resistance surveillance programs quantify the proportion of resistant bacteria as a means of monitoring emergence and control measures. The reason for monitoring these different populations are many, including interest in similar changes in resistance which might provide insight into emergence and control options.

Methods

In this research, we developed a method to quantify the correlation in antimicrobial resistance across populations, for the conventionally unnoticed mean shift of the susceptible bacteria. With the proposed Bayesian latent class mixture model with censoring and multivariate normal hierarchy, we address several challenges associated with analyzing the minimum inhibitory concentration data.

Results

Application of this approach to the surveillance data from National Antimicrobial Resistance Monitoring System led to a detection of positive correlation in the central tendency of azithromycin resistance of the susceptible populations from Salmonella serotype Typhimurium across food animal and human populations.

Conclusions

Our proposed approach has been shown to be accurate and superior to the commonly used naïve estimation by simulation studies. Further implementation of this Bayesian model could serve as a useful tool to indicate the co-existence of antimicrobial resistance, and potentially a need of clinical intervention.

Exploration of Clinical Breakpoint of Danofloxacin for Glaesserella parasuis in Plasma and in PELF

Background: In order to establish the clinical breakpoint (CBP) of danofloxacin against G. parasuis, three cutoff values, including epidemiological cutoff value (ECV), pharmacokinetic-pharmacodynamic (PK-PD) cutoff value (CO_PD) and clinical cutoff value (CO_CL), were obtained in the present study. Methods: The ECV was calculated using ECOFFinder base on the MIC distribution of danfloxacin against 347 G. parasuis collected from disease pigs. The CO_PD was established based on in vivo and ex vivo PK-PD modeling of danofloxacin both in plasma and pulmonary epithelial lining fluid (PELF) using Hill formula and Monte Carlo analysis. The CO_CL was established based on the relationship between the possibility of cure (POC) and MIC in the clinical trials using the "WindoW" approach, nonlinear regression and CART analysis. Results: The MIC₅₀ and MIC₉₀ of danofloxacin against 347 G. parasuis were 2 μg/mL and 8 μg/mL, respectively. The ECV value was set to 8 μg/mL using ECOFFinder. Concentration-time curves of danofloxacin were fitted with a two-compartment PK model. The PK parameters of the maximum concentration (C_max) and area under concentration-time curves (AUC) in PELF were 3.67 ± 0.25 μg/mL and 24.28 ± 2.70 h·μg/mL, higher than those in plasma (0.67 ± 0.01 μg/mL and 4.47 ± 0.51 h·μg/mL). The peak time (T_max) in plasma was 0.23 ± 0.07 h, shorter than that in PELF (1.61 ± 0.15 h). The CO_PD in plasma and PELF were 0.125 μg/mL and 0.5 μg/mL, respectively. The CO_CL calculated by WindoW approach, nonlinear regression and CART analysis were 0.125-4 μg/mL, 0.428 μg/mL and 0.56 μg/mL, respectively. The 0.5 μg/mL was selected as eligible CO_CL. The ECV is much higher than the CO_PD and CO_CL, and the clinical breakpoint based on data in plasma was largely different from that of PELF. Conclusions: Our study firstly established three cutoff values of danofloxacin against G. parasuis. It suggested that non-wild-type danofloxacin-resistant G. parasuis may lead to ineffective treatment by danofloxacin.

Early detection of drug-resistant Streptococcus pneumoniae and Haemophilus influenzae by quantitative flow cytometry

Early detection of drug resistance contributes to combating drug-resistant bacteria and improving patient outcomes. Microbial testing in the laboratory is essential for treating infectious diseases because it can provide critical information related to identifying pathogenic bacteria and their resistance profiles. Despite these clinical requirements, conventional phenotypic testing is time-consuming. Additionally, recent rapid drug resistance tests are not compatible with fastidious bacteria such as Streptococcus and Haemophilus species. In this study, we validated the feasibility of direct bacteria counting using highly sensitive quantitative flow cytometry. Furthermore, by combining flow cytometry and a nucleic acid intercalator, we constructed a highly sensitive method for counting viable fastidious bacteria. These are inherently difficult to measure due to interfering substances from nutrients contained in the medium. Based on the conventional broth microdilution method, our method acquired a few microliter samples in a time series from the same microplate well to exclude the growth curve inconsistency between the samples. Fluorescent staining and flow cytometry measurements were completed within 10 min. Therefore, this approach enabled us to determine antimicrobial resistance for these bacteria within a few hours. Highly sensitive quantitative flow cytometry presents a novel avenue for conducting rapid antimicrobial susceptibility tests.

A hierarchical Bayesian latent class mixture model with censorship for detection of linear temporal changes in antibiotic resistance

Identifying and controlling the emergence of antimicrobial resistance (AMR) is a high priority for researchers and public health officials. One critical component of this control effort is timely detection of emerging or increasing resistance using surveillance programs. Currently, detection of temporal changes in AMR relies mainly on analysis of the proportion of resistant isolates based on the dichotomization of minimum inhibitory concentration (MIC) values. In our work, we developed a hierarchical Bayesian latent class mixture model that incorporates a linear trend for the mean log2MIC of the non-resistant population. By introducing latent variables, our model addressed the challenges associated with the AMR MIC values, compensating for the censored nature of the MIC observations as well as the mixed components indicated by the censored MIC distributions. Inclusion of linear regression with time as a covariate in the hierarchical structure allowed modelling of the linear creep of the mean log2MIC in the non-resistant population. The hierarchical Bayesian model was accurate and robust as assessed in simulation studies. The proposed approach was illustrated using Salmonella enterica I,4,[5],12:i:- treated with chloramphenicol and ceftiofur in human and veterinary samples, revealing some significant linearly increasing patterns from the applications. Implementation of our approach to the analysis of an AMR MIC dataset would provide surveillance programs with a more complete picture of the changes in AMR over years by exploring the patterns of the mean resistance level in the non-resistant population. Our model could therefore serve as a timely indicator of a need for antibiotic intervention before an outbreak of resistance, highlighting the relevance of this work for public health. Currently, however, due to extreme right censoring on the MIC data, this approach has limited utility for tracking changes in the resistant population.

Pharmacokinetics and Dosing of Ceftobiprole Medocaril for the Treatment of Hospital- and Community-Acquired Pneumonia in Different Patient Populations

Hospital-acquired pneumonia (HAP) and community-acquired pneumonia (CAP) are among the most common infections treated in the hospital setting, and together they place a significant burden on healthcare systems. Successful management of HAP and CAP depends on rapid initiation of empirical antibiotic therapy with broad-spectrum antibiotics. Ceftobiprole is a new-generation, broad-spectrum cephalosporin antibiotic for the treatment of HAP (excluding ventilator-associated pneumonia) and CAP. It displays potent in vitro activity against a broad range of pathogens important in pneumonia. This review summarizes the pharmacokinetic profile of ceftobiprole, and considers the pharmacokinetic parameters and pharmacodynamics underlying the choice of dosing regimen. Ceftobiprole shows linear pharmacokinetics after single and multiple doses and is eliminated predominantly through the kidneys. Ceftobiprole is administered as a 500 mg intravenous infusion over 2 h every 8 h, and steady-state concentrations are reached on the first day of dosing. Dose adjustment is recommended for patients with moderate or severe renal impairment and for those with end-stage renal disease. Extending the infusion time of ceftobiprole to 4 h is recommended to optimize drug exposure in critically ill patients with augmented renal clearance. However, there is no need for dose adjustments based on age, sex or ethnicity, or for patients with severe obesity. Population pharmacokinetic modelling and Monte Carlo simulations were used to determine the optimal dosing regimen for ceftobiprole in special patient populations, including paediatric patients. Future studies of ceftobiprole in patients with HAP and CAP would be of interest.

Use of old antibiotics now and in the future from a pharmacokinetic/pharmacodynamic perspective

Because of the increase in bacterial resistance to commonly used antibacterial drugs, old antibiotics are being 'revived' and, once again, are attracting interest. Many of these old antibiotics were approved long ago, in an era when there was no clear process for development, and requirements for efficacy to be demonstrated in rigorous clinical trials did not exist. At the time of these approvals, pharmacokinetic and pharmacodynamic principles were largely unknown, and did not inform the dose-finding process or recommendations for optimal usage. Indeed, the task of generating basic vital information for these old antibiotics remains to be performed. In this review, we provide a brief overview of the most essential data needed for dose justification and optimization. An overview of the shortage of data for selected old antibiotics illustrates the scope of the problem. In order to prevent harming patients with clinical decisions based on inadequate evidence, a redevelopment procedure for old antibiotics is urgently needed, including a regulatory framework.

The minimal inhibitory concentration for sulbactam was not associated with the outcome of infections caused by carbapenem-resistant Acinetobacter sp. treated with ampicillin/sulbactam

OBJECTIVE

The objective of this study was to evaluate whether the outcomes of carbapenem-resistant Acinetobacter infections treated with ampicillin/sulbactam were associated with the in vitro susceptibility profiles.

METHODS

Twenty-two infections were treated with ampicillin/sulbactam. The median treatment duration was 14 days (range: 3-19 days), and the median daily dose was 9 g (range: 1.5-12 g). The median time between Acinetobacter isolation and treatment was 4 days (range: 0-11 days).

RESULTS

The sulbactam minimal inhibitory concentration (MIC) ranged from 2.0 to 32.0 mg/L, and the MIC was not associated with patient outcome, as 4 of 5 (80%) patients with a resistant infection (MIC≥16), 5 of 10 (50%) patients with intermediate isolates (MIC of 8) and only 1 of 7 (14%) patients with susceptible isolates (MIC ≤4) survived hospitalization.

CONCLUSION

These findings highlight the need to improve the correlation between in vitro susceptibility tests and clinical outcome.

Staphylococcus aureus in continuous culture: a tool for the rational design of antibiotic treatment protocols

In vitro measures of the pharmacodynamics of antibiotics that account for the factors anticipated for bacteria in infected patients are central to the rational design of antibiotic treatment protocols. We consider whether or not continuous culture devices are a way to obtain these measures. Staphylococcus aureus PS80 in high-density continuous cultures were exposed to oxacillin, ciprofloxacin, vancomycin, gentamicin, daptomycin and linezolid. Contrary to results from low density retentostats as well as to predictions of traditional PK/MIC ratios, daily dosing with up to 100× MIC did not clear these cultures. The densities of S. aureus in these cultures oscillated with constant amplitude and never fell below 10⁵ CFU per ml. Save for daptomycin “treated” populations, the densities of bacteria in these cultures remained significantly below that of similar antibiotic-free cultures. Although these antibiotics varied in their pharmacodynamic properties there were only modest differences in their mean densities. Mathematical models and experiments suggest that the dominant factor preventing clearance was wall-adhering subpopulations reseeding the planktonic population which can be estimated and corrected for. Continuous cultures provide a way to evaluate the potential efficacy of antibiotic treatment regimes in vitro under conditions that are more clinically realistic and comprehensive than traditional in vitro PK/PD indices.

The relative contributions of physical structure and cell density to the antibiotic susceptibility of bacteria in biofilms

For many bacterial infections, noninherited mechanisms of resistance are responsible for extending the term of treatment and in some cases precluding its success. Among the most important of these noninherited mechanisms of resistance is the ability of bacteria to form biofilms. There is compelling evidence that bacteria within biofilms are more refractory to antibiotics than are planktonic cells. Not so clear, however, is the extent to which this resistance can be attributed to the structure of biofilms rather than the physiology and density of bacteria within them. To explore the contribution of the structure of biofilms to resistance in a quantitative way, we developed an assay that compares the antibiotic sensitivity of bacteria in biofilms to cells mechanically released from these structures. Our method, which we apply to Escherichia coli and Staphylococcus aureus each with antibiotics of five classes, controls for the density and physiological state of the treated bacteria. For most of the antibiotics tested, the bacteria in biofilms were no more resistant than the corresponding populations of planktonic cells of similar density. Our results, however, suggest that killing by gentamicin, streptomycin, and colistin is profoundly inhibited by the structure of biofilms; these drugs are substantially more effective in killing bacteria released from biofilms than cells within these structures.

Challenges in assessing microbial susceptibility and predicting clinical response to newer-generation fluoroquinolones

PURPOSE

To determine the most appropriate methods for assessing the potential effectiveness of the newer topical fluoroquinolones gatifloxacin, moxifloxacin, and besifloxacin.

METHODS

This article is based on a literature search for published articles about the pharmacokinetic (PK) and pharmacodynamic (PD) characteristics of and measure of bacterial susceptibility to topical ophthalmic fluoroquinolones. Search terms included fluoroquinolones, susceptibility, resistance, minimal/minimum inhibitory concentration (MIC), PDs, PKs, and ocular, ophthalmic, or topical antibiotics.

RESULTS

Topical fluoroquinolones, particularly besifloxacin, gatifloxacin, and moxifloxacin, have become important treatment options for common ocular bacterial infections due to their broad-spectrum bactericidal activity and low toxicity. An important challenge in ophthalmology is identifying the most accurate in vivo and in vitro methods for evaluating the efficacy of these topical fluoroquinolones. The MIC is the most commonly used measure of in vitro susceptibility. In systemic therapy, this measure is combined with PK data of antibiotics to generate PD indices PK/PD whose breakpoints differentiate clinically susceptible from nonsusceptible bacterial pathogen populations. PD breakpoints are further tested in prospective studies for their ability to predict clinical efficacy. However, it is not known whether systemically derived breakpoints apply to the assessment of clinical susceptibility to ocular agents. Topical ocular antibiotics likely achieve higher concentrations at the target site than do systemically administered antibiotics, but these higher concentrations can be quickly reduced by reflex tearing and blinking induced by instillation. Hence, studies have been conducted in animals and humans to determine the PK concentrations of topically administered antibiotics in ocular compartments. When combined with MIC values for topical pathogens, the results have the potential to predict clinical efficacy after identification of the appropriate PK/PD target. Ocular studies incorporating PK/PD assessments have recently begun to be reported with newer fluoroquinolones, including besifloxacin, gatifloxacin, and moxifloxacin, whose prolonged contact time and potent bactericidal activity have translated into some of the most favorable PK/PD target values. However, the clinical relevance of these studies has yet to be determined.

CONCLUSION

There is still a clear need for predictive models to extend our understanding of the clinical susceptibility of ocular pathogens.

Emergence and spread of antibiotic resistance following exposure to antibiotics

Within a susceptible wild-type population, a small fraction of cells, even <10(-9) , is not affected when challenged by an antimicrobial agent. This subpopulation has mutations that impede antimicrobial action, allowing their selection during clinical treatment. Emergence of resistance occurs in the frame of a selective compartment termed a mutant selection window (MSW). The lower margin corresponds to the minimum inhibitory concentration of the susceptible cells, whereas the upper boundary, named the mutant prevention concentration (MPC), restricts the growth of the entire population, including that of the resistant mutants. By combining pharmacokinetic/pharmacodynamic concepts and an MPC strategy, the selection of resistant mutants can be limited. Early treatment avoiding an increase of the inoculum size as well as a regimen restricting the time within the MSW can reduce the probability of emergence of the resistant mutants. Physiological and, possibly, genetic adaptation in biofilms and a high proportion of mutator clones that may arise during chronic infections influence the emergence of resistant mutants. Moreover, a resistant population can emerge in a specific selective compartment after acquiring a resistance trait by horizontal gene transfer, but this may also be avoided to some extent when the MPC is reached. Known linkage between antimicrobial use and resistance should encourage actions for the design of antimicrobial treatment regimens that minimize the emergence of resistance. Emergence of a resistant bacterial subpopulation within a susceptible wild-type population can be restricted with a regimen using an antibiotic dose that is sufficiently high to inhibit both susceptible and resistant bacteria.

Susceptibility of bifidobacteria of animal origin to selected antimicrobial agents

Strains of the genus Bifidobacterium are frequently used as probiotics, for which the absence of acquired antimicrobial resistance has become an important safety criterion. This clarifies the need for antibiotic susceptibility data for bifidobacteria. Based on a recently published standard for antimicrobial susceptibility testing of bifidobacteria with broth microdilution method, the range of susceptibility to selected antibiotics in 117 animal bifidobacterial strains was examined. Narrow unimodal MIC distributions either situated at the low-end (chloramphenicol, linezolid, and quinupristin/dalfopristin) or high-end (kanamycin, neomycin) concentration range could be detected. In contrast, the MIC distribution of trimethoprim was multimodal. Data derived from this study can be used as a basis for reviewing or verifying present microbiological breakpoints suggested by regulatory agencies to assess the safety of these micro-organisms intended for the use in probiotics.

Population dynamics of antibiotic treatment: a mathematical model and hypotheses for time-kill and continuous-culture experiments

The objectives of the study were to develop a quantitative framework for generating hypotheses for and interpreting the results of time-kill and continuous-culture experiments designed to evaluate the efficacy of antibiotics and to relate the results of these experiments to MIC data. A mathematical model combining the pharmacodynamics (PD) of antibiotics with the population dynamics of bacteria exposed to these drugs in batch and continuous cultures was developed, and its properties were analyzed numerically (using computer simulations). These models incorporate details of (i) the functional form of the relationship between the concentrations of the antibiotics and rates of kill, (ii) the density of the target population of bacteria, (iii) the growth rate of the bacteria, (iv) byproduct resources generated from dead bacteria, (v) antibiotic-refractory subpopulations, persistence, and wall growth (biofilms), and (vi) density-independent and -dependent decay in antibiotic concentrations. Each of the factors noted above can profoundly affect the efficacy of antibiotics. Consequently, if the traditional (CLSI) MICs represent the sole pharmacodynamic parameter, PK/PD indices can fail to predict the efficacy of antibiotic treatment protocols. More comprehensive pharmacodynamic data obtained with time-kill and continuous-culture experiments would improve the predictive value of these indices. The mathematical model developed here can facilitate the design and interpretation of these experiments. The validity of the assumptions behind the construction of these models and the predictions (hypotheses) generated from the analysis of their properties can be tested experimentally. These hypotheses are presented, suggestions are made about how they can be tested, and the existing statuses of these tests are briefly discussed.

Pharmacokinetics of clindamycin in pregnant women in the peripartum period

The study presented here was performed to determine the pharmacokinetics of intravenously administered clindamycin in pregnant women. Seven pregnant women treated with clindamycin were recruited. Maternal blood and arterial and venous umbilical cord blood samples were obtained. Maternal clindamycin concentrations were analyzed by nonlinear mixed-effects modeling with the NONMEM program. The data were best described by a linear three-compartment model. The clearance and the volume of distribution at steady state were 10.0 liters/h and 6.32 x 10(3) liters, respectively. Monte Carlo simulations were performed to determine the area under the concentration curve (AUC) for the free (unbound) drug (f) in maternal serum for 24 h divided by the MIC (fAUC(0-24)/MIC). At a MIC of 0.5 mg/liter, which is the EUCAST breakpoint, the attainment at the lower 95% confidence interval (CI) was 24.6 if the level of protein binding was 65%, and this value concurred well with the target value of 27. However, for higher degrees of protein binding, as has been described in the literature, the attainment was lower, down to 10.2 for a protein binding level of 85% (lower 95% CI). The concentrations in umbilical cord blood were lower than those in maternal blood. The concentration-time profiles in maternal serum indicate that the level of exposure to clindamycin may be too low in these patients. Together with the lower concentrations in umbilical cord blood, this finding suggests that the current dosing regimen may not be adequate to protect all neonates from group B streptococcal disease.

Antimicrobial breakpoint estimation accounting for variability in pharmacokinetics

Background

Pharmacokinetic and pharmacodynamic (PK/PD) indices are increasingly being used in the microbiological field to assess the efficacy of a dosing regimen. In contrast to methods using MIC, PK/PD-based methods reflect in vivo conditions and are more predictive of efficacy. Unfortunately, they entail the use of one PK-derived value such as AUC or Cmax and may thus lead to biased efficiency information when the variability is large. The aim of the present work was to evaluate the efficacy of a treatment by adjusting classical breakpoint estimation methods to the situation of variable PK profiles.

Methods and results

We propose a logical generalisation of the usual AUC methods by introducing the concept of "efficiency" for a PK profile, which involves the efficacy function as a weight. We formulated these methods for both classes of concentration- and time-dependent antibiotics. Using drug models and in silico approaches, we provide a theoretical basis for characterizing the efficiency of a PK profile under in vivo conditions. We also used the particular case of variable drug intake to assess the effect of the variable PK profiles generated and to analyse the implications for breakpoint estimation.

Conclusion

Compared to traditional methods, our weighted AUC approach gives a more powerful PK/PD link and reveals, through examples, interesting issues about the uniqueness of therapeutic outcome indices and antibiotic resistance problems.

Use of Antimicrobial Agents in Veterinary Medicine and Implications for Human Health

This review discusses why veterinary usage of antimicrobial agents is wrongly accused of causing a substantial part of the problem of resistant human pathogens. Without doubt, resistant organisms in animals are selected by veterinary antimicrobials. However, these are not a major human health risk either because the role of veterinary usage in selection or propagation is insignificant, or because resistant populations selected by veterinary usage do not pose a substantial risk to human health. Indeed, resistant bacterial infections in humans causing serious quantitative and qualitative health consequences are rarely food-borne and are not the same as those selected by veterinary usage of antimicrobial agents. The available evidence for veterinary selection of resistance, transmission to humans, and subsequent health consequences are reviewed for food-borne zoonotic pathogens. A risk assessment strategy is proposed to quantify potential hazards in order to decide on the most effective risk management strategy.

Antimicrobial resistance in Salmonella enterica subspecies enterica serovar Dublin from dairy source calves in the central San Joaquin Valley, California (1998-2002)

The present study describes antimicrobial resistance patterns of Salmonella enterica subspecies enterica serovar Dublin (S. Dublin) in clinical submissions from calves and temporal and farm-type trends in antimicrobial resistance patterns of the isolates. A total of 300 isolates of S. Dublin were obtained from fecal or internal organs of calves fewer than 120 days of age originating from 84 dairies and 18 calf ranches from July 1998 to December 2002. The isolates were susceptibility tested to a panel of 10 antimicrobials using the disk diffusion assay. Temporal and farm-type trends in individual antimicrobial inhibition zone sizes were assessed and antimicrobial resistance patterns were described using cluster analysis. Isolates obtained from calf ranches compared with dairies exhibited decreased susceptibility to florfenicol, gentamicin, neomycin, sulfisoxazole, sulfamethoxazole/trimethoprim, and tetracycline. During the years 1998-2002, decreasing susceptibility was seen for ceftiofur, enrofloxacin, and sulfamethoxazole/trimethoprim. There were 20 different antimicrobial resistance patterns in the isolate set, indicating that S. Dublin has the ability to transfer and pick up resistance genes with relative ease. The trends seen in antimicrobial resistance in S. Dublin may likely be linked to antimicrobial drug use in young calves.

Pharmacokinetics of penicillin G in infants with a gestational age of less than 32 weeks

The pharmacokinetics of penicillin G were studied in 20 preterm neonates with a gestational age of less than 32 weeks on day 3 of life by using a population approach performed with the nonlinear mixed effects modeling program NONMEM. The derived population estimates and the correlation matrix of these estimates were used to perform Monte Carlo simulations and obtain the probability of target attainment (PTA). The pharmacokinetics of penicillin G were best described by a two-compartment pharmacokinetic model. The population estimates of the central volume of distribution, the peripheral volume of distribution, the intercompartmental clearance, and the total body clearance were 0.359 +/- 0.06 liter, 0.152 +/- 0.03 liter, 0.774 +/- 0.28 liter/h, and 0.103 +/- 0.01 liter/h (mean +/- standard error), respectively. The terminal half-life was 3.9 h. Clearance increased significantly with increasing birth weight. Assuming the percentage of time that the concentration of unbound drug remained above the MIC of 50% for preterm neonates, the susceptibility breakpoint based on a 100% PTA was < or =4 mg/liter, simulating the current dosing regimen of 50,000 U/kg every 12 h. This regimen is therefore adequate for the treatment of common infections in neonates on the third day of life.

Evaluating Empiric Treatment Options for Secondary Peritonitis Using Pharmacodynamic Profiling

BACKGROUND

Selecting an appropriate agent for empiric antibiotic therapy for secondary peritonitis is challenging. The pathogens responsible, aerobic gram-negative bacilli in particular, are becoming more resistant to antibiotics. The purpose of this study was to predict the ability of common antimicrobial regimens to achieve optimal pharmacodynamic exposure against aerobic bacteria implicated in secondary peritonitis, while considering current national resistance trends.

METHODS

Monte Carlo simulation was used to model pharmacodynamic endpoints and compare the cumulative fraction of response (CFR) for imipenem-cilastatin, meropenem, ertapenem, piperacillin/tazobactam, ceftazidime, ceftriaxone, ciprofloxacin, and levofloxacin against isolates of species associated with secondary peritonitis. Minimum inhibitory concentration (MIC) distributions for isolates collected in North America were obtained from the 2004 MYSTIC database. Pharmacokinetic parameters were derived from the literature; the endpoints evaluated included free drug time above the MIC (fT(>MIC)) and the area under the concentration-time curve to MIC ratio (AUC:MIC).

RESULTS

The simulation predicted that several compounds would have a superior probability of providing appropriate coverage of aerobic bacteria: Imipenem-cilastatin (98.6% CFR at 1 g q8h), meropenem (98.2% CFR at 1 g q8h), ertapenem (91.7% CFR at 1 g q24h), piperacillin/ tazobactam (93.7% CFR at 3.375 g q6h), ceftazidime (91.1% CFR at 2 g q8h), and cefepime (92.9% CFR at 1 g q12h and 95.8% CFR at 2 g q12h). Ceftriaxone, ciprofloxacin, and levofloxacin exhibited CFRs < 82%.

CONCLUSIONS

Considering contemporary susceptibility data for aerobic bacteria, monotherapy with any of the three carbapenems or piperacillin/tazobactam 3.375 g q6h would provide optimal exposure for the pathogens commonly encountered in secondary peritonitis. Cefepime (in combination with metronidazole to provide anti-anaerobic coverage) also would be an acceptable choice, as would ceftazidime given at 2 g q8h (again in combination with metronidazole). Despite the popularity of combination therapy based on ciprofloxacin, levofloxacin, or ceftriaxone with metronidazole, these choices appear to be inferior to the other options because of emerging antibiotic resistance, particularly in E. coli.

Antibiotic susceptibility of Bifidobacterium thermophilum and Bifidobacterium pseudolongum isolates from animal sources

The widespread use of antimicrobial substances has led to resistant populations of microorganisms in several ecosystems. In animal husbandry, the application of antibiotics has contributed to resistance development in pathogenic and commensal bacteria. These strains or their resistance genes can be spread along several ecological routes, including the food chain. Antibiotic resistance is important in terms of the safety of industrial strains, such as probiotics for food and feed. Bifidobacterium thermophilum and Bifidobacterium pseudolongum are known to comprise the major part of the bifidobacterial microbiota in the gut and feces of cattle and pigs. In this study, the antimicrobial susceptibility in bifidobacterial isolates of these species was investigated. Isolates from the beef and pork production chain were identified and typed to strain level, and the antimicrobial susceptibility level was tested to a set of antibiotics. Isolates with low susceptibility levels were screened by PCR for already described resistance genes. Strains atypically resistant to clindamycin, erythromycin, and tetracycline were determined. The resistance genes tet(O), tet(W), and erm(X) were detected in the bifidobacterial species that were examined.

Association of fluconazole area under the concentration-time curve/MIC and dose/MIC ratios with mortality in nonneutropenic patients with candidemia

The present study tested in vitro susceptibility of Candida bloodstream isolates to fluconazole to determine if the ratio of the fluconazole area under the concentration-time curve (AUC) or weight-normalized daily dose (dose(wn)) to MIC correlated with mortality. Fluconazole susceptibility and outcome data were determined for 77 patients with a positive Candida blood culture between 2002 and 2005. The most commonly isolated Candida species were C. albicans (64%), C. glabrata (14%), C. parapsilosis (8%), C. tropicalis (6%), and C. lusitaniae (4%). Only two isolates were classified as fluconazole resistant by the CLSI M27-A2 method. Fluconazole MICs were highest against C. glabrata relative to other Candida species. Overall the crude mortality assessed at hospital discharge was 19.4% (n = 15). Mortality rates by species were as follows: C. albicans, 16.3%; C. glabrata, 36.4%; C. parapsilosis, 0%; C. tropicalis, 0%; C. lusitaniae, 33.3%. A mortality rate of 50% was noted among patients infected with nonsusceptible isolates (MIC > or = 16 microg/ml) compared to 18% for patients infected with susceptible (MIC < or = 8 microg/ml) isolates (P = 0.17). The fluconazole dose(wn)/MIC (24-h) values were significantly higher for the 62 survivors (13.3 +/- 10.5 [mean +/- standard deviation]) compared to the 15 nonsurvivors (7.0 +/- 8.0) (P = 0.03). The fluconazole AUC/MIC (24 h) values also trended higher for survivors (775 +/- 739) compared to nonsurvivors (589 +/- 715) (P = 0.09). These data support the dose-dependent properties of fluconazole. Underdosing fluconazole against less-susceptible Candida isolates has the potential to increase the risk of mortality associated with candidemia.

In vitro susceptibility testing versus in vivo effectiveness

The clinical relevance of susceptibility testing has always been questioned because of the difficulty of correlating in vitro susceptibility testing with in vivo clinical effectiveness. Clearly there have always been host/pathogen factors that influence the clinical outcome that cannot be predicted by the results of susceptibility testing. However, improved understanding of pharmacodynamic and pharmacokinetic parameters has greatly improved the use of antimicrobial agents. Most importantly, the integration of these pharmacodynamic and pharmacokinetic indices has greatly improved the correlation between in vitro susceptibility testing and in vivo clinical effectiveness and allows more realistic breakpoints. Finally, the clinical microbiology laboratory has advanced with improved methods as well as the adaptation of breakpoints that are more realistic.

Principles of Antimicrobial Therapy

The most compelling reason for practicing judicious antimicrobial use is to facilitate therapeutic success. The definition of therapeutic success has changed in recent years, however; not only does success include eradication of infection, but it must now include avoidance of resistance. If the goal of antimicrobial therapy is to achieve sufficient concentrations at the site of infection such that the infecting organism is killed, therapy should be successful.

Population distributions of minimum inhibitory concentration - increasing accuracy and utility

AIMS

To generate continuous minimum inhibitory concentration (MIC) data that describes the discrete nature of experimentally derived population MIC data.

METHODS AND RESULTS

A logistic model was fitted to experimentally derived MIC population cumulative distributions from clinical isolates of Haemophilus influenzae, Moraxella catarrhalis, Streptococcus pneumoniae and Staphylococcus aureus (European Committee on Antimicrobial Susceptibility Testing, BSAC and MYSTIC population susceptibility databases). From the model continuous distributions of population susceptibility were generated. The experimentally observed population distributions based on discrete MIC could be reproduced from this underlying continuous distribution. Monte Carlo (MC) simulation was used to confirm findings. Where the discrete experimental data contained few or no isolates with MIC greater or less than the antimicrobial concentration range tested, the true mean MIC was a factor of 0.707 times that normally reported and may be of little clinical significance. Where data contained isolates beyond the range of concentration used, the true MIC was dependent on the SD and the number of isolates and could be clinically significant. Subpopulations of differing susceptibilities could be modelled successfully using a modified logistic equation: this allows a more accurate examination of the data from these databases.

CONCLUSIONS

The mean MIC and SD of population data currently reported are incorrect as the method of obtaining such parameters relies on normally distributed data which current MIC population data are not.

SIGNIFICANCE AND IMPACT OF THE STUDY

Obtaining the distribution parameters from the underlying continuous distribution of MIC can be carried out using a simple logistic equation. MC simulation using these values allows easy visualization of the discrete data. The analyses of subpopulations within the data should increase the usefulness of horizontal studies.

Statistical properties and inference of the antimicrobial MIC test

A common method for measuring the drug-specific minimum inhibitory concentration (MIC) of an antibacterial agent is via a two-fold broth dilution test known as the MIC test. Because this procedure implicitly rounds data upward, inference based on unadjusted measurements is biased and overestimates bacterial resistance to a drug. We detail this test procedure and its associated bias, which, in many cases, has an expected value of approximately 0.5 on the log(2) scale. In addition, new bias-corrected estimates of resistance are proposed. A numeric example is used to illustrate the extent to which the traditional resistance estimate can overestimate the true proportion of resistant strains, a phenomenon which is remedied by using the proposed estimates.

A novel approach to estimate in vitro antibacterial potency of Chinese medicine using a concentration-killing curve method

The antibacterial pharmacodynamics against E. coli of Chinese medicine (CM) Rhizoma coptidis (Coptis Root) and its formula Sanhuang, and the control antibiotics enoxacin, were analyzed by a concentration-killing curve (CKC) approach, and the novel parameters BC50 and r for antibacterial potency were proposed. Using the agar plate method, about 400 cells of E. coli were evenly inoculated into LB agar plates containing a series of different concentrations of CM or antibiotic, and after a 24 hour incubation at 37 degrees C, all the viable colonies were enumerated. This resulted in a sigmoid concentration-killing curve , in which No, that could be closely fitted (R2 > 0.9) with the function: N = 1 + e(r(x-BC50))/N0 in which N0, BC50 and r represent meaningfully inoculums size, median bactericidal concentration, and bactericidal intensity, respectively. N modeled the survival of colony-forming units on each plate (CFU/plate) in a concentration series x of the drug. The CKC was symmetrical about its single inflexion (BC50, N0/2). Therefore theoretically, 2BC50 can replace MBC (minimum bactericidal concentration). BC1 = BC50 + r/ln(N0-1), the drug concentration at r which only one colony survived, was the least critical value of MBC in CKC. The parameters 2BC50 and BC1 agreed more closely with the definition of MBC, and were little affected by either the biochemical basis of the antibacterial or the inoculum's size (200-400 CFU/plate), and were determined by a multi-point curve. As a result, these were more accurate, reproducible and practical as metrics than was the endpoint of MBC. The two-dimensional CKC, involving BC50 and r, captures the intrinsic dynamics of the antibacterial effect of CM/strain versus concentration, and it is consistent with the Logistic equation of the bacterial growth curve in the format. This verified approach has considerable value as a tool for the accurate and proper administration of CM. The CKC of CM, different from that of antibiotics, is likely to be the resultant force of each ingredient in certain CM, which provides a clue to solve the problem of antibiotic resistance.

Evaluating ciprofloxacin dosing for Pseudomonas aeruginosa infection by using clinical outcome-based Monte Carlo simulations

Pseudomonas aeruginosa causes serious infections whose outcome is highly dependent on antimicrobial therapy. The goal of this study was to predict the relative efficacies of three ciprofloxacin dosing regimens for P. aeruginosa infection using clinical outcome-based Monte Carlo simulations (MCS) with "real patient" demographics, pharmacokinetics, MICs, and pharmacodynamics (PDs). Each cohort consisted of 1,000 simulated study subjects. Three ciprofloxacin dosing regimens were studied, including (i) the recommended standard dose of 400 mg given intravenously (i.v.) every 12 h (q12h), (ii) the recommended high dose of 400 mg i.v. q8h, and (iii) a novel, PD-targeted regimen to attain a fAUC/MIC value of >86. Probability of target attainment (PTA) and probability of cure (POC) were determined for each regimen. POC with the standard dose was at least 0.90 if pathogen MICs were < or =0.25 microg/ml but only 0.59 or 0.27 if MICs were 0.5 or 1 microg/ml, respectively. Predicted cure rates in these MIC categories were significantly higher at 0.72 and 0.40 with the high dose and 0.91 and 0.72 with the PD-targeted regimen(P < 0.0001). Analyses based on the local susceptibility profile produced PTA and POC estimates of 0.44 and 0.74 with the standard ciprofloxacin dose, 0.58 and 0.81 with the high dose, and 0.84 and 0.93 with the PD-targeted regimen, respectively. In conclusion, as demonstrated by clinical outcome-based MCSs, the highest recommended ciprofloxacin dose of 400 mg i.v. q8h should be used in the treatment of P. aeruginosa infection to improve PD target attainment and clinical cure. However, even this appears ineffective if pathogen MICs are 1 mug/ml, warranting the consideration of a lower MIC breakpoint, < or =0.5 microg/ml.

A retrospective analysis using Monte Carlo simulation to evaluate recommended ceftazidime dosing regimens in healthy volunteers, patients with cystic fibrosis, and patients in the intensive care unit

BACKGROUND

Over the past decades, the relationship between the pharmacokinetic (PK) properties of antibiotics, MICs, and clinical effects has been increasingly well understood. Interpatient variability in the PK profile, however, has only recently been recognized as a major factor in predicting the outcome in individual patients and establishing breakpoints for clinical susceptibility. Most predictions to date have used data from healthy volunteers.

OBJECTIVE

The purpose of this study was to perform Monte Carlo simulations of the PK/pharmacodynamic relationships of ceftazidime to assess whether the probability of target attainment (PTA) differed significantly between 3 distinct populations. To that end, population PK models of ceftazidime were developed for the 3 populations.

METHODS

Serum concentration-time data from earlier studies in healthy volunteers (n = 8), patients with cystic fibrosis (CF) (n = 17), and patients in the intensive care unit (ICU) (n = 6) were used to obtain population PK parameter estimates and covariance matrices using the nonparametric adaptive grid program. The PTA for each group was obtained using 10,000 patient simulations for dosing regimens of 1000 and 2000 mg q8h over a range of MICs and percentages of time that concentrations of unbound drug remained above the MIC (%T > MIC).

RESULTS

The relationship between the MIC and the population mean %T > MIC, as well as the PTA profiles, differed markedly between the 3 groups as a result of both differences and variations in V(d) and Cl. Breakpoints based on a 100% PTA for a %T > MIC of 60% were < or = 4, 0.5, and 0.5 mg/L in healthy volunteers, patients with CF, and patients in the ICU, respectively. However, when PTA values between 90% and 100% were reevaluated and differences in clinical dosing regimens were accounted for, the resulting breakpoints were identical in the 3 groups.

CONCLUSIONS

PK parameter estimates for ceftazidime based on data from a small group of healthy volunteers resulted in a clinical susceptibility breakpoint comparable to those for patients with CF and patients in the ICU. Based on the study findings, this breakpoint would be < or = 4 mg/L. Patients suspected of having unusually high rates of clearance should be monitored closely.

The OPTAMA programme: utilizing MYSTIC (2002) to predict critical pharmacodynamic target attainment against nosocomial pathogens in Europe

OBJECTIVES

The Optimising Pharmacodynamic Target Attainment using the MYSTIC (Meropenem Yearly Susceptibility Test Information Collection) Antibiogram (OPTAMA) programme identifies antibiotic regimens with the highest probability of attaining critical pharmacodynamic targets, accounting for the inherent variability in pharmacokinetics, dosages and MIC distributions.

METHODS

European MIC data were obtained from the MYSTIC programme. Pharmacodynamic target attainment was calculated by Monte Carlo simulation for meropenem, imipenem, ceftazidime, cefepime, piperacillin/tazobactam and ciprofloxacin against Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii and Pseudomonas aeruginosa.

RESULTS

Significant differences in probability of target attainment were found, with Northern Europe demonstrating the highest probabilities of target attainment and Eastern Europe the lowest. The carbapenems had the highest target attainments and susceptibility levels across all regions and pathogens. The cephalosporins demonstrated high target attainments and susceptibility results against E. coli and K. pneumoniae in Northern and Southern Europe. Piperacillin/tazobactam and ciprofloxacin had the lowest levels for both parameters in all regions. Desirable target attainment was not achieved (except for carbapenems in Northern Europe) for A. baumannii and P. aeruginosa; thus, combination therapy may be appropriate empirical therapy for these pathogens in Southern and Eastern Europe. The closest correlations between target attainment and susceptibility were for meropenem 1 g every 8 h, imipenem 0.5 g every 6 h and ceftazidime 1 g every 8 h.

CONCLUSIONS

The study highlighted significant overestimations between the probability of target attainment and the reported percentage susceptibility, particularly for piperacillin/tazobactam and ciprofloxacin. The approach of the OPTAMA programme provides a novel tool which complements susceptibility data to help in the selection of appropriate empirical antibiotic therapy.

Novel concentration-killing curve method for estimation of bactericidal potency of antibiotics in an in vitro dynamic model

The bactericidal pharmacodynamics of antibiotics against Escherichia coli were analyzed by a concentration-killing curve (CKC) approach, and the novel parameters median bactericidal concentration (BC(50)) and bactericidal intensity (r) for bactericidal potency were proposed. By using the agar plate method, about 500 E. coli cells were inoculated onto Luria-Bertani plates containing a series of antibiotic concentrations, and after 24 h of incubation at 37 degrees C, all the viable colonies were enumerated. This resulted in a sigmoidal CKC that could be perfectly fitted (R(2) > 0.9) with the function N = N(0)/[1 + e(r(x - BC(50)))], where N is number of colonies surviving on each plate with an x series of concentrations of an antibiotic, and N(0) represents the meaningful inoculum size. Construction of the CKC method was based on the bactericidal effect of each antibiotic against the bacterial strain versus the concentration in two dimensions and may be a more valid, accurate, and reproducible method for estimating the bactericidal effect than the endpoint minimum bactericidal concentration (MBC) method. Mathematically, the CKC approach was point symmetrical toward its inflexion (BC(50), N(0)/2); thus, 2BC(50) could replace MBC. The parameter BC(1) can be defined as BC(50) + [ln(N(0) - 1)/r], which is the drug concentration at which only one colony survived and which is the least critical value of MBC in the CKC. The variate r, which determined the tangent slope on inflexion when N(0) was limited, could estimate the bactericidal intensity of an antibiotic. This verified that the CKC approach may be useful in studies with other classes of antibiotics and has considerable value as a tool for the accurate and proper administration of antibiotics.

Community-acquired pneumonia: new management strategies for evolving pathogens and antimicrobial susceptibilities

Community-acquired pneumonia (CAP) is still one of the leading causes of mortality and morbidity. The most common bacterial cause of CAP is Streptococcus pneumoniae. The increase in antimicrobial resistance has raised concerns about the efficacy of available therapies, and a call for the reassessment of both existing and newer therapeutic agents. Although microbiological breakpoints are useful for monitoring the emergence of resistance, the current National Committee for Clinical Laboratory Standards (NCCLS) guidelines make no distinction between clinical and microbiological breakpoints. Recent changes in NCCLS breakpoints for extended spectrum cephalosporins have provided a more meaningful approach to susceptibility testing and to consideration of the site of infection. Further controversy surrounds the clinical guidelines relating to CAP in terms of which antimicrobial agents should be given empirically to which types of patients. Within this review, the role of monotherapy versus the need for combination antimicrobial therapy, which often includes a macrolide and an extended spectrum cephalosporin such as ceftriaxone, is discussed. This review also discusses the various aspects of antimicrobial susceptibilities of S. pneumoniae, the drivers and influences of increasing resistance, the clinical relevance of this resistance and possible therapeutic options in the face of changing susceptibilities and mixed bacterial aetiologies. New guidelines from the IDSA attempt to embrace these changes.

Designing fluoroquinolone breakpoints for Streptococcus pneumoniae by using genetics instead of pharmacokinetics-pharmacodynamics

We determined fluoroquinolone microbiological resistance breakpoints for Streptococcus pneumoniae by using genetic instead of pharmacokinetic-pharmacodynamic parameters. The proposed microbiological breakpoints define resistance as the MIC at which >50% of the isolates carry quinolone resistance-determining region mutations and/or, if data are available, when Monte Carlo simulations demonstrate a <90% chance of bacteriological eradication. The proposed microbiological resistant breakpoints are as follows (in micrograms per milliliter): gatifloxacin, >0.25; gemifloxacin, >0.03; levofloxacin, >1; and moxifloxacin, >0.12. Monte Carlo simulations of the once daily 400-mg doses of gatifloxacin and 750-mg doses levofloxacin demonstrated a high level of target attainment (free-drug area under the concentration-time curve from 0 to 24 h/MIC ratio of 30) by using these new genetically derived breakpoints.

Optimizing pharmacodynamic target attainment using the MYSTIC antibiogram: data collected in North America in 2002

The OPTAMA Program is intended to examine typical antimicrobial regimens used in the treatment of common nosocomial pathogens and the likelihood of these regimens attaining appropriate pharmacodynamic exposure in different parts of the world. A 5,000-subject Monte Carlo simulation was used to estimate pharmacodynamic target attainment for meropenem, imipenem, ceftazidime, cefepime, piperacillin-tazobactam, and ciprofloxacin against Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa. Standard dosing regimens from North America were used. Pharmacokinetic parameter variability was derived from existing healthy volunteer data, and MIC data came from the 2002 MYSTIC Program. Ciprofloxacin displayed the lowest target attainment against all bacterial species (41 to 46% for A. baumannii, 53 to 59% for P. aeruginosa, and 80 to 85% for the Enterobacteriaceae). Increasing the dose to 400 mg every 8 h did not significantly increase target attainment against nonfermenters. Piperacillin-tazobactam target attainments were similar to that of ceftazidime against all pathogens. Higher doses of both compounds were needed to achieve better target attainments against P. aeruginosa. Overall, meropenem, imipenem, and cefepime attained the highest probabilities of attainment against the Enterobacteriaceae (99 to 100%). The carbapenems appear to be the most useful agents against A. baumannii (88 to 92%), and these agents, along with higher doses of any of the beta-lactams, would be the most appropriate choices for empirical therapy for P. aeruginosa infection. Given the lack of agreement between percent susceptibility and probability of target attainment for certain antimicrobial regimens, a methodology employing stochastic pharmacodynamic analyses may be a more useful tool for differentiating the most-optimal compounds and dosing regimens in the clinical setting of initial empirical therapy.

Use of Monte Carlo simulations to select therapeutic doses and provisional breakpoints of BAL9141

BAL9141, a new antimicrobial agent belonging to the class of parenteral pyrrolidinone-3-ylidenemethyl cephalosporins, is active against most gram-positive microorganisms, including methicillin-resistant variants (methicillin-resistant Staphylococcus aureus [MRSA] and methicillin-resistant Staphylococcus epidermidis [MRSE]), as well as against penicillin-resistant pneumococci (PRP) and many gram-negative microorganisms. BAL9141 is administered as the prodrug BAL5788, which is rapidly converted to BAL9141 by plasma esterases. Pharmacokinetic (PK) data obtained in a previous multiple ascending dose study were used to fit a population PK model to using the NPEM2 program, yielding PK parameter estimates and its covariance matrix for BAL9141. These estimates and matrix were used to perform Monte Carlo simulations (MCSs) and obtain unbiased target attainment rates (TARs) for various time periods during which the concentration remains above the MIC (T(>MIC)). Assuming a T(>MIC) of 40%, TARs of 100% were reached with a dose of 500 mg/liter every 12 h for pathogens with MICs of 2 mg/liter and with a dose of 750 mg/liter every 12 h for pathogens with MICs of 4 mg/liter. Because MICs are </=2 mg/liter for most strains of MRSA, MRSE, and PRP (with some strains showing an MIC of 4 mg/liter), a dosing regimen of 750 mg every 12 h is proposed for clinical studies. The corresponding provisional breakpoint is S (susceptible) </= 4 mg/liter.

How does susceptibility prevalence impact on the performance of disk diffusion susceptibility testing?

Antimicrobial disk diffusion susceptibility testing, devoted in a clinical context to predicting whether an antibiotic regimen will be effective, should be evaluated through predictive values. This approach implies that the susceptibility prevalence (frequency of susceptible, intermediate, and resistant isolates) affects the predictive value of a result. We quantified the influence of the susceptibility prevalence variation on the disk diffusion method performance through a modeling approach. Simulations based on a resampling procedure from two distinct minimum inhibitory concentration/diameter data sets were performed. Experimental variability on minimum inhibitory concentration and diameters was taken into account in the simulations. Results show that the susceptibility prevalence impact depends on the antibiotic and may be significant when prevalence variation is high enough. Consequences of these results on zone diameter breakpoint determination policy are discussed. This implies that the following should be done: (i) consider more rigorously the susceptibility prevalence in studies dealing with zone diameter breakpoint determination and performance evaluation, (ii) re-evaluate disk diffusion breakpoint consistency when the weight of prevalence variation is noteworthy, (iii) estimate consequences of a breakpoint international consensus on prediction quality and appropriate patient management.

Quinolone Resistance among Pneumococci: Therapeutic and Diagnostic Implications

Fluoroquinolones are widely recommended as empirical monotherapy for community-acquired pneumonia. Since 1999, case reports of failure of levofloxacin therapy due to levofloxacin-resistant strains of Streptococcus pneumoniae have started to appear. Most worrying is that, in some cases, levofloxacin resistance has been acquired by pneumococci within days of the initiation of therapy. Because use of current clinical antimicrobial resistance breakpoints fail to identify the majority of S. pneumoniae isolates with only first-step mutations, current treatment guidelines not only may have implications with regard to the ability of surveillance programs to detect emerging resistance but may have therapeutic implications as well.

Laboratory detection of Haemophilus influenzae with decreased susceptibility to nalidixic acid, ciprofloxacin, levofloxacin, and moxifloxacin due to GyrA and ParC mutations

The detection of clinical isolates with decreased fluoroquinolone susceptibilities and a resistance mechanism is of epidemiological and clinical interest. We studied the susceptibilities of 62 clinical isolates and 2 American Type Culture Collection reference strains of Haemophilus influenzae to ciprofloxacin, levofloxacin, moxifloxacin, and nalidixic acid by the microdilution and disk diffusion methods. The ciprofloxacin MICs for 34 of the isolates were >/=0.12 micro g/ml (range, 0.12 to 32 micro g/ml), and the ciprofloxacin MICs for 28 matched control isolates were /=0.5 micro g/ml and the vast majority of those for which nalidixic acid MICs were >/=32 micro g/ml exhibited amino acid changes in GyrA and ParC. Nalidixic acid and the other three fluoroquinolones studied could be used to screen H. influenzae isolates for the detection of decreased susceptibilities to quinolones due to the acquisition of two amino acid changes in the QRDRs of GyrA and ParC (sensitivity, >95%; specificity, >80%).

Impact of pharmacodynamics on breakpoint selection for susceptibility testing

In their report, Ericsson and Sherris stated that "adequately described sensitivity categorization schemes have been based on four main concepts" and went on to describe the advantages and inherent limitations of those four criteria. These limitations are very comparable with what is increasingly viewed as limitations to the current breakpoint systems because those four concepts have always been tried to be brought together in one breakpoint system. In line of this development, the European Committee on Antimicrobial Susceptibility Testing has recently recognized that clinical breakpoints based on PK-PD relationships confer a different meaning to resistance than early detection of microorganisms that do not belong to the natural bacterial population, but somehow have acquired resistance mechanisms and have introduced a wild-type cutoff breakpoint conveying a different meaning than clinical breakpoint. Similarly, there is increasing worry on the setting of clinical breakpoints based on frequency distributions because they do not bear an optimal relationship between dose and bacteriologic or clinical effect. This is especially apparent for older drugs because PK-PD was not used until a few years ago. Because dose-effect relationships have now been reasonably well established for most drugs, these should now be used to reappraise current clinical breakpoints.

Antimicrobial susceptibility breakpoints and first-step parC mutations in Streptococcus pneumoniae: redefining fluoroquinolone resistance

Clinical antimicrobial susceptibility breakpoints are used to predict the clinical outcome of antimicrobial treatment. In contrast, microbiologic breakpoints are used to identify isolates that may be categorized as susceptible when applying clinical breakpoints but harbor resistance mechanisms that result in their reduced susceptibility to the agent being tested. Currently, the National Committee for Clinical Laboratory Standards (NCCLS) guidelines utilize clinical breakpoints to characterize the activity of the fluoroquinolones against Streptococcus pneumoniae. To determine whether levofloxacin breakpoints can identify isolates that harbor recognized resistance mechanisms, we examined 115 S. pneumoniae isolates with a levofloxacin MIC of >2 mg/mL for first-step parC mutations. A total of 48 (59%) of 82 isolates with a levofloxacin MIC of 2 mg/mL, a level considered susceptible by NCCLS criteria, had a first-step mutation in parC. Whether surveillance programs that use levofloxacin data can effectively detect emerging resistance and whether fluoroquinolones can effectively treat infections caused by such isolates should be evaluated.