Breakpoint beware: reliance on historical breakpoints for Enterobacteriaceae leads to discrepancies in interpretation of susceptibility testing for carbapenems and cephalosporins and gaps in detection of carbapenem-resistant organisms

Comparison of BD Phoenix and disk diffusion to broth microdilution for determining cefepime susceptibility among carbapenem-resistant Enterobacterales

There are increasing reports of carbapenem-resistant Enterobacterales (CRE) that test as cefepime-susceptible (S) or susceptible-dose dependent (SDD). However, there are no data to compare the cefepime testing performance of BD Phoenix automated susceptibility system (BD Phoenix) and disk diffusion (DD) relative to reference broth microdilution (BMD) against carbapenemase-producing (CPblaKPC-CRE) and non-producing (non-CP CRE) isolates. Cefepime susceptibility results were interpreted according to CLSI M100Ed32. Essential agreement (EA), categorical agreement (CA), minor errors (miEs), major errors (MEs), and very major errors (VMEs) were calculated for BD Phoenix (NMIC-306 Gram-negative panel) and DD relative to BMD. Correlates were also analyzed by the error rate-bounded method. EA and CA for CPblaKPC-CRE isolates (n = 64) were <90% with BD Phoenix while among non-CP CRE isolates (n = 58), EA and CA were 96.6%, and 79.3%, respectively. CA was <90% with DD for both cohorts. No ME or VME was observed for either isolate cohort; however, miEs were >10% for CPblaKPC-CRE and non-CP CRE with BD Phoenix and DD tests. For error rate-bounded method, miEs were <40% for IHigh + 1 to ILow - 1 ranges for CPblaKPC-CRE and non-CP CRE with BD Phoenix. Regarding disk diffusion, miEs were unacceptable for all MIC ranges among CPblaKPC-CRE. For non-CP CRE isolates, only IHigh + 1 to ILow - 1 range was acceptable at 37.2%. Using this challenge set of genotypic-phenotypic discordant CRE, the BD Phoenix MICs and DD susceptibility results trended higher (toward SDD and resistant phenotypes) relative to reference BMD results yielding lower CA. These results were more prominent among CPblaKPC-CRE than non-CP CRE.

Cefepime in vivo activity against carbapenem-resistant Enterobacterales that test as cefepime susceptible or susceptible-dose dependent in vitro: implications for clinical microbiology laboratory and clinicians

BACKGROUND

Carbapenem-resistant Enterobacterales (CRE) are a public health concern. Among these isolates, there are reports of isolates that test as cefepime susceptible or susceptible-dose dependent (SDD) in vitro despite presence of a carbapenemase. This study aimed to evaluate the pharmacokinetic/pharmacodynamic profile of cefepime against carbapenemase-producing (CP-CRE) and non-producing (non-CP-CRE) isolates with a range of cefepime MICs.

METHODS

Reference broth microdilution and modified carbapenem inactivation method (mCIM) were performed on genotypically characterized clinical CRE isolates. Ultimately, CP-CRE (n = 21; blaKPC) and non-CP-CRE (n = 19) isolates with a distribution of cefepime MICs (≤0.5 to >256 mg/L) were utilized in the murine thigh infection model. Mice were treated with cefepime human-simulated regimens (HSRs) representative of a standard dose (1 g q12h 0.5 h infusion) or the SDD dose (2 g q8h 0.5 h infusion). Efficacy was assessed as the change in bacterial growth at 24 h compared with 0 h control, where ≥1 log bacterial reduction is considered translational value for clinical efficacy.

RESULTS

Among both cohorts of CRE isolates, i.e. CP-CRE and non-CP-CRE, that tested as SDD to cefepime in vitro, 1 log bacterial reduction was not attainable with cefepime. Further blunting of cefepime efficacy was observed among CP-CRE isolates compared with non-CP-CRE across both susceptible and SDD categories.

CONCLUSIONS

Data indicate to avoid cefepime for the treatment of serious infections caused by CRE isolates that test as cefepime susceptible or SDD. Data also provide evidence that isolates with the same antibiotic MIC may have different pharmacokinetic/pharmacodynamic profiles due to their antimicrobial resistance mechanism.

Synergistic action of indole-3-carbinol with membrane-active agents against multidrug-resistant Gram-negative bacteria

The purpose of this study was to evaluate the antimicrobial activity of indole-3-carbinol (I3C) with membrane-active agents, namely carbonyl cyanide 3-chlorophenylhydrazone (CCCP) and ethylenediaminetetraacetic acid (EDTA) against multidrug-resistant (MDR) Gram-negative bacteria and bacterial persisters. The determination of minimal inhibitory concentration (MIC) showed that I3C was effective against Acinetobacter baumannii (3.13‒6.25 × 10-3 mol l-1), Klebsiella pneumoniae (8 × 10-3 mol l-1), Pseudomonas aeruginosa (6.25‒12.5 × 10-3 mol l-1), and Escherichia coli (6.25‒12.5 × 10-3 mol l-1). Our study demonstrated that EDTA synergistically enhanced the bactericidal activity of I3C against most MDR Gram-negative bacteria isolates and contributed to an 8- to 64-fold MIC reduction compared with that of I3C alone, yet CCCP only displayed synergy with I3C against P. aeruginosa and A. baumannii. The EDTA-I3C combination also significantly reduced the viable number of testing bacteria (P = 7.2E-05), effectively reduced bacterial persisters, and repressed bacterial growth compared with that the use of I3C alone. Our data demonstrate that use of EDTA as adjuvant molecules can effectively improve the antibacterial activity of I3C and may help to reduce the development of antimicrobial resistance.

Development of Modernized Acinetobacter baumannii Susceptibility Test Interpretive Criteria for Recommended Antimicrobial Agents Using Pharmacometric Approaches

Acinetobacter baumannii-Acinetobacter calcoaceticus complex (referred to herein as A. baumannii) treatment guidelines contain numerous older antimicrobial agents with susceptibility test interpretive criteria (STIC, also known as susceptibility breakpoints) set using only epidemiological data. We utilized a combination of in vitro surveillance data, preclinical murine thigh and lung infection models, population pharmacokinetics, simulation, and pharmacokinetic/pharmacodynamic (PK/PD) target attainment analyses to evaluate A. baumannii STIC for four commonly recommended antimicrobials from different classes (amikacin, ceftazidime, ciprofloxacin, and minocycline). Antimicrobial in vitro surveillance data were based on 1,647 clinical A. baumannii isolates obtained from 109 centers in the United States and Europe. Among these isolates, 5 were selected for evaluation in murine infection models based on fitness and MIC variability. PK and dose-ranging studies were conducted using neutropenic murine thigh and lung infection models The MIC ranges for the 5 isolates evaluated were as follows: amikacin, 2 to 32 μg/mL; ceftazidime, 4 to 16 μg/mL; ciprofloxacin, 0.12 to 2 μg/mL; minocycline, 0.25 to 4 μg/mL. All organisms grew ≥1.5 log10 CFU in both models in untreated controls. Plasma and epithelial lining fluid (ELF) pharmacokinetics for all drugs were determined in mice using liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods. For each isolate, 5 dose levels of each drug were tested individually in the thigh and lung infection model. The inoculum ranged from 7.9 to 8.4 and 6.8 to 7.7 log10 CFU/mL for the lung and thigh models, respectively. PK/PD targets associated with net bacterial stasis and 1- and 2-log10 CFU reductions from baseline were identified for each organism/infection model using Hill-type models. Population pharmacokinetic models for each agent were identified from the literature. Using demographic variables for simulated patients with hospital-acquired or ventilator-associated bacterial pneumonia or urinary tract infections (including acute pyelonephritis) who were administered maximal dosing regimens of each agent, estimates of protein binding, and ELF penetration ratios based on data from the literature, free-drug plasma and total-drug concentration-time profiles were generated, and PK/PD indices by MIC were calculated. Percent probabilities of attaining median and randomly assigned PK/PD targets associated with the above-described endpoints were determined. Recommended susceptible breakpoints for each agent were those representing the highest MIC at which the percent probabilities of achieving PK/PD targets associated with a 1-log10 CFU reduction from baseline approached or were ≥90%. The following susceptible breakpoints for A. baumannii were identified: amikacin, ≤8 μg/mL for pneumonia; ceftazidime, ≤32 and ≤8 μg/mL for pneumonia; ciprofloxacin, ≤1 μg/mL; and minocycline, ≤0.5/≤1 μg/mL which correspond to the standard and high minocycline dosing regimens of 200 mg per day and 200 mg every 12 h, respectively. Implementation of appropriate STIC will help clinicians optimally use the above-described agents and improve the likelihood of successful patient outcomes.

The Practical Problem With Carbapenem Testing and Reporting Accurate Bacterial Susceptibilities

Background: Antibiotic resistance is an evolving issue which requires constant review. Susceptibility breakpoints are revised in line with new microbiological and pharmacological data. Susceptibility breakpoints for carbapenems and Enterobacterales were revised in response to the rise in resistance and the potential for standard doses of carbapenems to provide the necessary antibiotic exposure and to accurately identify rates of carbapenem resistance.

Objectives: This review sought to identify real-world implications associated with lack of testing and reporting current carbapenem breakpoints and potential barriers that may impede implementation of these strategies.

Methods: A literature review was conducted using PubMed and Google Scholar electronic databases.

Results: The failure to adopt revised breakpoints incurs negative clinical outcomes and carries increased cost implications. However, there were several impediments highlighted which are barriers for laboratories to implement breakpoint updates.

Conclusion: Possible practical steps to implement revised breakpoints which apply to carbapenems and Enterobacterales are proposed. The challenge for laboratories is to be aware and implement these changes to provide accurate and relevant susceptibility results for clinicians.

The Collateral Effects of COVID-19 Pandemic on the Status of Carbapenemase-Producing Pathogens

The serious challenge of antimicrobial resistance continues to threaten public health and lingers in the era of the coronavirus disease 2019 (COVID-19), declared pandemic by the World Health Organization. While the pandemic has triggered the importance of infection control practices and preventive measures such as physical distancing, hand hygiene, travel reduction and quarantine, the ongoing alarm of antimicrobial resistance seems to accompany the pandemic too. Antimicrobial resistance has been fostered during COVID-19, possibly due to high rate of empirical antibiotic utilization in COVID-19 patients, increased use of biocides, and the disruption of proper healthcare for other conditions. Specifically, carbapenemase-producing Gram-negative bacteria have shown to cause secondary bacterial infections in patients hospitalized for COVID-19. Clinical and microbiological evidence of such infections is accumulating in different parts of the world. With the resilient nature of carbapenemases, their association with mortality, and the limited treatment options available, concerns regarding this group of antibiotic-hydrolyzing enzymes during the pandemic are expected to upsurge. While the additional burden carbapenemases exert on healthcare is worrisome, it remains hidden or abandoned among the various health consequences of the pandemic. The purpose of this minireview is to shed a light on carbapenemase-associated infections during such unprecedented time of COVID-19. A focused insight shall be made into carbapenemases, their implications for COVID-19 patients, and the features and consequences of co-infection, with a review of available evidence from pertinent literature. The importance of increased surveillance for carbapenemase-producers and optimizing their management in relation to the pandemic, shall be addressed as well.

Raising the Bar: Improving Antimicrobial Resistance Detection by Clinical Laboratories by Ensuring Use of Current Breakpoints

Background

Antimicrobial resistance (AMR) is a pressing global challenge detected by antimicrobial susceptibility testing (AST) performed by clinical laboratories. AST results are interpreted using clinical breakpoints, which are updated to enable accurate detection of new and emerging AMR. Laboratories that do not apply up-to-date breakpoints impede global efforts to address the AMR crisis, but the extent of this practice is poorly understood.

Methods

A total of 1490 clinical laboratories participating in a College of American Pathologists proficiency testing survey for bacterial cultures were queried to determine use of obsolete breakpoints.

Results

Between 37.9% and 70.5% of US laboratories reported using obsolete breakpoints for the antimicrobials that were queried. In contrast, only 17.7%-43.7% of international laboratories reported using obsolete breakpoints (P < .001 for all comparisons). Use of current breakpoints varied by AST system, with more laboratories reporting use of current breakpoints in the US if the system had achieved US Food and Drug Administration clearance with current breakpoints. Among laboratories that indicated use of obsolete breakpoints, 55.9% had no plans to update to current standards. The most common reason cited was manufacturer-related issues (51.3%) and lack of internal resources to perform analytical validation studies to make the update (23.4%). Thirteen percent of laboratories indicated they were unaware of breakpoint changes or the need to update breakpoints.

Conclusions

These data demonstrate a significant gap in the ability to detect AMR in the US, and to a lesser extent internationally. Improved application of current breakpoints by clinical laboratories will require combined action from regulatory agencies, laboratory accreditation groups, and device manufacturers.

The antibiogram: key considerations for its development and utilization

The antibiogram is an essential resource for institutions to track changes in antimicrobial resistance and to guide empirical antimicrobial therapy. In this Viewpoint, data and examples from literature are presented that suggest institutions have not completely adopted the standardized approach in developing antibiograms, as variations in the development methodologies of antibiograms exist despite consensus guidelines (M39) published by CLSI. We emphasize developing antibiograms in line with the M39 recommendations will help ensure that they are accurate, reliable and valid, and highlight that understanding the limitations of antibiogram data is critical to ensuring appropriate interpretation and application to clinical decision-making. We also stress the importance of easy accessibility and education on antibiogram use, to allow for prescribers to select the most optimal empirical treatment regimens and propose the creation of an abbreviated antibiogram for frontline users. Multidisciplinary antimicrobial stewardship programmes are vital to accomplishing these goals.