False non-susceptible results of tigecycline susceptibility testing against Enterobacteriaceae by an automated system: a multicentre study

New Option for Antibiotic Susceptibility Testing in Clinical Practice: Performance Evaluation of AutoMic-i600 Automatic System Based on Broth Microdilution Method

Background

The emergence of multidrug-resistant bacteria limits antibiotic efficacy, making accurate antimicrobial susceptibility testing (AST) essential for clinical decisions. Broth microdilution (BMD) is the gold standard but is impractical for routine application. Current automated AST systems improve efficiency but face accuracy or operational challenges, highlighting the need for reliable and user-friendly solutions.

Objective

This study aims to evaluate the performance of a novel automated AST system (AutoMic-i600) based on the BMD method for AST of common clinical bacteria.

Methods

A total of 229 clinical isolates (150 Gram-negative and 79 Gram-positive) were prospectively collected from microbiology laboratory between June 2023 and August 2023. We reported the comparison of the AutoMic-i600 and Vitek 2 systems for routine antibiotics, and also validated the detection performance of AutoMic-i600 for novel antibiotics, based on the BMD method.

Results

The overall essential agreement (EA) and categorical agreement (CA) between AutoMic-i600 and BMD were 93.2% and 93.5% for Gram-negative bacteria and 98.5% and 97.8% for Gram-positive bacteria, respectively. The overall EA and CA between Vitek 2 and BMD were 92.6% and 93.5% for Gram-negative bacteria and 97.9% and 97.4% for Gram-positive bacteria. Importantly, for drug-resistant bacteria, AutoMic-i600 demonstrated a higher overall agreement than Vitek 2 (EA: 98.1% vs 94.8%, CA: 97.5% vs 92.0%), especially in Gram-negative bacteria (EA: 97.7% vs 93.5%, CA: 97.7% vs 89.3%). The VME rate for Gram-negative bacteria using AutoMic-i600 was significantly lower than that of Vitek 2 (1.0% vs 2.9%). Novel antibiotics detected by AutoMic-i600 exhibited EA and CA rates exceeding 90.0%.

Conclusion

Based on these findings, we recommend that the AutoMic-i600 system could be a new option for routine AST testing in a clinical setting. Particularly for drug-resistant bacteria and novel antibiotics, detection with AutoMic-i600 may be more reliable, which could further contribute to the prevention and treatment of drug-resistant bacteria.

A modified Kirby-Bauer disc diffusion (mKB) method for accurately testing tigecycline susceptibility: a nation-wide multicenter comparative study

Introduction. Tigecycline is one of the important antibiotics available for treating infection caused by multiple-drug resistant pathogens. However, the conventional AST methods which are commonly used in clinical microbiology laboratories usually lead to false intermediate or resistant results in testing tigecycline susceptibility, and further mislead clinical antimicrobial therapies.Hypothesis. The modified Kirby-Bauer disc diffusion (mKB) method was performed based on the traditional standard Kirby-Bauer disc diffusion (sKB) method.Aim. To evaluate a modified Kirby-Bauer disc diffusion (mKB) method for tigecycline susceptibility testing, for the purpose of providing accurate tigecycline susceptibility results in clinical practice.Methodology. A total of 4271 nonduplicate clinical strains were isolated from 37 hospitals across China to perform the mKB method, standard Kirby-Bauer disc diffusion (sKB) method, comparing with the reference broth microdilution (BMD) according to the CLSI. Parameters of categorical agreement (CA), minor errors (mE), major errors (ME), and very major errors (VME) were used in this methodological evaluation research.Results. BMD testing showed that 91.3-98.9 % of the A. baumannii, K. pneumoniae, E. coli, E. cloacae, S. marcescens, and C. freundii strains were susceptible, while 0-3.1% strains were resistant to tigecycline. When testing A. baumannii, mKB demonstrated higher CA than sKB (90.6 % vs 44.8 %) compared to reference BMD. The mE (9.0 % vs 45.2 %), ME (0.5 % vs 10.6 %) and VME (both 0 %) all satisfied the acceptability criteria. mKB also showed higher CA (87.2 % vs 52.0 %) than sKB in comparison with BMD when testing Enterobacterales (mainly K. pneumonia). The ME (0.45 % vs 8.1 %) and VME (both 0 %) but not mE (12.4 % vs 40.4 %) met the acceptability criteria.Conclusion. The mKB method can test bacterial susceptibility to tigecycline more accurately than sKB. For the tigecycline-intermediate or -resistant strains by sKB method, BMD or mKB method should be used to verify the results and report reliable tigecycline susceptibility results.

Comparative Evaluation of Seven Tigecycline Susceptibility Testing Methods for Carbapenem-Resistant Enterobacteriaceae

Purpose

Carbapenem-resistant Enterobacteriaceae (CRE) strains are extensively resistant to most antibiotics. Tigecycline is one of the few effective drugs that can be used to treat infections caused by CRE. The aim of this study was to evaluate the accuracy of different methods for detecting the susceptibility of CRE to tigecycline.

Methods

Seven commonly used drug susceptibility testing methods were compared and evaluated for the ability to determine CRE tigecycline susceptibility: broth microdilution (BMD), agar dilution method (ADM), disk diffusion method, Etest, MicroScan, Vitek2 COMPACT, and BD Phoenix 100.

Results

The minimum inhibitory concentration (MIC) of tigecycline to inhibit 50% and 90% of CRE growth (MIC₅₀ and MIC₉₀, respectively) assessed by ADM and BD Phoenix 100 was the same as that determined by the reference method, BMD. The MIC₅₀ was 2 µg/mL, and the MIC₉₀ was 4 µg/mL. The highest number of susceptible strains was detected by MicroScan, followed by BMD, Etest, ADM, BD Phoenix 100, Vitek2 COMPACT, and disk diffusion method, in descending order. No significant differences were observed among the tigecycline susceptibility results (P > 0.05) obtained from MicroScan, Etest, BD Phoenix 100, and BMD. BMD confirmed that 82.0% of strains were susceptible to tigecycline. ADM, MicroScan, and BD Phoenix 100 yielded the categorical agreement of 96%, 92%, and 93%, respectively. No method was found to present any very major errors (VMEs), and only the Vitek2 COMPACT yielded major errors (MEs) greater than 3%.

Conclusion

Among the seven methods tested, the ADM, MicroScan, and BD Phoenix 100 methods were accurate for determining the tigecycline susceptibility of CRE. MicroScan was acceptable with better performance than other methods.

Performance of VITEK 2, E-test, Kirby-Bauer disk diffusion, and modified Kirby-Bauer disk diffusion compared to reference broth microdilution for testing tigecycline susceptibility of carbapenem-resistant K. pneumoniae and A. baumannii in a multicenter study in China

Tigecycline is an alternative antibiotic for managing carbapenem-resistant Gram-negative bacterial infections. However, disk diffusion and automated testing often show false-intermediate or false-resistant results in tigecycline susceptibility, misleading clinical antimicrobial therapy. Broth microdilution (BMD) is the reference method for testing tigecycline susceptibility, but it is labor intensive and time consuming to perform in clinical laboratories. Therefore, a simple and accurate method is urgently needed. We evaluated the performance of VITEK 2, E-test, Kirby-Bauer disk diffusion (KB), and modified KB disk diffusion (mKB) versus BMD in testing tigecycline susceptibility of 372 strains of carbapenem-resistant Klebsiella pneumoniae (CRKP) and 346 strains of carbapenem-resistant Acinetobacter baumannii (CRAB). BMD confirmed that 96.8% of CRKP and 91% of CRAB strains were susceptible to tigecycline. E-test, VITEK 2, KB, and mKB yielded categorical agreement of 96.7/59.3%, 69.9/54.3%, 78.5/87.3%, and 96.5%/91% for CRKP/CRAB, respectively. No very major error was found for either CRKP or CRAB by any method. No major error was found for CRKP or CRAB by the mKB method. The mKB method enhanced by R-buffer is simple, accurate, and inexpensive for clinical laboratories to test the susceptibility of CRKP and CRAB isolates to tigecycline.

Bad bug, no test: Tigecycline susceptibility testing challenges and way forward

Tigecycline is a reserve antibiotic increasingly used for the treatment of multidrug-resistant bacteria, especially Klebsiella pneumoniae and Acinetobacter baumannii. At present, there are concerns regarding the testing and interpretation of tigecycline susceptibility to bugs such as K. pneumoniae and A. baumannii, which limit clinicians in appropriate usage. Use of appropriate method for testing such as broth microdilution is essential. In addition, tigecycline susceptibility testing is a challenge due to inconsistent results from various antimicrobial susceptibility testing automated platforms. There is a great need to define a suitable methodology along with interpretive criteria, especially for K. pneumoniae and A. baumannii. The European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the Food and Drug Administration (FDA) breakpoints show wide variation and are defined for different set of organisms. Non-species-related pharmacokinetic/pharmacodynamic (PK/PD) breakpoints defined by the EUCAST can be used for organisms such as K. pneumoniae and A. baumannii.

How to accelerate antimicrobial susceptibility testing

BACKGROUND

Antimicrobial susceptibility testing (AST) results are crucial for timely administration of effective antimicrobial treatment, and, thus, should be made available to clinicians as fast as possible. In particular, increasing rates of multidrug-resistant organisms emphasize the need for rapid AST (rAST).

OBJECTIVES

This article aims to provide microbiologists and clinicians with a critical overview of the current state of possibilities to accelerate AST. We also intend to discuss technical and strategic aspects of rAST, which may be helpful to academic researchers and assay developers in the industry.

SOURCES

We have reviewed literature on rAST methods and their implementation in routine diagnostics.

CONTENT

Phenotypic rAST is universal, mechanism-independent and allows exact categorization, but it demands time for the microorganisms to start the growth and to express the response to antibiotics. Detection of selected resistance mechanisms is more rapid, but the interpretation of its clinical impact is limited. Technical challenges of phenotypic rAST include inoculum effect, delayed expression of resistance, lag phase and initial biomass increase in susceptible isolates. Criteria for a successful rAST assay are ease of use, random access, capacity for simultaneous testing of multiple specimens, affordability and financial attractiveness for industry. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS)-based AST seems to be particularly promising, as it can optimally be combined with MALDI-TOF MS identification. Direct testing from clinical specimens provides particularly early findings, with positive blood cultures being the most suitable specimen type. Polymicrobial samples and inoculum effect are serious obstacles for direct AST from other clinical specimens. Next to the technology improvement, optimization of pre-analytics and laboratory organization is essential.

IMPLICATIONS

It appears feasible to generate an AST report within the same working shift; however, only affordable and easy-to-use rAST technologies have a chance to enter broad diagnostic routine. Efforts should be made by industry, authorities and academia to enable wide dissemination of rAST in clinical diagnostics.

Susceptibility of MDR Pseudomonas aeruginosa to ceftolozane/tazobactam and comparison of different susceptibility testing methods

Background

Infections caused by MDR Pseudomonas aeruginosa are on the rise, particularly in critically ill patients. Therefore, there is a need to evaluate new antimicrobial regimens. The objectives of this study were to investigate the ceftolozane/tazobactam resistance rates of MDR and XDR P. aeruginosa, the underlying resistance genes, the clonal structure and different antimicrobial susceptibility testing (AST) methods regarding their accuracy for ceftolozane/tazobactam testing.

Methods

In total, 112 MDR and XDR P. aeruginosa (from infection and colonization) from one German tertiary care hospital were included (2013-16). AST was done using broth microdilution (BMD), gradient diffusion test strips and disc diffusion. Resistance genes were screened by PCR. A randomly selected subset of 77 isolates was subjected to WGS to assess the clonal structure.

Results

In total, 38 isolates (33.9%) were resistant to ceftolozane/tazobactam according to the BMD reference method. Resistance was significantly lower in MDR P. aeruginosa (4.8%) compared with XDR P. aeruginosa (50%, P < 0.0001). The underlying mechanism in carbapenemase-positive ceftolozane/tazobactam-resistant isolates (n = 38) was blaIMP (n = 25), blaVIM (n = 4) and blaGES (n = 1). The resistance mechanism of the remaining eight ceftolozane/tazobactam-resistant isolates remained unclear. Although our strain collection was diverse, resistance to ceftolozane/tazobactam was almost exclusively associated with MLST ST235. The disc diffusion method was accurate for ceftolozane/tazobactam AST (no false-susceptible results, categorical agreement = 92.9%).

Conclusions

Ceftolozane/tazobactam resistance was low in MDR P. aeruginosa, but higher in XDR P. aeruginosa. The disc diffusion method showed an acceptable accuracy for ceftolozane/tazobactam AST.

Comparison of tigecycline susceptibility testing methods for multidrug-resistant Acinetobacter baumannii

Automated systems Vitek 2, BD Phoenix, MicroScan WalkAway and gradient diffusion assays Etest and MIC Test Strip were compared against broth microdilution for susceptibility testing of tigecycline against Acinetobacter baumannii. BD Phoenix and MIC Test Strip seemed to provide the most accurate determination of tigecycline susceptibility.

In vitro susceptibility to 19 agents other than β-lactams among third-generation cephalosporin-resistant Enterobacteriaceae recovered on hospital admission

Objectives

As part of the multicentre Antibiotic Therapy Optimisation Study, MIC values of 19 non-β-lactam agents were determined for third-generation cephalosporin-resistant Escherichia coli , Klebsiella species and Enterobacter species (3GCREB) isolates collected in German hospitals.

Methods

A total of 328 E. coli , 35 Klebsiella spp. (1 Klebsiella oxytoca and 34 Klebsiella pneumoniae ) and 16 Enterobacter spp. (1 Enterobacter aerogenes and 15 Enterobacter cloacae ) isolates were submitted to broth microdilution antimicrobial susceptibility testing with the MICRONAUT system. MICs of fluoroquinolones (levofloxacin and moxifloxacin), aminoglycosides (gentamicin, tobramycin, amikacin, streptomycin, neomycin and paromomycin), tetracyclines (tetracycline, minocycline and tigecycline), macrolides (erythromycin, clarithromycin and azithromycin) and miscellaneous agents [trimethoprim/sulfamethoxazole, chloramphenicol, nitrofurantoin, colistin and fosfomycin intravenous (iv)] were determined and reviewed against 2016 EUCAST breakpoints.

Results

The MIC of levofloxacin was >2 mg/L for 128 of 328 E. coli and 8 of 35 Klebsiella spp., but only 1 of 16 Enterobacter spp. Rates of resistance to trimethoprim/sulfamethoxazole were high (>70%), except for Enterobacter spp. Rates of resistance to colistin and fosfomycin iv were still low. About 20% of the tested isolates were resistant to chloramphenicol. Only 1 (of 328) E. coli isolate had an MIC of amikacin >16 mg/L and only 33 of 328 E. coli and 1 of 35 Klebsiella spp. had an MIC of tobramycin >4 mg/L, whereas average gentamicin MICs were in general more elevated. A tigecycline MIC >2 mg/L was only found for 1 of 16 Enterobacter spp., but in none of the E. coli or Klebsiella spp. isolates.

Conclusions

Our study gives insight into previously unreported non-β-lactam MIC distributions of 3GCREB isolates.

Activity of minocycline against Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae clinical isolates, with comparison to doxycycline and tigecycline

We examined the in vitro activity of minocycline against 103 Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae isolates and found approximately half had susceptible (26%) or intermediate (26%) MICs. For a subset of 35 isolates, susceptibility was highest to tigecycline (71% FDA vs. 20% EUCAST) followed by minocycline (14%) and then doxycycline (6%).