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

Prevalence of difficult-to-treat resistance in ESKAPE pathogens in a third level hospital in Mexico

Background

Antimicrobial resistance and difficult-to-treat resistance (DTR) in ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) is a threat to human health. The aim of this study was to determine the prevalence of antimicrobial resistance and DTR rates in ESKAPE pathogens over six years in a third-level hospital from Monterrey, Mexico.

Methods

Antimicrobial susceptibility testing was determined by either disk diffusion or broth microdilution in strains from 2018 to 2023. Isolates were screened for carbapenemase genes. Multidrug resistance (MDR), extensively drug resistance (XDR), carbapenem resistance (CR), extended-spectrum cephalosporin-resistance (ESCR), fluoroquinolone resistance (FQR), and DTR were determined.

Results

From 3,239 strains, 48.5% were from respiratory infections, resistance was 87.5% to meticillin in Staphylococcus spp. and 39.8% in S. aureus, and 13.9% to vancomycin in Enterococcus spp. MDR, FQR and ESCR rates were between 54-90% in A. baumannii, 20-60% in Enterobacterales and 17-25% in P. aeruginosa. CR was 85.7% in A. baumannii, 33.3% in P. aeruginosa and <5% in Enterobacterales. Most frequent CR genes were OXA-24/40-like in A. baumannii and NDM and OXA-48 in carbapenem-resistant Enterobacterales. DTR rates were 59.7% in A. baumannii (49.2% in 2018 vs 62.9% in 2023), 8.9% in P. aeruginosa and <3% in Enterobacterales. XDR in A. baumannii was 14.4%.

Conclusions

Antimicrobial resistance rates were high in Gram-negative pathogens. CR and DTR rates were higher in A. baumannii than P. aeruginosa and Enterobacterales. DTR surveillance in healthcare providers should be continuous updating local and regional DTR trends among Gram-negative bacteria.

Tigecycline-Rifampicin Restrains Resistance Development in Carbapenem-Resistant Klebsiella pneumoniae

The goal of this study was to clarify the synergistic antibacterial activity of the combination of tigecycline (TGC) and rifampicin (RIF). Additionally, the study sought to investigate the impact of this combination on the development of mutational resistance and to assess its efficacy in an in vivo model using Galleria mellonella. Through a checkerboard test, we found that the combination of TGC and RIF showed synergistic antibacterial activity against carbapenem-resistant Klebsiella pneumoniae (CRKP). The fractional inhibition concentration index (FICI) was found to be ≤0.5, confirming the potency of the combination. Additionally, this synergistic effect was further validated in vivo using the G. mellonella infection model. TGC-RIF treatment had a lower mutant prevention concentration (MPC) than that of monotherapy, indicating its potential to reduce the development of mutational resistance. We observed a substantial variation in the MPCs of TGC and RIF when they were measured at different proportions in the combinations. Furthermore, during the resistant mutant selection window (MSW) test, we noticed a correlation between strains with low FICI and low MSW. The expression of efflux-pump-related genes, namely rarA and acrB, is significantly decreased in the combination therapy group. This indicates that altered expression levels of certain efflux pump regulator genes are associated with a combined decrease in bacterial mutation resistance. In conclusion, the combination of TGC and RIF effectively suppresses antibiotic resistance selection in CRKP. This study establishes a paradigm for evaluating drug-resistant mutant suppression in antimicrobial combination therapy.

Evaluation of VITEK® 2 AST cards (AST-N376 and AST-N397) for susceptibility testing of challenging Gram negatives

Due to the increasing diffusion of MDR/XDR Gram-negatives it is necessary to offer reliable antibiotic susceptibility testing (AST), which also include new drugs. Here we evaluated the performances of the VITEK®2 AST-N376 and the AST-N397 cards. A collection of 180 clinical Gram-negative bacteria, producing relevant resistance mechanisms, were tested using VITEK 2 and MERLIN, in parallel. Discrepancies between the 2 systems were solved by the reference broth microdilution method. The workflow timing of the VITEK®2 system was also assessed. Overall, the VITEK®2 cards proved to be reliable in determining AST for the molecules evaluated, even if compliance with ISO acceptance criteria for accuracy assessment was not reached for some combinations and showed a short hands-on time for panels preparation. In conclusion, VITEK®2 is a valid system that ensures accurate results for AST of the molecules evaluated in this study and speeds up the workflow in the laboratory of diagnostic microbiology.

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.