Rapid colorimetric polymyxin B microelution directly from positive blood bottles: because patients with serious infections should not have to wait for results of culture-based methodologies.

PURPOSE: To evaluate the performance of the rapid colorimetric polymyxin B microelution (RCPEm) in determining polymyxin B resistance directly from Enterobacterales-positive blood cultures. METHODS: A set volume of positive blood culture bottles (diluted 1:10) was inoculated into a glucose-broth-phenol red solution (NP solution), where a polymyxin B disk was previously eluted (final concentration of 3 µg/mL). Test was read each 1 h for up to 4 h. Color change from red/orange to yellow indicated resistant isolates. Results were compared to the reference method, broth microdilution (BMD), performed from colonies grown on solid media from the same blood culture bottle. RESULTS: One hundred fifty-two Enterobacterales-positive blood cultures were evaluated, 22.4% (34/152) of them resistant to polymyxin B (including 6.6% with borderline MICs). When performing directly from positive blood cultures (RCPEm-BC), specificity and sensitivity were 99.1% and 94.1%, respectively. Of note, 79.4% (27/34) of truly resistant isolates required 3 h of incubation, compared to the 18 ± 2 h incubation that microtiter plates of BMD demand before reading can be performed. CONCLUSIONS: RCPEm directly from blood cultures has great potential to be part of the routine of clinical microbiology laboratories to establish polymyxin B susceptibility, impacting outcome of patients with bloodstream infections caused by carbapenem-resistant Enterobacterales.

Detection of polymyxins resistance among Enterobacterales: evaluation of available methods and proposal of a new rapid and feasible methodology.

BACKGROUND: Fast and accurate detection of polymyxins resistance is necessary as they remain the last resources to treat infections caused by Carbapenem-resistant Enterobacterales in many regions. We evaluated the rapid colorimetric polymyxin B elution (RCPE) and developed its miniaturized version, RCPE microelution (RCPEm), aiming to detect polymyxins resistance among Enterobacterales. METHODS: The methodologies consist of exposing the bacterial population in a solution (NP solution) where polymyxin B disks were previously eluted to obtain a concentration of 2 µg/mL for RCPE and 3 µg/mL for RCPEm. RESULTS: Two hundred sixty-seven Enterobacterales were evaluated, 90 (33.7%) resistant to polymyxin B by broth microdilution. It was observed 0.6% of major error (ME) by RCPE, with a specificity of 99.4%. The miniaturized version (RCPEm) presented the same ME and specificity values, but slightly higher sensitivity (97.8% vs. 95.6%) with 2.2% of very major error (VME). CONCLUSIONS: RCPE and RCPEm proved to be useful alternatives to determine polymyxin B susceptibility in clinical microbiology laboratories, presenting low cost, being easy to perform, and demanding short incubation time.

Determination of aztreonam/ceftazidime-avibactam synergism and proposal of a new methodology for the evaluation of susceptibility in vitro.

We proposed a new methodology, the microelution ATM/CZA (mATM/CZA), based on the antibiotic disc elution and the use of resazurin, for rapid (<4h) determination of in vitro susceptibility to aztreonam combined with ceftazidime-avibactam among Enterobacterales. The mATM/CZA presented excellent accuracy with 1.9 %, 98.1 % and 100 % of major error, specificity and sensitivity, respectively. Furthermore, we assessed synergism between aztreonam and ceftazidime-avibactam in Enterobacterales and Pseudomonas aeruginosa, which was observed in 37/55 Enterobacterales and 31/56 P. aeruginosa. As reference methodologies (checkerboard, time-kill curve) are not compatible with the routine of the clinical microbiology laboratories, mATM/CZA is an important alternative to evaluate susceptibility of the combination in a scenario where its clinical use is increasingly important.

Detection of KPC directly from positive blood cultures by MALDI-TOF: From research to the clinical microbiology laboratory routine.

Bloodstream infections (BSI) caused by carbapenem-resistant Gram-negative bacilli (CR-GNB) are a subject of major clinical concern, mainly those associated with carbapenemase-producing isolates. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) has been proposed to detect specific ß-lactamases, including KPC. We aimed to detect KPC enzyme directly from positive blood cultures using MALDI-TOF MS. Overall, 146 clinical Gram-negative bacilli (46 CR-GNB) recovered from consecutive blood cultures were evaluated. Proteins were extracted using formic acid, isopropyl alcohol, and water and spotted onto a steel target plate using the double-layer sinapinic acid method. The relative ions intensity ≥120 arbitrary units (a.u.) of a peak close to 28,700 m/z indicated the presence of KPC. The results were compared to HRM-qPCR methodology. This specific peak was observed in 11/14 blood bottles with blaKPC positive isolates (78.6% sensitivity), with 3 false-positive results (97.7% specificity). Analysis from colonies reached identical sensitivity (78.6%), but higher specificity (100%). The detection of KPC peaks directly from positive blood cultures using MALDI-TOF MS is feasible and rapid. It's excellent specificity indicates that positive results are consistently associated with the presence of a KPC producer in positive blood culture.

Evaluation of EDTA and Dipicolinic Acid in Broth Microdilution with Polymyxin B as a Phenotypic Test to Detect the mcr-1 Gene.

Polymyxins (colistin and polymyxin B) have recently regained significant importance as last-line drugs to treat infectious diseases due to multidrug-resistant gram-negative bacteria. However, resistance to polymyxins has increased, and the recognition of plasmid-mediated resistance (by the mcr gene) has led to an epidemiological concern. We aimed to evaluate the reduction of the polymyxin B minimum inhibitory concentration (MIC) in the presence of EDTA or dipicolinic acid (DPA) by using the broth microdilution (BMD) method for phenotypic screening of acquired polymyxin resistance mediated by the mcr-1 gene. Overall, 94 Enterobacterales (48 polymyxin-resistant and 46 polymyxin-susceptible) were evaluated: 47 mcr-1 positive (36 Escherichia coli, 2 Klebsiella pneumoniae, and 9 Salmonella spp.) and 47 mcr-1 negative (3 E. coli and 44 K. pneumoniae-27 isolates with MIC from ≤0.125 to 8 µg/mL and 20 isolates with MIC from 16 to 64 µg/mL). Results were categorized as positive when the chelator decreased the original BMD MIC by ≥2 logs. The majority (95.7%) of mcr-1 positive isolates displayed at least a 3 log dilution decrease in the MIC of polymyxin B with EDTA or DPA. The EDTA-based BMD assay detected 45 mcr-1-positive isolates, with only one false-positive among the mcr-1-negative isolates (sensitivity [SN], 95.7%; specificity [SP], 97.9%), whereas the DPA-based BMD assay detected 44 mcr-1-positive isolates (SN, 93.6%; SP, 95.7%), with two false-positive results. The accuracy of EDTA- and DPA-based BMD assays were 97% and 95%, respectively. The EDTA- and DPA-based assays were demonstrated to be reliable methods to detect mcr-1 positive isolates with excellent accuracy.