Biomolecules capturing live bacteria from clinical samples.

Rapid phenotypic antimicrobial susceptibility testing (AST) requires the enrichment of live bacteria from patient samples, which is particularly challenging in the context of life-threatening bloodstream infections (BSIs) due to low bacterial titers. Over two decades, an extensive array of pathogen-specific biomolecules has been identified to capture live bacteria. The prevailing biomolecules are immune proteins of the complement system, antibodies, aptamers, phage proteins, and antimicrobial peptides. These biomolecules differ by their binder generation technologies and exhibit highly variable specificities, ranging from bacterial strains to most pathogenic bacteria. Here, we summarize how these diverse biomolecules were identified, list examples of successfully reported capture assays, and provide an outlook on the use of nanobodies raised against conserved surface-accessible proteins as promising biomolecules for pathogen capture.

Disseminated, fatal reactivation of bovine tuberculosis in a patient treated with adalimumab: a case report and review of the literature.

PURPOSE: Tumor necrosis factor inhibitors (TNFi) are known to increase the risk of tuberculosis (TB) reactivation, though cases involving Mycobacterium bovis are rarely reported. CASE PRESENTATION/RESULTS: We describe a case of disseminated TB with M. bovis in a 78-year-old woman with a negative Interferon-Gamma-Release Assay (IGRA), taking adalimumab due to rheumatoid polyarthritis, which resulted in a fatal outcome. The atypical clinical and histopathological features were initially interpreted as sarcoidosis. The case occurred in Switzerland, an officially bovine tuberculosis-free country. The whole genome sequence of the patient's cultured M. bovis isolate was identified as belonging to the animal lineage La1.2, the main genotype in continental Europe, but showed significant genetic distance from previously sequenced Swiss cattle strains. In a literature review, four cases of bovine tuberculosis reactivation under TNFi treatment were identified, with pulmonal, oral and intestinal manifestations. Similar to our patient, two cases presented a negative IGRA before TNFi initiation, which later converted to positive upon symptomatic presentation of M. bovis infection. CONCLUSION: This case highlights the diagnostic challenges of TB in immunosuppressed patients, the limited sensitivity of IGRA, and the importance of considering TB reactivation even in regions declared free of bovine tuberculosis. Detailed patient histories, including potential exposure to unpasteurized dairy products, are essential for guiding preventive TB treatment before TNFi initiation.

Evaluation of Phenotypic Tests to Detect Extended-Spectrum ß-Lactamase (ESBL)-Producing Klebsiella oxytoca Complex Strains.

Klebsiella oxytoca complex (KoC) species may overproduce their chromosomal class A OXY ß-lactamases, conferring reduced susceptibility to piperacillin-tazobactam, expanded-spectrum cephalosporins and aztreonam. Moreover, since clavulanate maintains its ability to inhibit these enzymes, the resulting resistance phenotype may falsely resemble the production of acquired extended-spectrum ß-lactamases (ESBLs). In this work, a collection of 44 KoC strains of human and animal origin was characterized with whole-genome sequencing (WGS) and broth microdilution (BMD) susceptibility testing. Comparison of ESBL producers (n = 11; including CTX-M-15 [n = 6] and CTX-M-1 [n = 5] producers) and hyperproducers of OXYs (n = 21) showed certain phenotypic differences: piperacillin-tazobactam (MIC90s: 16 versus >64 µg/mL), cefotaxime (MIC90s: 64 versus 4 µg/mL), ceftazidime (MIC90s: 32 versus 4 µg/mL), cefepime (MIC90s: 8 versus 4 µg/mL) and associated resistance to non-ß-lactams (e.g., trimethoprim-sulfamethoxazole: 90.9% versus 14.3%, respectively). However, a clear phenotype-based distinction between the two groups was difficult. Therefore, we evaluated 10 different inhibitor-based confirmatory tests to allow such categorization. All tests showed a sensitivity of 100%. However, only combination disk tests (CDTs) with cefepime/cefepime-clavulanate and ceftazidime/ceftazidime-clavulanate or the double-disk synergy test (DDST) showed high specificity (100%, 95.5%, and 100%, respectively). All confirmatory tests in BMD or using the MIC gradient strip did not perform well (specificity, ≤87.5%). Of note, ceftazidime/ceftazidime-avibactam tests also exhibited low specificity (CDT, 87.5%; MIC gradient strip, 77.8%). Our results indicate that standard antimicrobial susceptibility profiles can raise some suspicion, but only the use of cefepime/cefepime-clavulanate CDT or DDST can guarantee distinction between ESBL-producing KoC strains and those hyperproducing OXY enzymes.

Antibacterial Effects of X-ray and MRI Contrast Media: An In Vitro Pilot Study.

Some radiological contrast agents have been shown to have effects on bacterial growth. In this study, the antibacterial effect and mechanism of action of iodinated X-ray contrast agents (Ultravist 370, Iopamiro 300, Telebrix Gastro 300 and Visipaque) and complexed lanthanide MRI contrast solutions (MultiHance and Dotarem) were tested against six different microorganisms. Bacteria with high and low concentrations were exposed to media containing different contrast media for various lengths of time and at pH 7.0 and 5.5. The antibacterial effect of the media was examined in further tests using agar disk diffusion analysis and the microdilution inhibition method. Bactericidal effects were found for microorganisms at low concentrations and low pH. Reductions were confirmed for Staphylococcus aureus and Escherichia coli.

Drug-resistant tuberculosis: advances in diagnosis and management.

PURPOSE OF REVIEW: Diagnosis and treatment of drug-resistant tuberculosis (DR-TB) is undergoing substantial changes, owing availability of new diagnostic tools and drugs, coupled with global underdiagnosis and undertreatment. Recent developments are reviewed. RECENT FINDINGS: Molecular diagnostics, for Mycobacterium tuberculosis complex detection and prediction of drug resistance, implemented in the last decade, accelerated TB diagnosis with improved case detection. Nevertheless, access and coverage of drug-resistance testing remain insufficient. Genome sequencing-technologies, based on targeted next-generation sequencing show early potential to mitigate some of the challenges in the future. The recommendation to use an all oral, bedaquiline based regimen for treatment of multidrug-resistant/rifampicin-resistant TB is major advancement in DR-TB care. TB regimen using new and repurposed TB drugs demonstrate in recent clinical trials like, NIX-TB, ZeNIX and TB PRACTECAL considerable treatment success, shorten treatment duration and reduce toxicity. Their optimal use is threatened by the rapid occurrence and spread of strains, resistant to new drugs. Children benefit only very slowly from the progress. SUMMARY: There is notable progress in improved diagnosis and treatment of drug-resistant TB, but complicated by the COVID-19 pandemic the majority of TB patients worldwide don't have (yet) access to the advances.

Mycobacterium helveticum sp. nov., a novel slowly growing mycobacterial species associated with granulomatous lesions in adult swine.

The occurrence of nontuberculous mycobacteria in different hosts and their implication as obligate or opportunistic pathogens remain mainly unclear. Mycobacteriosis in pigs is usually associated with members of the Mycobacterium avium complex and, in particular, with 'Mycobacterium avium subsp. hominissuis'. Here we describe a novel slow-growing mycobacterial species isolated from lymph nodes obtained from two sows housed in different Swiss farms. The animals presented chronic inappetence and mild diarrhoea. Gross pathology revealed focal caseous lymphadenopathy of the mesenteric lymph nodes. Complete genome sequencing of the two isolates from the two sows was performed. The genomes comprised 5.76 Mb and an average nucleotide identity score of 99.97 %. Whole genome sequence, mycolic acid and matrix-assisted laser desorption ionization-time of flight mass spectrometry analyses revealed that the two isolates were not related to any previously described Mycobacterium species. The closest related species was Mycobacterium parmense, a slow-growing scotochromogenic mycobacterium first isolated from a cervical lymph node of a 3-year-old child. The name proposed for the new species is Mycobacterium helveticum sp. nov. and 16-83T (=DSM 109965T= LMG 2019-02457T) is the type strain.

Rapid Increase of CTX-M-Producing Shigella sonnei Isolates in Switzerland Due to Spread of Common Plasmids and International Clones.

The Swiss Centre for Antibiotic Resistance (ANRESIS) has recently noted an increase of extended-spectrum cephalosporin-resistant (ESC-R) Shigella sonnei isolates nationwide (3.8% in 2016 versus 37.5% in 2019). To understand this phenomenon, we analyzed 25 representative isolates (of which 14 were ESC-R) collected in Switzerland during 2016 to 2019. Whole-genome sequencing was achieved using both the Illumina and the Nanopore platforms. Both ESC-R and extended-spectrum cephalosporin-susceptible isolates belonged to sequence type 152 (ST152). The ESC-R isolates carried blaCTX-M-3 in IncI1-pST57 (n = 5), blaCTX-M-15 in IncFII (F2:A-:B-) (n = 5), blaCTX-M-15 in IncI1-pST16, and blaCTX-M-27, blaCTX-M-55, or blaCTX-M-134 in other IncFII plasmids (n = 1 each). Plasmids having the same bla and Inc group exhibited high degrees of genetic identity to each other but also to plasmids previously reported in other Enterobacterales Core-genome analysis showed that there were 4 main clusters, each of which included strains that differed by <58 single nucleotide variants (SNVs) and that consisted of both blaCTX-M-positive and blaCTX-M-negative isolates. Moreover, most isolates belonging to the same cluster shared an identical core-genome sequence type (cgST). For instance, cluster 1 included 4 isolates of cgST113036, of which only 3 harbored the IncI1-pST57 blaCTX-M-3-positive plasmid. The 25 S. sonnei isolates were also subjected to phylogenetic comparison with deposited international strains. As a result, matching isolates (isolates that had the same cgST and that differed by <8 SNVs) have been reported in the United Kingdom, the United States, France, and the Netherlands. Overall, our results suggest that some common S. sonnei clusters can spread between continents and can be imported into other nations after international trips. Such clusters include, in part, isolates that do not possess blaESBL-harboring plasmids, indicating their tendency to acquire them from other Enterobacterales.

Natural Polymorphisms in Mycobacterium tuberculosis Conferring Resistance to Delamanid in Drug-Naive Patients.

Mutations in the genes of the F420 signaling pathway of Mycobacterium tuberculosis complex, including dnn, fgd1, fbiA, fbiB, fbiC, and fbiD, can lead to delamanid resistance. We searched for such mutations among 129 M. tuberculosis strains from Asia, South America, and Africa using whole-genome sequencing; 70 (54%) strains had at least one mutation in one of the genes. For 10 strains with mutations, we determined the MIC of delamanid. We found one strain from a delamanid-naive patient carrying the natural polymorphism Tyr29del (ddn) that was associated with a critical delamanid MIC.

Mycoplasma penetrans bacteremia in an immunocompromised patient detected by metagenomic sequencing: a case report.

BACKGROUND: Mycoplasma sp. are well recognized as etiological agents of respiratory and sexually transmitted disease. Mycoplasma penetrans, a species of Mycoplasma sp., has been frequently detected in HIV-positive patients and associated with the progression of HIV-associated disease. To date, there is only a single case report describing M. penetrans as the causative agent of a severe respiratory tract infection in a HIV-negative patient. CASE PRESENTATION: In this report, we describe the case of M. penetrans bacteremia in a HIV-negative, 38-year-old, female, immunocompromised, solid organ transplant patient (combined kidney and pancreas transplantation in 2016), who was admitted to our hospital with anemic uterine bleeding and fever of 38.3 °C. Several hours before her admission at our university hospital, a latex bladder catheter was inserted into her uterus and she complained about fatigue, dizziness and ongoing vaginal bleeding. Laboratory examination showed severe anemia, but microbiological examination was inconspicuous (culture negative vaginal and cervical smears, negative urine culture). Bacterial blood cultures showed a growth signal after 4 h, but microscopic examination with Gram staining and subcultures on different agar media did not identify bacterial pathogens. To identify the bacterial cause of malignancy in the patient, metagenomic sequencing of the blood culture was performed that identified M. penetrans. CONCLUSION: Metagenomic sequencing identified M. penetrans in an immunosuppressed patient with culture-negative bacteremia. Clinicians should be aware of the opportunistic potential of M. penetrans that may cause severe infections in certain vulnerable patient populations and the limitations of culture and Gram staining for confirming the presence of fastidious bacterial pathogens like Mycoplasma spp.

Phenotypic and Genomic Analyses of Burkholderia stabilis Clinical Contamination, Switzerland.

A recent hospital outbreak related to premoistened gloves used to wash patients exposed the difficulties of defining Burkholderia species in clinical settings. The outbreak strain displayed key B. stabilis phenotypes, including the inability to grow at 42°C; we used whole-genome sequencing to confirm the pathogen was B. stabilis. The outbreak strain genome comprises 3 chromosomes and a plasmid, sharing an average nucleotide identity of 98.4% with B. stabilis ATCC27515 BAA-67, but with 13% novel coding sequences. The genome lacks identifiable virulence factors and has no apparent increase in encoded antimicrobial drug resistance, few insertion sequences, and few pseudogenes, suggesting this outbreak was an opportunistic infection by an environmental strain not adapted to human pathogenicity. The diversity among outbreak isolates (22 from patients and 16 from washing gloves) is only 6 single-nucleotide polymorphisms, although the genome remains plastic, with large elements stochastically lost from outbreak isolates.

First Clinical Case of In Vivo Acquisition of DHA-1 Plasmid-Mediated AmpC in a Salmonella enterica subsp. enterica Isolate.

A pan-susceptible Salmonella enterica serovar Worthington isolate was detected in the stool of a man returning from Sri Lanka. Under ceftriaxone treatment, a third-generation cephalosporin (3GC)-resistant Salmonella Worthington was isolated after 8 days. Molecular analyses indicated that the two isolates were identical. However, the latter strain acquired a blaDHA-1-carrying IncFII plasmid probably from a Citrobacter amalonaticus isolate colonizing the gut. This is the first report of in vivo acquisition of plasmid-mediated resistance to 3GCs in S. enterica.

Whole-Genome Sequencing for Drug Resistance Profile Prediction in Mycobacterium tuberculosis.

Whole-genome sequencing allows rapid detection of drug-resistant Mycobacterium tuberculosis isolates. However, the availability of high-quality data linking quantitative phenotypic drug susceptibility testing (DST) and genomic data have thus far been limited. We determined drug resistance profiles of 176 genetically diverse clinical M. tuberculosis isolates from the Democratic Republic of the Congo, Ivory Coast, Peru, Thailand, and Switzerland by quantitative phenotypic DST for 11 antituberculous drugs using the BD Bactec MGIT 960 system and 7H10 agar dilution to generate a cross-validated phenotypic DST readout. We compared DST results with predicted drug resistance profiles inferred by whole-genome sequencing. Classification of strains by the two phenotypic DST methods into resistotype/wild-type populations was concordant in 73 to 99% of cases, depending on the drug. Our data suggest that the established critical concentration (5 mg/liter) for ethambutol resistance (MGIT 960 system) is too high and misclassifies strains as susceptible, unlike 7H10 agar dilution. Increased minimal inhibitory concentrations were explained by mutations identified by whole-genome sequencing. Using whole-genome sequences, we were able to predict quantitative drug resistance levels for the majority of drug resistance mutations. Predicting quantitative levels of drug resistance by whole-genome sequencing was partially limited due to incompletely understood drug resistance mechanisms. The overall sensitivity and specificity of whole-genome-based DST were 86.8% and 94.5%, respectively. Despite some limitations, whole-genome sequencing has the potential to infer resistance profiles without the need for time-consuming phenotypic methods.

Francisella tularensis Periprosthetic Joint Infections Diagnosed with Growth in Cultures.

Tularemia caused by Francisella tularensis is a zoonotic infection of the Northern Hemisphere that mainly affects the skin, lymph nodes, bloodstream, and lungs. Other manifestations of tularemia are very rare, especially those with musculoskeletal involvement. Presenting in 2016, we diagnosed two cases of periprosthetic knee joint infections (PJI) caused by Francisella tularensis in Europe (one in Switzerland and one in the Czech Republic). We found only two other PJI cases in the literature, another knee PJI diagnosed 1999 in Ontario, Canada, and one hip PJI in Illinois, USA, in 2017. Diagnosis was made in all cases by positive microbiological cultures after 3, 4, 7, and 12 days. All were successfully treated, two cases by exchange of the prosthesis, one with debridement and retention, and one with repeated aspiration of the synovial fluid only. Antibiotic treatment was given between 3 weeks and 12 months with either ciprofloxacin-rifampin or with doxycycline alone or doxycycline in combination with gentamicin. Zoonotic infections should be considered in periprosthetic infections in particular in culture-negative PJIs with a positive histology or highly elevated leukocyte levels in synovial aspiration. Here, we recommend prolonging cultivation time up to 14 days, performing specific PCR tests, and/or conducting epidemiologically appropriate serological tests for zoonotic infections, including that for F. tularensis.

Aortic Endograft Infection with Mycobacterium chimaera and Granulicatella adiacens, Switzerland, 2014.

We describe an aortic endograft infection caused by Mycobacterium chimaera and Granulicatella adiacens, successfully treated with prolonged antimicrobial drug therapy after complete explantation of the infected endoprosthesis and extra-anatomical reconstruction. Whole-genome sequencing analysis did not indicate a close relationship to bacterial strains known to cause infections after cardiac surgery.

Acquired Resistance to Bedaquiline and Delamanid in Therapy for Tuberculosis.

Treatment of multidrug-resistant Mycobacterium tuberculosis is a challenge. This letter describes the emergence of resistance to new therapies, bedaquiline and delamanid.

Pathogen Identification by Multiplex LightMix Real-Time PCR Assay in Patients with Meningitis and Culture-Negative Cerebrospinal Fluid Specimens.

Acute bacterial meningitis is a medical emergency, and delays in initiating effective antimicrobial therapy result in increased morbidity and mortality. Culture-based methods, thus far considered the "gold standard" for identifying bacterial microorganisms, require 24 to 48 h to provide a diagnosis. In addition, antimicrobial therapy is often started prior to clinical sample collection, thereby decreasing the probability of confirming the bacterial pathogen by culture-based methods. To enable a fast and accurate detection of the most important bacterial pathogens causing meningitis, namely, Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis, Streptococcus agalactiae, and Listeria monocytogenes, we evaluated a commercially available multiplex LightMix real-time PCR (RT-PCR) in 220 cerebrospinal fluid (CSF) specimens. The majority of CSF samples were collected by lumbar puncture, but we also included some CSF samples from patients with symptoms of meningitis from the neurology department that were recovered from shunts. CSF samples were analyzed by multiplex RT-PCR enabling a first diagnosis within a few hours after sample arrival at our institute. In contrast, bacterial identification took between 24 and 48 h by culture. Overall, a high agreement of bacterial identification between culture and multiplex RT-PCR was observed (99%). Moreover, multiplex RT-PCR enabled the detection of pathogens, S. pneumoniae (n = 2), S. agalactiae (n = 1), and N. meningitidis (n = 1), in four culture-negative samples. As a complement to classical bacteriological CSF culture, the LightMix RT-PCR assay proved to be valuable by improving the rapidity and accuracy of the diagnosis of bacterial meningitis.

Rapid disc diffusion antibiotic susceptibility testing for Pseudomonas aeruginosa, Acinetobacter baumannii and Enterococcus spp.

Background: We investigated the feasibility of rapid disc diffusion antibiotic susceptibility testing (rAST) with reading of inhibition zones after 6 and/or 8 h of incubation for Enterococcus faecalis, Enterococcus faecium, Pseudomonas aeruginosa and Acinetobacter baumannii. In addition, we evaluated discrimination of resistant populations from the WT populations at early timepoints and the requirement for clinical breakpoint adaptations for proper interpretation of rAST data. Methods: In total, 815 clinical strains [E. faecalis (n = 135), E. faecium (n = 227), P. aeruginosa (n = 295) and A. baumannii (n = 158)] were included in this study. Disc diffusion plates were streaked, incubated and imaged using the WASPLabTM automation system. WT populations and non-WT populations were defined using epidemiological cut-offs. Results and conclusions: rAST at 6 and 8 h was possible for A. baumannii and enterococci with readability of inhibition zones >90%. Overall categorical agreement of rAST at 6 h with AST at 18 h was 97.2%, 97.4% and 95.3% for E. faecalis, E. faecium and A. baumannii, respectively. With few exceptions, major categorization error rates were <1% for A. baumannii, and vancomycin-resistant E. faecium were clearly separated from the WT at 6 h. For P. aeruginosa the average readability of inhibition zones was 68.9% at 8 h and we found an overall categorical agreement of 94.8%. Adaptations of clinical breakpoints and/or introduction of technical buffer zones, preferably based on aggregated population data from various epidemiological settings, are required for proper interpretation of rAST.

Detection of respiratory bacterial pathogens causing atypical pneumonia by multiplex Lightmix® RT-PCR.

Pneumonia is a severe infectious disease. In addition to common viruses and bacterial pathogens (e.g. Streptococcus pneumoniae), fastidious respiratory pathogens like Chlamydia pneumoniae, Mycoplasma pneumoniae and Legionella spp. can cause severe atypical pneumonia. They do not respond to penicillin derivatives, which may cause failure of antibiotic empirical therapy. The same applies for infections with B. pertussis and B. parapertussis, the cause of pertussis disease, that may present atypically and need to be treated with macrolides. Moreover, these fastidious bacteria are difficult to identify by culture or serology, and therefore often remain undetected. Thus, rapid and accurate identification of bacterial pathogens causing atypical pneumonia is crucial. We performed a retrospective method evaluation study to evaluate the diagnostic performance of the new, commercially available Lightmix® multiplex RT-PCR assay that detects these fastidious bacterial pathogens causing atypical pneumonia. In this retrospective study, 368 clinical respiratory specimens, obtained from patients suffering from atypical pneumonia that have been tested negative for the presence of common agents of pneumonia by culture and viral PCR, were investigated. These clinical specimens have been previously characterized by singleplex RT-PCR assays in our diagnostic laboratory and were used to evaluate the diagnostic performance of the respiratory multiplex Lightmix® RT-PCR. The multiplex RT-PCR displayed a limit of detection between 5 and 10 DNA copies for different in-panel organisms and showed identical performance characteristics with respect to specificity and sensitivity as in-house singleplex RT-PCRs for pathogen detection. The Lightmix® multiplex RT-PCR assay represents a low-cost, time-saving and accurate diagnostic tool with high throughput potential. The time-to-result using an automated DNA extraction device for respiratory specimens followed by multiplex RT-PCR detection was below 4 h, which is expected to significantly improve diagnostics for atypical pneumonia-associated bacterial pathogens.

MASTER: a model to improve and standardize clinical breakpoints for antimicrobial susceptibility testing using forecast probabilities.

Objectives: The procedure for setting clinical breakpoints (CBPs) for antimicrobial susceptibility has been poorly standardized with respect to population data, pharmacokinetic parameters and clinical outcome. Tools to standardize CBP setting could result in improved antibiogram forecast probabilities. We propose a model to estimate probabilities for methodological categorization errors and defined zones of methodological uncertainty (ZMUs), i.e. ranges of zone diameters that cannot reliably be classified. The impact of ZMUs on methodological error rates was used for CBP optimization. Methods: The model distinguishes theoretical true inhibition zone diameters from observed diameters, which suffer from methodological variation. True diameter distributions are described with a normal mixture model. The model was fitted to observed inhibition zone diameters of clinical Escherichia coli strains. Repeated measurements for a quality control strain were used to quantify methodological variation. Results: For 9 of 13 antibiotics analysed, our model predicted error rates of < 0.1% applying current EUCAST CBPs. Error rates were > 0.1% for ampicillin, cefoxitin, cefuroxime and amoxicillin/clavulanic acid. Increasing the susceptible CBP (cefoxitin) and introducing ZMUs (ampicillin, cefuroxime, amoxicillin/clavulanic acid) decreased error rates to < 0.1%. ZMUs contained low numbers of isolates for ampicillin and cefuroxime (3% and 6%), whereas the ZMU for amoxicillin/clavulanic acid contained 41% of all isolates and was considered not practical. Conclusions: We demonstrate that CBPs can be improved and standardized by minimizing methodological categorization error rates. ZMUs may be introduced if an intermediate zone is not appropriate for pharmacokinetic/pharmacodynamic or drug dosing reasons. Optimized CBPs will provide a standardized antibiotic susceptibility testing interpretation at a defined level of probability.

Effect of ß-lactamase production and ß-lactam instability on MIC testing results for Mycobacterium abscessus.

OBJECTIVES: Limited treatment options available for Mycobacterium abscessus infections include the parenteral ß-lactam antibiotics cefoxitin and imipenem, which show moderate in vitro activity. Other ß-lactam antibiotics (except meropenem) have no considerable in vitro activity, due to their rapid hydrolysis by a broad-spectrum ß-lactamase (Bla_Mab). We here addressed the impact of ß-lactamase production and ß-lactam in vitro stability on M. abscessus MIC results and determined the epidemiological cut-off (ECOFF) values of cefoxitin, imipenem and meropenem. METHODS: By LC high-resolution MS (LC-HRMS), we assessed the in vitro stability of cefoxitin, imipenem and meropenem. M. abscessus ATCC 19977 strain and its isogenic blaMab deletion mutant were used for MIC testing. Based on MIC distributions for M. abscessus clinical strains, we determined ECOFFs of cefoxitin, imipenem and meropenem. RESULTS: A functional Bla_Mab increased MICs of penicillins, ceftriaxone and meropenem. LC-HRMS data showed significant degradation of cefoxitin, imipenem and meropenem during standard antibiotic susceptibility testing procedures. MIC, MIC50 and ECOFF values of cefoxitin, imipenem and meropenem are influenced by incubation time. CONCLUSIONS: The results of our study support administration of imipenem, meropenem and cefoxitin, for treatment of patients infected with M. abscessus. Our findings on in vitro instability of imipenem, meropenem and cefoxitin explain the problematic correlation between in vitro susceptibility and in vivo activity of these antibiotics and question the clinical utility of susceptibility testing of these chemotherapeutic agents.