Effect of incremental tazobactam concentrations on the bactericidal activity of piperacillin against ESBL-producing enterobacterales clinical isolates causing bacteraemia.

We evaluated the activity of piperacillin in relation to INCREASING TAZOBACTAM CONCENTRATION against ESBL-producing Enterobacterales collected from patients with bacteraemia. Increasing tazobactam concentration (4, 12 or 24 mg/L) exerted a reduction of piperacillin MICs under the clinical breakpoint in a concentration-dependent manner (0%, 60% and 90% of clinical isolates). Also, activity of piperacillin/tazobactam based at higher achievable serum concentrations (123/14 mg/L) is needed to reduce the bacterial growth in 92% of ESBL-producers. CHANGES IN THE PIPERACILLIN MIC IN RELATION TO INCREASING TAZOBACTAM SUGGEST THAT REALTIME TDM COULD BE USED FOR DRIVEN ANTIMICROBIAL THERAPY WITH PIPERACILLIN/TAZOBACTAM IN BSI DUE TO ESBL STRAINS.

Antimicrobial resistance surveillance of Bacteroides fragilis isolated from blood cultures, Europe, 2022 (ReSuBacfrag).

OBJECTIVES: Bacteroides fragilis is the most frequent cause of anaerobic bacteraemia. Although recent data suggest a rise in antimicrobial resistance (AMR) of this and other anaerobic bacteria, surveillance remains limited due to a lack of both data availability and comparability. However, a newly introduced standardised method for antimicrobial susceptibility testing (AST) of anaerobic bacteria has made larger scale surveillance possible for the first time. The aim of this study was to investigate phenotypic AMR of Bacteroides fragilis isolates from bacteraemia across Europe in 2022. METHODS: In a multicentre approach, clinical microbiology laboratories in Europe were invited to contribute results of AST for Bacteroides fragilis blood culture isolates (including only the first isolate per patient and year). AST of a selection of four antibiotics was performed locally by participating laboratories in a prospective or retrospective manner, using the new EUCAST disc diffusion method on fastidious anaerobe agar (FAA-HB). RESULTS: A total of 16 European countries reported antimicrobial susceptibilities in 449 unique isolates of Bacteroides fragilis from blood cultures in 2022. Clindamycin demonstrated the highest resistance rates (20.9%, range 0 - 63.6%), followed by piperacillin-tazobactam (11.1%, 0-54.5%), meropenem (13.4%, 0-45.5%), and metronidazole (1.8%, 0-20.0%), all with wide variation between countries. CONCLUSION: Considering that the mean resistance rates across Europe were higher than expected for three of the four anti-anaerobic antibiotics under surveillance, both local AST of clinically relevant isolates of Bacteroides fragilis and continued surveillance on an international level is warranted.

Clinical outcomes in patients with piperacillin/tazobactam-non-susceptible but ceftriaxone-susceptible E. coli or K. pneumoniae bloodstream infection.

BACKGROUND: A small proportion of Escherichia coli and Klebsiella pneumoniae demonstrate in vitro non-susceptibility to piperacillin/tazobactam but retain susceptibility to ceftriaxone. Uncertainty remains regarding how best to treat these isolates. OBJECTIVES: We sought to compare clinical outcomes between patients with piperacillin/tazobactam-non-susceptible but ceftriaxone-susceptible E. coli or K. pneumoniae bloodstream infection receiving definitive therapy with ceftriaxone versus an alternative effective antibiotic. METHODS: We retrospectively identified patients with a positive blood culture for piperacillin/tazobactam-non-susceptible but ceftriaxone-susceptible E. coli or K. pneumoniae between 1 January 2013 and 31 December 2022. Patients were divided into one of two definitive treatment groups: ceftriaxone or alternative effective antibiotic. Our primary outcome was a composite of 90 day all-cause mortality, hospital readmission, or recurrence of infection. We used Cox proportional hazards models to compare time with the composite outcome between groups. RESULTS: Sixty-two patients were included in our analysis. Overall, median age was 63 years (IQR 49.5-71.0), the most common source of infection was intra-abdominal (25/62; 40.3%) and the median total duration of therapy was 12.0 days (IQR 9.0-16.8). A total of 9/22 (40.9%) patients in the ceftriaxone treatment group and 18/40 (45.0%) patients in the alternative effective antibiotic group met the composite endpoint. In an adjusted time-to-event analysis, there was no difference in the composite endpoint between groups (HR 0.67, 95% CI 0.30-1.50). The adjusted Bayesian posterior probability that the HR was less than or equal to 1 (i.e. ceftriaxone is as good or better than alternative therapy) was 85%. CONCLUSIONS: These findings suggest that ceftriaxone can be used to effectively treat bloodstream infections with E. coli or K. pneumoniae that are non-susceptible to piperacillin/tazobactam but susceptible to ceftriaxone.

Possible impact of revisions in disc diffusion breakpoints for aminoglycosides and piperacillin/tazobactam in the 33rd edition of CLSI M100 document on clinical reporting and use in Indian settings with low susceptibility.

PURPOSE: The study explores the impact of significant interpretative breakpoint changes for aminoglycosides and piperacillin-tazobactam in Enterobacterales and Pseudomonas aeruginosa, considering PK/PD, clinical data, and susceptibility on clinical reporting and use. PROCEDURE: Between January 2021 and June 2023, a total of 189,583 samples were processed for bacterial pathogens and antimicrobial susceptibility testing was performed using disc diffusion method/VITEK® 2 Compact system/broth microdilution. WHONET software was utilised to capture and analyse the changes in the interpretation of disc diffusion method, following updates to CLSI M100 documents in comparison to previous editions. Antimicrobial consumption data was collected and interpreted as DDD/100 bed days using AMC tool software. Here, we present data for 13,615 members of Order Enterobacterales and 1793 Pseudomonas aeruginosa isolates. FINDING: Enterobacterales exhibited a significant susceptibility drop of 14.7% for gentamicin and 21.7% for amikacin. Pseudomonas aeruginosa showed an increase in isolates with intermediate tobramycin susceptibility, from 0.6% to 29.7%, with relatively minor changes in piperacillin-tazobactam interpretation. CONCLUSION: The changes indicate a shift toward increased 'resistance' and 'intermediate susceptibility' for these antibiotics, emphasizing the need for cautious use and leveraging PK/PD knowledge for improved antibiotic utilization, patient outcomes, and antimicrobial stewardship.

Evaluation of the activity of cefepime/enmetazobactam against Enterobacterales bacteria collected in Europe from 2019 to 2021, including third-generation cephalosporin-resistant isolates.

OBJECTIVES: This study was performed to investigate the activity of the novel ß-lactam/ß-lactamase inhibitor combination cefepime/enmetazobactam, against recently circulating Enterobacterales isolates from Europe from 2019 to 2021. METHODS: A total of 2627 isolates were collected, and antimicrobial susceptibility was determined according to the European Committee on Antimicrobial Susceptibility Testing guidelines. Isolates with phenotypic resistance to ceftriaxone and ceftazidime (but susceptible to meropenem) and isolates nonsusceptible to meropenem were screened for the presence of ß-lactamases. RESULTS: Overall, susceptibility to third-generation cephalosporins was 77%, and 97.3% were susceptible to meropenem. Cefepime/enmetazobactam susceptibility was 97.9% (72% of these isolates were Klebsiella pneumoniae from Italy), compared with 80.0% susceptibility to piperacillin/tazobactam and 99.4% to ceftazidime/avibactam. A total of 320 isolates (12.2%) were resistant to third-generation cephalosporins but susceptible to meropenem, and virtually all (96.3%) carried an extended-spectrum ß-lactamase with or without an AmpC and these were all susceptible to cefepime/enmetazobactam. Most meropenem-nonsusceptible isolates carried a KPC (68%), which were not inhibited by cefepime/enmetazobactam but were inhibited by ceftazidime/avibactam. Additionally, most meropenem-nonsusceptible isolates carrying OXA-48 (9/12 isolates) were susceptible to cefepime/enmetazobactam. CONCLUSIONS: Cefepime/enmetazobactam was highly active against Enterobacterales isolates, especially those resistant to third-generation cephalosporins. These data suggest that cefepime/enmetazobactam could be used as a carbapenem-sparing agent to replace piperacillin/tazobactam.

A Day Saved is a Life Saved: Direct Antimicrobial Susceptibility Testing from Positively Flagged Blood Culture Bottles and their Concordance with the Routine Method.

BACKGROUND: Sepsis is a major health problem worldwide and is associated with high morbidity and mortality with every hour delay in initiation of therapy. A conventional method of blood culture and Antimicrobial Susceptibility Testing (AST) takes around 48-72 hours. Empirical antibiotics need to be administered until the sensitivity report is made available. It has been estimated that 20-50% of the empirical antibiotics are inappropriate, resulting in prolonged hospital stays, adverse effects, and emergence of drug resistance. Additionally, this also puts an extra financial burden on both the patients and healthcare settings. Performing direct Antimicrobial Sensitivity Testing (dAST) is an important tool to reduce turn-around time (TAT) by at least 18-24 hours, thus reducing morbidity and mortality among critically ill patients. METHODS: Direct AST (dAST) was performed from the positively flagged blood culture bottles received between December, 2021 to May, 2022 from Intensive Care Units (ICUs) on Mueller- Hinton Agar (MHA) using four drops of withdrawn blood. dAST was performed for six drugs: Ceftriaxone-30 µg (CTR), Piperacillin/Tazobactam-100/10 µg (PIT), Meropenem-10 µg (MRP), Ciprofloxacin-5 µg (CIP), Aztreonam-30 µg (AT), and Colistin (CL). The zone of inhibition was interpreted as per CLSI M100 ed32, 2022 guidelines. A parallel conventional method was also performed to examine for categorical agreement and disagreement. Identification was carried out using MALDI-TOF MS from the colonies that appeared on the dAST plate on the subsequent day. RESULTS: A total of 162 positively flagged blood culture bottles were included in the study. The majority of the Gram-negative organisms were from Enterobacterales (n=109), followed by Acinetobacter spp. (n=28) and Pseudomonas aeruginosa (n=25). Out of the 972 isolate-antimicrobial combinations, overall Categorical Agreement (CA) was seen in 936 (96.3%), whereas disagreement was observed in 36 with minor error (mE) in 21 (2.2%), major error (ME) in 7 (0.7%), and very major error (VME) in 8 (0.8%) when compared to the routine method. Categorical agreement (CA) of > 99% was seen in ceftriaxone (CTR) and ciprofloxacin (CIP). In comparison, the lowest CA was observed with meropenem (MRP) at 92%. Colistin dAST was performed using the E-strip method, and the result obtained was highly convincing, with an overall disagreement of only 1.2%. CONCLUSION: Rapid dAST from positively flagged blood culture bottles proved to significantly reduce the TAT from the time of sample collection to the first availability of antimicrobial susceptibility report with excellent categorical agreement of > 95% using the conventional disc diffusion method. Results obtained were within the acceptance criteria set by U. S. Food and Drug Administration (FDA) guidelines of > 90% categorical agreement for a new method. We were able to obtain excellent concordance for colistin using the E-strip method. Performing dAST not only saves a "day", but its proper implementation would save a "life".

Nationwide genome surveillance of carbapenem-resistant Pseudomonas aeruginosa in Japan.

Japan is a country with an approximate 10% prevalence rate of carbapenem-resistant Pseudomonas aeruginosa (CRPA). Currently, a comprehensive overview of the genotype and phenotype patterns of CRPA in Japan is lacking. Herein, we conducted genome sequencing and quantitative antimicrobial susceptibility testing for 382 meropenem-resistant CRPA isolates that were collected from 78 hospitals across Japan from 2019 to 2020. CRPA exhibited susceptibility rates of 52.9%, 26.4%, and 88.0% against piperacillin-tazobactam, ciprofloxacin, and amikacin, respectively, whereas 27.7% of CRPA isolates was classified as difficult-to-treat resistance P. aeruginosa. Of the 148 sequence types detected, ST274 (9.7%) was predominant, followed by ST235 (7.6%). The proportion of urine isolates in ST235 was higher than that in other STs (P = 0.0056, χ2 test). Only 4.1% of CRPA isolates carried the carbapenemase genes: blaGES (2) and blaIMP (13). One ST235 isolate carried the novel blaIMP variant blaIMP-98 in the chromosome. Regarding chromosomal mutations, 87.1% of CRPA isolates possessed inactivating or other resistance mutations in oprD, and 28.8% showed mutations in the regulatory genes (mexR, nalC, and nalD) for the MexAB-OprM efflux pump. Additionally, 4.7% of CRPA isolates carried a resistance mutation in the PBP3-encoding gene ftsI. The findings from this study and other surveillance studies collectively demonstrate that CRPA exhibits marked genetic diversity and that its multidrug resistance in Japan is less prevailed than in other regions. This study contributes a valuable data set that addresses a gap in genotype/phenotype information regarding CRPA in the Asia-Pacific region, where the epidemiological background markedly differs between regions.

Lessons learned from an external quality assurance program in applying CLSI interpretive criteria for reporting piperacillin/tazobactam susceptibility.

We evaluated the performance of automated susceptibility testing for piperacillin/tazobactam (PTZ) MICs against the reference microbroth dilution method. The Minimum Inhibitory Concentration of PTZ against a clinical isolate of Klebsiella pneumoniae was determined by reference broth micro-dilution method in 10 replicates which yielded a modal MIC of 16 mg/L (susceptible dose-dependent). Out of 434 laboratories who obtained MIC of 16 mg/L correctly, only 301 interpreted the result as susceptible dose dependent as per 2022 revised CLSI criteria. Educating the clinical laboratories in validating AST methods as per latest CLSI guidelines is of utmost important.

In vitro development of resistance against antipseudomonal agents: comparison of novel ß-lactam/ß-lactamase inhibitor combinations and other ß-lactam agents.

We subjected seven P. aeruginosa isolates to a 10-day serial passaging against five antipseudomonal agents to evaluate resistance levels post-exposure and putative resistance mechanisms in terminal mutants were analyzed by whole-genome sequencing analysis. Meropenem (mean, 38-fold increase), cefepime (14.4-fold), and piperacillin-tazobactam (52.9-fold) terminal mutants displayed high minimum inhibitory concentration (MIC) values compared to those obtained after exposure to ceftolozane-tazobactam (11.4-fold) and ceftazidime-avibactam (5.7-fold). Fewer isolates developed elevated MIC values for other ß-lactams and agents belonging to other classes when exposed to meropenem in comparison to other agents. Alterations in nalC and nalD, involved in the upregulation of the efflux pump system MexAB-OprM, were common and observed more frequently in isolates exposed to ceftazidime-avibactam and meropenem. These alterations, along with ones in mexR and amrR, provided resistance to most ß-lactams and levofloxacin but not imipenem. The second most common gene altered was mpl, which is involved in the recycling of the cell wall peptidoglycan. These alterations were mainly noted in isolates exposed to ceftolozane-tazobactam and piperacillin-tazobactam but also in one cefepime-exposed isolate. Alterations in other genes known to be involved in ß-lactam resistance (ftsI, oprD, phoP, pepA, and cplA) and multiple genes involved in lipopolysaccharide biosynthesis were also present. The data generated here suggest that there is a difference in the mechanisms selected for high-level resistance between newer ß-lactam/ß-lactamase inhibitor combinations and older agents. Nevertheless, the isolates exposed to all agents displayed elevated MIC values for other ß-lactams (except imipenem) and quinolones tested mainly due to alterations in the MexAB-OprM regulators that extrude these agents.

Win Ratio Analyses of Piperacillin-Tazobactam Versus Meropenem for Ceftriaxone-Nonsusceptible Escherichia coli or Klebsiella pneumoniae Bloodstream Infections: Post Hoc Insights From the MERINO Trial.

BACKGROUND: Clinical trials of treatments for serious infections commonly use the primary endpoint of all-cause mortality. However, many trial participants survive their infection and this endpoint may not truly reflect important benefits and risks of therapy. The win ratio uses a hierarchical composite endpoint that can incorporate and prioritize outcome measures by relative clinical importance. METHODS: The win ratio methodology was applied post hoc to outcomes observed in the MERINO trial, which compared piperacillin-tazobactam with meropenem. We quantified the win ratio with a primary hierarchical composite endpoint, including all-cause mortality, microbiological relapse, and secondary infection. A win ratio of 1 would correspond to no difference between the 2 antibiotics, while a ratio <1 favors meropenem. Further analyses were performed to calculate the win odds and to introduce a continuous outcome variable in order to reduce ties. RESULTS: With the hierarchy of all-cause mortality, microbiological relapse, and secondary infection, the win ratio estimate was 0.40 (95% confidence interval [CI], .22-.71]; P = .002), favoring meropenem over piperacillin-tazobactam. However, 73.4% of the pairs were tied due to the small proportion of events. The win odds, a modification of the win ratio accounting for ties, was 0.79 (95% CI, .68-.92). The addition of length of stay to the primary composite greatly minimized the number of ties (4.6%) with a win ratio estimate of 0.77 (95% CI, .60-.99; P = .04). CONCLUSIONS: The application of the win ratio methodology to the MERINO trial data illustrates its utility and feasibility for use in antimicrobial trials.

Introducing the new face of CLSI M100 in 2023: An explanatory review.

BACKGROUND: The CLSI annual update of its M100 document is eagerly awaited every year. This year's update, the M100-Ed33, was published in February, and will significantly affect clinical practices. OBJECTIVE: To highlight and explain the rationale of the changes and their clinical impact. CONTENT: The major changes this year are mostly focused on PK/PD data, selective and cascade reporting of the antibiotics and therapy related comments. The CLSI has moved away from its classical grouping of antibiotics (A, B, U, O) to a tier-based approach (Tier 1, 2, 3, 4) which will aid in cascade reporting during an antibiotic susceptibility testing (AST). Rather than non-fastidious, fastidious and anaerobe grouping, the tables have been made organism specific. The aminoglycosides breakpoints have been changed for both Enterobacterales and Pseudomonas aeruginosa while for P. aeruginosa, the breakpoints of piperacillin - tazobactam (TZP) are also updated. These updates are mostly based on attainment of drug plasma level for bacterial stasis rather than bactericidal effect of the antibiotics. It is noteworthy, that these breakpoint changes are made, keeping in view that the aminoglycosides for all organisms should be used in combination therapy. For P. aeruginosa, gentamicin has been removed, while amikacin has been restricted for urinary isolates only.

Identification of antibiotic consumption targets for the management of Clostridioides difficile infection in hospitals- a threshold logistic modelling approach.

BACKGROUND: This study aims to demonstrate the utility of a threshold logistic approach to identifying thresholds for specific antibiotic use associated with Clostridioides difficile infection (CDI) in an English teaching hospital. METHODS: A combined approach of nonlinear modeling and logistic regression, named threshold logistic, was used to identify thresholds and risk scores in hospital-level antibiotic use associated with hospital-onset, healthcare-associated (HOHA) CDI cases. RESULTS: Using a threshold logistic regression approach, an incidence greater than 0.2645 cases/1000 occupied bed-days (OBD; 85th percentile) was determined as the cutoff rate to define a critical (high) incidence rate of HOHA CDI. Fluoroquinolones and piperacillin-tazobactam were found to have thresholds at 84.8 and 54 defined daily doses (DDD)/1000 OBD, respectively. Analysis of data allowed calculating risk scores for HOHA CDI incidence rates exceeding the 85th percentile, i.e. entering critical incidence level. The threshold-logistic model also facilitated performing 'what-if scenarios' on future values of fluoroquinolones and piperacillin-tazobactam use to understand how HOHA CDI incidence rates may be affected. CONCLUSION: Using threshold logistic analysis, critical incidence levels and antibiotic use targets to control HOHA CDI were determined. Threshold logistic models can be used to inform and enhance the effective design and implementation of antimicrobial stewardship programs.

Strategies for Updating Piperacillin-Tazobactam Breakpoints.

In this issue of the Journal of Clinical Microbiology, C. Manuel, R. Maynard, A. Abbott, K. Adams, et al. (J Clin Microbiol 61:e01617-22, 2023, https://doi.org/10.1128/JCM.01617-22) describe a multisite study evaluation of piperacillin-tazobactam (TZP) MIC testing on three U.S. Food and Drug Administration (FDA)-cleared antimicrobial susceptibility testing (AST) devices compared to the reference broth microdilution method for organisms belonging to Enterobacterales. Although overall performance of each of the three devices was comparable when applying either FDA or Clinical and Laboratory Standards Institute (CLSI) TZP breakpoints, failure to update to the current CLSI breakpoints may result in falsely categorizing as many as 20% of the TZP-resistant isolates as susceptible. The impact of not updating clinical breakpoints and strategies for implementation of updated breakpoints are discussed.

A Placebo-Controlled Trial to Evaluate Two Locally Delivered Antibiotic Gels (Piperacillin Plus Tazobactam vs. Doxycycline) in Stage III-IV Periodontitis Patients.

Background and objectives: this study aims to evaluate the clinical and microbiological effects of a single subgingival administration of a locally delivered antibiotic gel containing piperacillin plus tazobactam and compare it with a slow-release doxycycline (14%) gel and a placebo gel, following subgingival instrumentation (SI) in patients with severe periodontitis. Materials and methods: sixty-four patients diagnosed with stage III-IV periodontitis were enrolled, were randomly assigned into three groups, and were treated additionally with a single subgingival administration of piperacillin plus tazobactam gel (group A); doxycycline gel (group B); and placebo gel (group C). The primary outcome variable was the change in mean probing pocket depth (PPD) 6 months after the intervention. Secondary outcome variables were changes in mean full-mouth bleeding score (FMBS); full-mouth plaque score (FMPS); overall bleeding index (BOP); pocket closure; and clinical attachment level (CAL), along with changes in the numbers of five keystone bacteria: Aggregatibacter actinomycetemcomitans (A.a.), Porphyromonas gingivalis (P.g.), Prevotella intermedia (P.i.), Tannerella forsythia (T.f.), and Treponema denticola (T.d.). Intergroup and intragroup differences were evaluated at 3 and 6 months. Results: at baseline, the three groups were comparable. An improvement in clinical parameters such as PPD, CAL, and BOP between groups was observed at 3 and 6 months, but without statistical significance (p > 0.05). At 6 months, the intragroup analysis showed a significant reduction in clinical parameters. Even though the piperacillin plus tazobactam group showed slightly higher PPD reduction, this was not statistically significant when compared to both control groups. Conclusions: The groups had similar results, and subgingival instrumentation can be executed without adjunctive antimicrobials, reducing the costs for the patient and the working time/load of the professional.

Antimicrobial Resistance Pattern of Pseudomonas aeruginosa Isolates from Tertiary Care Hospitals in Kathmandu.

Background Antimicrobial resistance, caused by Pseudomonas aeruginosa (P. aeruginosa), poses a global health threat, limiting treatment options and increasing morbidity and mortality rates due to its intrinsic and multidrug resistance. Objective To determine the antimicrobial resistance patterns of P. aeruginosa isolates from patients visiting or admitted to tertiary care hospitals in Kathmandu. Method A cross-sectional study was conducted at Bir Hospital and Tribhuvan University Teaching Hospital (TUTH) from December 2021 to December 2022. Isolates were identified and tested for antibiotic susceptibility following standard microbiological guidelines. Result The antimicrobial resistance of 200 P. aeruginosa isolates increased from low to high levels, as per the recommended anti-pseudomonal antibiotics by the Clinical and Laboratory Standards Institute (CLSI), from 0% to 94%. piperacillin/tazobactam exhibited significantly lower resistance at 18(9%) and while considerably higher resistance was observed with ceftazidime at 188(94%) compared to different antibiotics, followed by amikacin 34(17%), imipenem 58(29%), ciprofloxacin 42(21%), aztreonam 51(25.5%), and fosfomycin 44(22%). No resistance was observed to colistin and polymyxin B. P. aeruginosa resistant to carbapenem was accounted for 33.5% of the total, and multidrug resistance categories included multidrug resistance (MDR) at 39.0%, extensively drug resistance (XDR) at 13.5%, and P. aeruginosa difficult-to-treat (DTR PA) at 4.6%. Conclusion Most of the isolates were resistant to anti-pseudomonal antibiotics; however, colistin, polymyxin B, amikacin, doripenem, piperacillin/tazobactam, and fosfomycin were effective against MDR P. aeruginosa. Regular surveillance measures are essential to manage antimicrobial resistance.

Evaluation of Piperacillin-Tazobactam ETEST for the Detection of OXA-1 Resistance Mechanism among Escherichia coli and Klebsiella pneumoniae.

Globally, piperacillin-tazobactam resistance among Escherichia coli and Klebsiella pneumoniae is driven by OXA-1 beta-lactamases. Expression of blaOXA-1 yields piperacillin-tazobactam MICs of 8 to 16 µg/mL, which straddle the susceptible/susceptible-dose dependent breakpoint set by the Clinical and Laboratory Standards Institute in 2022. Variability of the reference broth microdilution method (BMD) was evaluated by manufacturing BMD panels using 2 brands of piperacillin, 2 brands of tazobactam and 2 brands of cation-adjusted Mueller-Hinton broth. In addition, ETEST, which harbors an intermediate dilution of 12 µg/mL was evaluated for the ability to differentiate isolates with and without blaOXA-1. A collection of 200 E. coli and K. pneumoniae, of which 82 harbored a blaOXA-1 gene, were tested. BMD variability was on average 1.3-fold, within the accepted 2-fold variability of MICs. However, categorical agreement (CA) between BMD reads was 74.0% for all isolates and 63.4% for those with a blaOXA-1 gene and 81.3% for those without blaOXA-1 detected (P = 0.004, Pearson's Chi Square). ETEST overall CA with the BMD mode was 68.0% and essential agreement (EA) was 80.5%. For isolates with blaOXA-1, CA was 50.0% and EA was 69.5%, versus 80.5% and 88.1%, respectively, for isolates without blaOXA-1 (P < 0.0001 for both comparisons). All ETEST errors were major errors (false resistance) compared to BMD mode. However, the negative predictive value of the ETEST for the presence of blaOXA-1 was 94.1%, compared to only 74.2% negative predictive value for BMD. Clinicians and microbiologists should be aware of the challenges associated with testing piperacillin-tazobactam in regions where blaOXA-1 is prevalent.