Accurate antimicrobial susceptibility testing (AST) and reporting are essential for guiding appropriate therapy for patients and direction for public health prevention and control actions. A critical feature of AST reporting is the interpretation of AST results using clinical breakpoints for reporting as susceptible, susceptible-dose dependent, intermediate, or resistant. Breakpoints are subject to continuous adjustment and updating to best reflect current clinical data. These breakpoint changes can benefit patients and public health only if adopted in a timely manner. A recent survey identified that up to 70% of College of American Pathologists (CAP)-accredited U.S. laboratories and 45% of CAP-accredited laboratories outside the U.S. use various obsolete clinical breakpoints to interpret AST results to guide patient care. The reason for the ongoing use of obsolete breakpoints is multifactorial, including barriers encountered by laboratories, commercial AST device manufacturers, standards development organizations, and regulatory bodies alike. To begin to address this important patient safety issue, CAP implemented checklist requirements for CAP-accredited laboratories to ensure up-to-date clinical breakpoint use. Furthermore, the topic was discussed at the June 2022 American Society for Microbiology Clinical Microbiology Open (CMO) with various stakeholders to identify potential solutions. This minireview summarizes the breakpoint setting process in the U.S. and highlights solutions to close the gap between breakpoint revisions and implementation in clinical and public health laboratories. Solutions discussed include clarification of data requirements and minimum inhibitory concentration only reporting for regulatory clearance of AST devices, clinical data generation to close breakpoints gaps, advocacy, education, and greater dialogue between stakeholders.

BACKGROUND: Aztreonam/avibactam is a combination agent that shows promise in treating infections caused by highly antibiotic-resistant MBL-producing Enterobacterales. This combination can be achieved by combining two FDA-approved drugs: ceftazidime/avibactam and aztreonam. It is unknown whether ceftazidime in the combination ceftazidime/aztreonam/avibactam has a synergistic or antagonistic effect on the in vitro activity of aztreonam/avibactam by significantly increasing or decreasing the MIC. OBJECTIVES: To determine whether increasing ceftazidime concentrations affect the MICs of aztreonam/avibactam alone. METHODS: A custom 8 × 8 chequerboard broth microdilution (BMD) panel was made using a digital dispenser (Hewlett-Packard, Corvallis, OR, USA). The panel included orthogonal 2-fold dilution series of aztreonam and ceftazidime ranging from 0.5 to 64 mg/L. Avibactam concentration was kept constant at 4 mg/L throughout the chequerboard. Thirty-seven Enterobacterales isolates from the CDC & FDA Antibiotic Resistance Isolate Bank or CDC's internal collection with intermediate or resistant interpretations to aztreonam and ceftazidime/avibactam were included for testing. All isolates harboured at least one of the following MBL genes: blaIMP, blaNDM or blaVIM. RESULTS: Regardless of the concentration of ceftazidime, aztreonam/avibactam with ceftazidime MICs for all 37 isolates were within one 2-fold doubling dilution of the aztreonam/avibactam MIC. CONCLUSIONS: Ceftazidime, in the combination ceftazidime/avibactam/aztreonam, did not affect the in vitro activity of aztreonam/avibactam in this sample of isolates. These findings can help assure clinical and public health laboratories that testing of aztreonam/avibactam by BMD can act as a reliable surrogate test when the combination of ceftazidime/avibactam and aztreonam is being considered for treatment of highly antibiotic-resistant MBL-producing Enterobacterales.

On behalf of the Clinical and Laboratory Standards Institute (CLSI), the Expert Panel on Microbiology would like to respond to the recent commentary by Kirby and colleagues voicing concerns related to verification of commercial antimicrobial susceptibility testing (AST) for new drugs that are introduced into the clinical laboratory (1).....

Aztreonam-avibactam is a combination antimicrobial agent with activity against carbapenemase-producing Enterobacteriaceae (CPE) with metallo-beta-lactamases (MbetaLs). Although aztreonam-avibactam is not yet approved by the U.S. Food and Drug Administration (FDA), clinicians can administer this combination by using two FDA-approved drugs: aztreonam and ceftazidime-avibactam. This combination of drugs is recommended by multiple experts for treatment of serious infections caused by MbetaL-producing CPE. At present, in vitro antimicrobial susceptibility testing (AST) of aztreonam-avibactam is not commercially available; thus, most clinicians receive no laboratory-based guidance that can support consideration of aztreonam-avibactam for serious CPE infections. Here, we report our internal validation for aztreonam-avibactam AST by reference broth microdilution (BMD) according to Clinical and Laboratory Standards Institute (CLSI) guidelines. The validation was performed using custom, frozen reference BMD panels prepared in-house at the Centers for Disease Control and Prevention (CDC). In addition, we took this opportunity to evaluate a new panel-making method using a digital dispenser, the Hewlett Packard (HP) D300e. Our studies demonstrate that the performance characteristics of digitally dispensed panels were equivalent to conventionally prepared frozen reference BMD panels for a number of drugs, including aztreonam-avibactam. We found the HP D300e liquid handler to be easy-to-use and to provide the capacity to prepare complex drug panels. Our findings will assist other clinical and public health laboratories implement susceptibility testing for aztreonam-avibactam.

Vancomycin-resistant Staphylococcus aureus (VRSA) are thought to occur by in vivo conjugative transfer of a vanA plasmid from Enterococcus to S. aureus. We studied bacterial isolates from VRSA cases that occurred in the United States to identify microbiological factors which may contribute to this plasmid transfer. First, vancomycin-susceptible, methicillin-resistant S. aureus (MRSA) isolates from five VRSA cases were tested for their ability to accept foreign DNA by conjugation in mating experiments with E. faecalis JH2-2 containing pAM378, a pheromone-response conjugative plasmid. All of the MRSA isolates accepted the plasmid DNA with similar transfer efficiencies (approximately 10(-7)/donor CFU) except one isolate, MRSA8, for which conjugation was not successful. MRSA isolates were also tested as recipients in mating experiments between an E. faecalis isolate with an Inc18-like vanA plasmid that was isolated from a VRSA case patient. Conjugative transfer was successful for 3/5 MRSA isolates. Successful MRSA recipients carried a pSK41-like plasmid, a staphylococcal conjugative plasmid, whereas the two unsuccessful MRSA recipients did not carry pSK41. Transfer of a pSK41-like plasmid from a successful MRSA recipient to the two unsuccessful recipients resulted in conjugal transfer of the Inc18-like vanA plasmid from E. faecalis at a frequency of 10(-7)/recipient CFU. In addition, conjugal transfer could be achieved for pSK41-negative MRSA in the presence of a cell-free culture filtrate from S. aureus carrying a pSK41-like plasmid at a frequency of 10(-8)/recipient CFU. These results indicated that a pSK41-like plasmid can facilitate the transfer of an Inc18-like vanA plasmid from E. faecalis to S. aureus possibly via an extracellular factor produced by pSK41-carrying isolates.

Over the past 10 years, dissemination of Klebsiella pneumoniae carbapenemase (KPC) has led to an increase in the prevalence of carbapenem-resistant Enterobacteriaceae (CRE) in the United States. Infections caused by CRE have limited treatment options and have been associated with high mortality rates. In the previous year, other carbapenemase subtypes, including New Delhi metallo-beta-lactamase, have been identified among Enterobacteriaceae in the United States. Like KPC, these enzymes are frequently found on mobile genetic elements and have the potential to spread widely. As a result, preventing both CRE transmission and CRE infections have become important public health objectives. This review describes the current epidemiology of CRE in the United States and highlights important prevention strategies.

Utility of Etest for antimicrobial susceptibility testing of Yersinia pestis was evaluated in comparison with broth microdilution and disk diffusion for eight agents. Four laboratories tested 26 diverse strains and found Etest to be reliable for testing antimicrobial agents used to treat Y. pestis, except for chloramphenicol and trimethoprim-sulfamethoxazole. Disk diffusion testing is not recommended.

We describe clinical and laboratory characteristics of invasive methicillin-resistant Staphylococcus aureus (MRSA) infections with vancomycin MICs of 2 mug/mL and compare hVISA to non-hVISA. Infections were most often healthcare-associated community-onset, demonstrated frequent complications and relapses. hVISA patients were more likely to have been hospitalized in the year prior to MRSA culture.

Staphylococcus aureus clinical isolates with vancomycin MICs of 2 mug/mL have been associated with vancomycin therapeutic failure and the heteroresistant vancomycin-intermediate S. aureus (hVISA) phenotype. Population analysis profile (PAP) with an area under the curve (AUC) ratio of ≥ 0.9 compared to hVISA strain Mu3 is most often used for determining hVISA, but it is time consuming and labor-intensive. A collection of 140 MRSA blood isolates with vancomycin MICs of 2 mug/mL by reference broth microdilution and screened for hVISA using PAP-AUC (21/140 [15%] hVISA) were tested by additional methods to detect hVISA. Methods included: 1) Etest macromethod using vancomycin and teicoplanin test strips, Brain Heart Infusion (BHI) agar and a 2.0 McFarland inoculum; 2) Etest GRD using vancomycin-teicoplanin double-sided gradient test strips on Mueller Hinton Agar (MHA) with 5% sheep's blood and 0.5 McFarland inoculum; and 3) BHI screen agar plates containing 4mug/mL vancomcyin and 16 g/L casein using 0.5 and 2.0 McFarland inocula. Each method was evaluated using PAP-AUC as the reference method. The sensitivity of each method for detecting hVISA was higher when read at 48 h. Etest macromethod was 57% sensitive and 96% specific, Etest GRD was 57% sensitive and 97% specific, BHI screen agar was 90% sensitive and 95% specific with 0.5 McFarland inoculum and 100% sensitive and 68% specific with 2.0 McFarland inoculum. BHI screen agar with 4 mug/mL vancomycin and casein and 0.5 McFarland inoculum had the best sensitivity and specificity combination, was easy to perform and may be useful for clinical detection of hVISA.

USA300 methicillin-resistant Staphylococcus aureus (MRSA) isolates are usually resistant only to oxacillin, erythromycin, and, increasingly, levofloxacin. Of these, oxacillin and levofloxacin resistances are chromosomally encoded. Plasmid-mediated clindamycin, mupirocin, and/or tetracycline resistance has been observed among USA300 isolates, but these descriptions were limited to specific patient populations or isolated occurrences. We examined the antimicrobial susceptibilities of invasive MRSA isolates from a national surveillance population in order to identify USA300 isolates with unusual, possibly emerging, plasmid-mediated antimicrobial resistance. DNA from these isolates was assayed for the presence of resistance determinants and the presence of a pSK41-like conjugative plasmid. Of 823 USA300 isolates, 72 (9%) were tetracycline resistant; 69 of these were doxycycline susceptible and tetK positive, and 3 were doxycycline resistant and tetM positive. Fifty-one (6.2%) isolates were clindamycin resistant and ermC positive; 22 (2.7%) isolates were high-level mupirocin resistant (mupA positive); 5 (0.6%) isolates were trimethoprim-sulfamethoxazole (TMP-SMZ) resistant, of which 4 were dfrA positive; and 7 (0.9%) isolates were gentamicin resistant and aac6'-aph2'' positive. Isolates with pSK41-like plasmids (n = 24) were positive for mupA (n = 19), dfrA (n = 6), aac6'-aph2'' (n = 6), tetM (n = 2), and ermC (n = 8); 20 pSK41-positive isolates were positive for two or more resistance genes. Conjugative transfer of resistance was demonstrated between four gentamicin- and mupirocin-resistant and three gentamicin- and TMP-SMZ-resistant USA300 isolates; transconjugants harbored a single pSK41-like plasmid, which was PCR positive for aac6'-aph2'' and either mupA and/or dfrA. USA300 and USA100 isolates from the same state with identical resistance profiles contained pSK41-like plasmids with indistinguishable restriction and Southern blot profiles, suggesting horizontal plasmid transfer between USA100 and USA300 isolates.

In the United States, the most prevalent mechanism of carbapenem resistance among Enterobacteriaceae is the production of a Klebsiella pneumoniae carbapenemase (KPC). KPC-producing isolates often exhibit a range of carbapenem MICs. To better understand the factors that contribute to overall carbapenem resistance, we analyzed 27 KPC-producing K. pneumoniae isolates with different levels of carbapenem resistance: 11 with low-level (i.e., meropenem or impenem MIC ≤ 4 mug/ml), 2 with intermediate-level (i.e., meropenem and imipenem MIC = 8 mug/ml), and 14 with high-level carbapenem resistance (i.e., imipenem or meropenem MIC ≥ 16 mug/ml) that were received from throughout the U.S. Among 14 isolates that exhibited high-level carbapenem resistance, Western blot analysis indicated that 10 produced an elevated amount of KPC. These isolates contained either an increased blaKPC gene copy number (n = 3) or the presence of deletions directly upstream of the blaKPC gene (n = 7). Four additional isolates lacked elevated KPC production but had high-level carbapenem resistance. Porin sequencing analysis identified 22 isolates potentially lacking a functional OmpK35, and three isolates potentially lacking a functional OmpK36. The highest carbapenem MICs were found in two isolates that lacked both functioning porins and contained elevated KPC production. The 11 isolates with low-level carbapenem resistance contained neither an upstream deletion nor increased blaKPC copy number. These results suggest that both blaKPC copy number and deletions in the upstream genetic environment affect the level of KPC production, and may contribute to high-level carbapenem resistance in KPC-producing K. pneumoniae, particularly when coupled with OmpK36 porin-loss.

Of the 9 vancomycin-resistant Staphylococcus aureus (VRSA) cases reported to date in the literature, 7 occurred in Michigan (MI). In 5 out of 7 of the Michigan VRSA cases, an Inc18-like vanA plasmid was identified in the VRSA isolate and/or an associated vancomycin-resistant Enterococcus (VRE) isolate from the same patient. This plasmid may play a critical role in the emergence of VRSA and analysis of the geographical distribution of VRE containing this plasmid may explain the observed prevalence of VRSA in MI. A total of 1,641 VRE from three separate collections were tested for the Inc18-like vanA plasmid by PCR for traA, repR, and vanA. All VRE from 2 MI institutions (N= 386), and between 60 to 70 VRE (N=883) from 17 institutions in 13 other states were tested. Fifteen isolates (3.9%) from MI were positive for an Inc18-like vanA plasmid (9 E. faecalis [12.5%], 3 E. faecium [1.0%], 2 E. avium, and 1 E. raffinosus). Six isolates (0.6%) from outside of MI were positive (3 E. faecalis [2.7%] and 3 E. faecium [0.4%]). Fourteen of the 15 plasmid-positive isolates from MI had the same Tn1546 insertion site location as those of the VRSA-associated Inc18 plasmid whereas 5 out of 6 plasmid-positive isolates from outside of MI differed in this characteristic. The one exception was an E. faecium isolate with a pulsed-field gel electrophoresis (PFGE) pattern that was indistinguishable from a plasmid-positive isolate from MI. Most plasmid-positive E. faecalis demonstrated diverse patterns by PFGE, with the exception of three pairs with indistinguishable patterns, suggesting the plasmid is mobile. Although VRE with the VRSA-associated Inc18-like vanA plasmid were more common in MI, they remain rare. Periodic surveillance of VRE for the plasmid may be useful in predicting occurrence of VRSA.

Mupirocin susceptibility testing of Staphylococcus aureus has become more important as mupirocin is used more widely to suppress or eliminate S. aureus colonization and prevent subsequent healthcare- and community-associated infections. This multi-center study evaluated two susceptibility testing screening methods to detect high-level mupirocin resistance (HLR), broth microdilution (BMD) MIC of ≥ 512 mug/ml and a 6 mm zone diameter for a 200-mug disk diffusion (DD) test. Initial testing indicated that with Clinical and Laboratory Standards Institute methods for BMD and DD, optimal conditions for detection of mupirocin HLR were 24 hours of incubation and reading DD zone diameters with transmitted light. Using the presence or absence of mupA as the gold standard for HLR, the sensitivity and specificity of a single-well 256 mug/ml BMD test were 97 and 99%, and for the 200-mug disk test were 98 and 99%, respectively. Testing with two disks, 200 mug and 5 mug, was evaluated for distinguishing HLR isolates (MIC ≥ 512 mug/ml), low-level resistant (LLR) isolates (MIC 8-256 mug/ml), and susceptible isolates (MIC ≤ 4 mug/ml). Using no zone with both disks as an indication of HLR, and no zone with the 5-mug disk plus any zone with the 200-mug disk as LLR, only 3 of the 340 isolates were misclassified, with 3 susceptible isolates being classified as LLR. Use of standardized MIC or disk tests could enable the detection of emerging high- and low-level mupirocin resistance in S. aureus.

Since some strains of Brucella species may require carbon dioxide for growth, a multilaboratory study was conducted to compare broth microdilution susceptibility results using ambient air (AA) and 5% CO2 incubation conditions. Six antimicrobial agents were tested against 39 Brucella isolates. Aminoglycoside MICs tended to be 1 log2 dilution higher in CO2 than in AA; tetracycline-class MICs to be 1 log2 dilution lower in CO2.

Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae have become more common in the United States and throughout the world. We used pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST) to examine the molecular epidemiology of KPC-producing K. pneumoniae isolates sent to the Centers for Disease Control and Prevention (CDC) for reference testing from 1996 to 2008. A dominant strain, sequence type 258 (ST 258), was found and likely accounts for 70% of the CDC's K. pneumoniae PFGE database. Isolates with PFGE patterns related to ST 258 were identified in 10 of the 19 U.S. states currently reporting KPC-producing K. pneumoniae, in addition to one isolate from Israel. KPC subtyping and analysis of the surrounding genetic environment were subsequently performed on 23 representative isolates. Thirteen isolates identified as ST 258 possessed either bla(KPC-2) or bla(KPC-3) and some variability in the Tn4401 element upstream of the bla(KPC) gene. Escherichia coli DH10B was successfully transformed by electroporation with KPC-encoding plasmid DNA from 20 of the 23 isolates. Restriction analysis of plasmid DNA prepared from transformants revealed a diversity of band patterns, suggesting the presence of different plasmids harboring the bla(KPC) gene, even among isolates of the same ST.

We compared the results obtained with six commercial MIC test systems (Etest, MicroScan, Phoenix, Sensititre, Vitek Legacy, and Vitek 2 systems) and three reference methods (agar dilution, disk diffusion, and vancomycin [VA] agar screen [VScr]) with the results obtained by the Clinical and Laboratory Standards Institute broth microdilution (BMD) reference method for the detection of VA-intermediate Staphylococcus aureus (VISA). A total of 129 S. aureus isolates (VA MICs by previous BMD tests, <or=1 microg/ml [n = 60 strains], 2 microg/ml [n = 24], 4 microg/ml [n = 36], or 8 microg/ml [n = 9]) were selected from the Centers for Disease Control and Prevention strain collection. The results of BMD with Difco Mueller-Hinton broth were used as the standard for data analysis. Essential agreement (percent +/-1 dilution) ranged from 98 to 100% for all methods except the method with the Vitek Legacy system, for which it was 90.6%. Of the six commercial MIC systems tested, the Sensititre, Vitek Legacy, and Vitek 2 systems tended to categorize VISA strains as susceptible (i.e., they undercalled resistance); the MicroScan and Phoenix systems and Etest tended to categorize susceptible strains as VISA; and the Vitek Legacy system tended to categorize VISA strains as resistant (i.e., it overcalled resistance). Disk diffusion categorized all VISA strains as susceptible. No susceptible strains (MICs <or= 2 microg/ml) grew on the VScr, but all strains for which the VA MICs were 8 microg/ml grew on the VScr. Only 12 (33.3%) strains for which the VA MICs were 4 microg/ml grew on VScr. The differentiation of isolates for which the VA MICs were 2 or 4 microg/ml was difficult for most systems and methods, including the reference methods.

This report describes the results of an 11-laboratory study to determine if a cefoxitin broth microdilution MIC test could predict the presence of mecA in staphylococci. Using breakpoints of < or = 4 microg/ml for mecA-negative and > or = 6 or 8 microg/ml for mecA-positive isolates, sensitivity and specificity based on mecA or presumed mecA for Staphylococcus aureus at 18 h of incubation were 99.7 to 100% in three cation-adjusted Mueller-Hinton broths tested. For coagulase-negative strains at 24 h of incubation, breakpoints of < or = 2 microg/ml for mecA-negative and > or = 4 microg/ml for mecA-positive isolates gave sensitivity and specificity of 94 to 99% and 69 to 80%, respectively.

Production of a Klebsiella pneumoniae carbapenemase (KPC) is the most common mechanism of carbapenem resistance in the U.S.; however, until now KPC-producing isolates have not been found in western Michigan. Molecular typing of two KPC-producing K. pneumoniae isolates from Michigan showed their similarity to other Midwestern isolates. They were also unrelated to the dominant sequence type observed throughout the U.S., multilocus sequence type 258. This could represent regional dissemination of another KPC-producing K. pneumoniae strain.

With increasing pressure to prevent methicillin-resistant Staphylococcus aureus (MRSA) infection, it is possible that there will be increased use of mupirocin for nasal decolonization of MRSA. Understanding the mechanisms, clinical significance, and epidemiology of mupirocin resistance is important for predicting how changes in mupirocin use may affect bacterial populations and MRSA control. High-level mupirocin resistance in S. aureus is mediated by a plasmid-encoded mupA gene. This gene can be found on conjugative plasmids that carry multiple resistance determinants for other classes of antimicrobial agents. High-level resistance has been associated with decolonization failure, and increased resistance rates have been associated with increased mupirocin use. Low-level mupirocin resistance is mediated via mutation in the native ileS gene, and the clinical significance of this resistance is unclear. Laboratory tests to detect and distinguish between these types of resistance have been described but are not widely available in the United States. Institutions that are considering the implementation of widespread mupirocin use should consider these resistance issues and develop strategies to monitor the impact of mupirocin use.