Antimicrobial susceptibility testing (AST) is performed to assess the in vitro activity of antimicrobial agents against various bacteria. The AST results, which are expressed as minimum inhibitory concentrations (MICs) are used in research for antimicrobial development and monitoring of resistance development and in the clinical setting for antimicrobial therapy guidance. Dalbavancin is a semi-synthetic lipoglycopeptide antimicrobial agent that was approved in May 2014 by the Food and Drug Administration (FDA) for the treatment of acute bacterial skin and skin structure infections caused by Gram-positive organisms. The advantage of dalbavancin over current anti-staphylococcal therapies is its long half-life, which allows for once-weekly dosing. Dalbavancin has activity against Staphylococcus aureus (including both methicillin-susceptible S. aureus [MSSA] and methicillin-resistant S. aureus [MRSA]), coagulase-negative staphylococci, Streptococcus pneumoniae, Streptococcus anginosus group, β-hemolytic streptococci and vancomycin susceptible enterococci. Similar to other recent lipoglycopeptide agents, optimization of CLSI and ISO broth susceptibility test methods includes the use of dimethyl sulfoxide (DMSO) as a solvent when preparing stock solutions and polysorbate 80 (P80) to alleviate adherence of the agent to plastic. Prior to the clinical studies and during the initial development of dalbavancin, susceptibility studies were not performed with the use of P-80 and MIC results tended to be 2-4 fold higher and similarly higher MIC results were obtained with the agar dilution susceptibility method. Dalbavancin was first included in CLSI broth microdilution methodology tables in 2005 and amended in 2006 to clarify use of DMSO and P-80. The broth microdilution (BMD) procedure shown here is specific to dalbavancin and is in accordance with the CLSI and ISO methods, with step-by-step detail and focus on the critical steps added for clarity.

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The performance of the Liofilchem omadacycline MIC Test Strip (MTS) was evaluated in a multi-site study. Three testing sites collected/tested clinical isolates and one site tested challenge isolates that totaled 175 S. aureus , 70 S. lugdunensis , 121 E. faecalis , 100 E. faecium , 578 Enterobacterales, 142 Haemophilus spp., 181 S. pneumoniae , 45 S. anginosus group, 35 S. pyogenes and 20 S. agalactiae . MIC testing was performed by CLSI broth microdilution (BMD) and MTS. Fastidious isolates testing included BMD and MTS testing with both CLSI and EUCAST Mueller Hinton Fastidious (MH-F). In addition, each site performed reproducibility for non-fastidious and fastidious isolates and QC by MTS and BMD. All BMD and MTS results for the QC strains were within expected ranges, with exception of one MTS HTM result for H. influenzae ATCC 49247. Among reproducibility isolates, omadacycline MTS results were within one dilution of the modal MIC for 95.2% of non-fastidious Gram-positive, 100% of Gram-negative, 99.3% and 98.5% of fastidious isolates tested on CLSI and EUCAST media, respectively. MTS results for all study isolates were within one doubling dilution of the CLSI BMD MIC for 98.9% of S. aureus , 100% of S. lugdunensis , 98.3% of E. faecalis , 100% of E. faecium , and 99.6% of Enterobacterales. Essential agreement rates for CLSI and EUCAST MH-F agar compared to CLSI BMD were 98.2% and 98.2%, for H. influenzae , 91.1% and 73.6%, for S. pneumoniae and 100% and 85-91.7% for other streptococcus species, respectively. Based on CLSI media, all categorical errors were minor errors and categorical agreement rates were >90% with exception of C. freundii , S. lugdunensis, E. faecalis , S. anginosus and S. constellatus.

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