Escherichia coli ATCC 25922, Enterobacter cloacae ATCC 13047, Klebsiella pneumoniae ATCC 13883, Pseudomonas aeruginosa ATCC 27853, and Acinetobacter baumannii 17978 were obtained from the American Type Culture Collection (Manassas, VA). Strains were stored at −80°C in tryptic soy broth (BD Diagnostics, Franklin Lakes, NJ) containing 50% glycerol (Sigma-Aldrich, St. Louis, MO).
Meropenem was from Ark Pharm (Arlington Heights, IL), cefepime was from Chem Impex (Wood Dale, IL), gentamicin was from Alfa Aesar (Haverhill, MA), and ciprofloxacin was from US Biological (Salem, MA). Antibiotic stock solutions were prepared in water for manual dilution-based testing or in water containing 0.3% polysorbate-20 as required for liquid handling by the HP D300 digital dispenser (HP Inc., Palo Alto, CA).9 We previously determined through extensive analysis that polysorbate-20 at the concentrations used in assay wells has no effect on MIC determinations for all antibiotics examined.5 (link), 10 (link)
Meropenem was from Ark Pharm (Arlington Heights, IL), cefepime was from Chem Impex (Wood Dale, IL), gentamicin was from Alfa Aesar (Haverhill, MA), and ciprofloxacin was from US Biological (Salem, MA). Antibiotic stock solutions were prepared in water for manual dilution-based testing or in water containing 0.3% polysorbate-20 as required for liquid handling by the HP D300 digital dispenser (HP Inc., Palo Alto, CA).9 We previously determined through extensive analysis that polysorbate-20 at the concentrations used in assay wells has no effect on MIC determinations for all antibiotics examined.5 (link), 10 (link)