title: Comparison of fixed concentration and fixed ratio options for dilution susceptibility testing of gram-negative bacilli to ampicillin and ampicillin/sulbactam.
abstract: Ampicillin combined with sulbactam was tested at both fixed ratio (2:1 and 1:1) and fixed sulbactam concentrations (4 micrograms/ml, 8 micrograms/ml and 16 micrograms/ml) against 2440 consecutively isolated gram-negative bacilli. Sulbactam significantly enhanced the spectrum of ampicillin activity. Overall, at 8 micrograms/ml ampicillin inhibited 50% of the Enterobacteriaceae isolates, whereas 69% to 84% of the isolates were inhibited by the various sulbactam combinations. The widest spectrum of activity for ampicillin/sulbactam was achieved by testing at a fixed sulbactam concentration of 16 micrograms/ml, followed by the 1:1 ratio and the fixed 8 micrograms/ml (84%, 76% and 74% inhibited, respectively). The amount of sulbactam at the susceptible breakpoint concentrations of ampicillin markedly affected the percentage of susceptible strains. Combinations that include 8 micrograms/ml of sulbactam are suggested for consideration.
authors.affiliation: Department of Pathology L471, Oregon Health Sciences University, Portland 97201.
authors.forename: M A
authors.initials: MA
authors.lastname: Pfaller
authors.forename: A L
authors.initials: AL
authors.lastname: Barry
authors.forename: P C
authors.initials: PC
authors.lastname: Fuchs
authors.forename: E H
authors.initials: EH
authors.lastname: Gerlach
authors.forename: D J
authors.initials: DJ
authors.lastname: Hardy
authors.forename: J C
authors.initials: JC
authors.lastname: McLaughlin
mesh_terms: D000667:Ampicillin / Q000494:pharmacology*; D000668:Ampicillin Resistance; D015169:Colony Count, Microbial; D004305:Dose-Response Relationship, Drug; D004352:Drug Resistance, Microbial; D004359:Drug Therapy, Combination / Q000494:pharmacology; D006090:Gram-Negative Bacteria / Q000187:drug effects*; D008826:Microbial Sensitivity Tests / Q000379:methods*; D013407:Sulbactam / Q000494:pharmacology*
chemical_list: C035444:sultamicillin; D000667:Ampicillin; D013407:Sulbactam