Ampicillin sulbactam

The susceptibilities to seven categories of antibiotics were determined. Specifically, susceptibility to amikacin, gentamicin, meropenem, imipenem, ampicillin-sulbactam, cefoperazone-sulbactam, ceftazidime, and ciprofloxacin (all antibiotics were obtained from Sigma) was determined using the standard agar dilution method (31 (link)) with breakpoints suggested by the Clinical and Laboratory Standards Institute (CLSI) (26 ). Briefly, bacterial suspensions of each strain were adjusted to a 0.5 McFarland standard and dispensed into the seeding-tray wells of a Cathra replicator system (32 wells). Mueller-Hinton agar plates supplemented with various concentrations of antibiotics were seeded using 1-mm pins. Susceptibility to colistin was determined by measuring MICs using the standard agar dilution method as previously described (31 (link)) and interpreted according to CLSI guidelines (26 ). Susceptibility to tigecycline and colistin was determined using the standard broth microdilution assay.

The efflux pump substrates, ethidium bromide (EtBr) and carbonyl cyanide m-chlorophenylhydrazine (CCCP), and the antibiotics oxacillin, ampicillin/sulbactam, and penicillin, were obtained from Sigma-Aldrich Co. Ltd, San Luis, MO, USA. The 1,8-naphthyridine sulfonamides and 1,8-naphthyridinone used in the study are listed in Table 2. All compounds were synthesized and provided by Luiz Everson da Silva, Ph.D., from the Federal University of Paraná, and by researchers Iêda Maria Begnini, Ph.D., Ricardo Andrade Rebelo, Ph.D., Sandro Lucio Mireski, Ph.D., Michele Caroline Nasato, M.Sc. and Maria Isabel Lacowicz Krautler, B.ASc., from the Regional University of Blumenau. The synthesis and full characterization of the listed compounds have been previously described in the literature [102 (link)]. The antibiotics and heterocycles were dissolved in dimethyl sulfoxide (DMSO) and diluted in sterile distilled water to a concentration of 1024 μg/mL. Compounds CCCP and EtBr were diluted directly in sterile distilled water to 1024 μg/mL.

Salmonella isolates were serotyped via the slide agglutination assay according to the instructions provided by the manufacturer of the Salmonella antisera (Statens Serum Institute, Copenhagen, Denmark). The serovars of the isolates were determined using the Kauffmann-White scheme (Grimont and Weill, 2007 ).
Antimicrobial susceptibility testing of Salmonella isolates was performed using the agar dilution method for ampicillin, ampicillin-sulbactam, piperacillin, piperacillin-tazobactam, cefuroxime, cefotetan, ceftazidime, ceftriaxone, cefepime, aztreonam, imipenem, amikacin, gentamicin, and tobramycin (Sigma, St. Louis, MO, United States). Escherichia coli ATCC 25922 was used as a quality control strain in each run. The breakpoints for determining susceptibility or resistance to antimicrobials were determined by the guidelines of the Clinical Laboratory Standard Institute (CLSI, 2018 ).