To explore the interaction effects of ciprofloxacin and ampicillin with nickel on the growth of bacteria, Staphylococcus aureus strain ATCC 29213, Enterococcus faecalis ATCC 29212 and Escherichia coli ATCC 25922 were used. Minimum inhibitory concentrations (MICs) were determined for nickel, ciprofloxacin and ampicillin, and the checkerboard method was used to assess their cumulative effects on bacterial growth. The interactions between the metal and antibiotics were assessed by the fractional inhibitory concentration (FIC). The MICs for ciprofloxacin and ampicillin were 0.31 and 1 mg/L for E. faecalis, 0.62 and 1 mg/L for S. aureus and 0.005 and 2.5 for E. coli, respectively. The MIC for nickel was 1000 mg/L for all bacteria. The FIC results for ciprofloxacin and nickel demonstrated an antagonistic effect of the two agents on the growth of E. coli and E. faecalis and an additive effect on S. aureus. The FICs for ampicillin and nickel demonstrated a synergistic effect on the growth of E. faecalis and E. coli. Different interactions of metals and antibiotics were observed depending on the bacteria and the type of antibiotic.

Department of Dental Medicine Faculty of Dental Medicine and Health J J Strossmayer University of Osijek Crkvena 21 31000 Osijek Croatia

Department of Medical Chemistry Biochemistry and Clinical Chemistry Faculty of Medicine University of Rijeka Braće Branchetta 20 51000 Rijeka Croatia

Department of Microbiology and Parasitology Faculty of Medicine University of Rijeka Brace Branchetta 20 51000 Rijeka Croatia

Department of Orthodontics Faculty of Dental Medicine University of Rijeka Kresimirova 42 51000 Rijeka Croatia

Department of Paediatric Dentistry Faculty of Dental Medicine University of Rijeka Kresimirova 42 51000 Rijeka Croatia