In vitro evaluation of synergistic activity between ciprofloxacin and broad snouted caiman serum against Escherichia coli

• We studied the activity of ciprofloxacin on Escherichia coli in the presence of serum of broad snouted caiman.
• The antibacterial activity was evaluated by in vitro assays of growth and kill-curves.
• The bacterial rate of elimination was higher at sub-inhibitory concentrations of ciprofloxacin.
• Ciprofloxacin showed higher values of bacterial elimination rate in the presence of caiman serum.
• The humoral immune factors present in caiman serum determined an increase on bacterial elimination.

The in vitro synergistic activity between ciprofloxacin and serum of broad snouted caiman on Escherichia coli was studied. The estimated MIC value of ciprofloxacin was 0.0188 µg/ml, and two assays of kill curve during 5 hours were performed: the first one in a standard culture medium and the second one in the presence of caiman serum. Different concentrations of ciprofloxacin were tested. Ciprofloxacin showed higher values of bacterial elimination rate in the presence of caiman serum in all concentrations tested. The combined activity of sub-inhibitory concentrations of ciprofloxacin and the humoral immune factors present in caiman serum determined an increase in the bacterial elimination observed in this assay. We suggest that the antibacterial activity of complement and natural antibodies present in caiman serum, which can bind to both Gram-negative and Gram-positive bacteria and acting through the classical complement pathway, can inhibit bacterial growth of Escherichia coli by lysis.