In vitro bactericidal activity of the N-terminal fragment of the frog peptide esculentin-1b (Esc 1-18) in combination with conventional antibiotics against Stenotrophomonas maltophilia

In this study the bactericidal effect of the N-terminal fragment of the frog skin peptide esculentin-1b [Esc(1-18)] in combination with clinically used antimicrobial agents was evaluated against Stenotrophomonas maltophilia, either in standard conditions (phosphate buffer) or in the presence of human serum. A synergistic bactericidal effect was observed after a 24 h incubation when combinations of Esc(1-18) and amikacin or colistin were used against clinical strains of S. maltophilia with or without resistance to these antibiotics, both in buffer and in the presence of serum. An indifferent effect was observed when the peptide was combined with levofloxacin or ceftazidime. A synergistic effect was also observed at earlier time points when the peptide was used in combination with colistin. Sequential exposure of bacterial cells to Esc(1-18) and amikacin or colistin, or vice versa, indicated that while Esc(1-18) and colistin cooperated in enhancing the bactericidal effect of their combination, when Esc(1-18) was combined with amikacin, the peptide had a major role in initiating the bactericidal effect, while amikacin was required for the subsequent effector phase. Altogether, the results obtained indicate that exposure of S. maltophilia to sub-bactericidal concentrations of Esc(1-18) increases its susceptibility to amikacin or colistin and may also render resistant strains susceptible to these antibiotics.