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In vitro synergistic effects of a short cationic peptide and clinically used antibiotics against drug‐resistant isolates of Brucella melitensis

Purpose. In the last few decades, increasing microbial resistance to common antibiotics has attracted researchers' attention to the development of new classes of antibiotics such as antimicrobial peptides. Accordingly, the aim of the current study was to evaluate antimicrobial effects of the CM11 peptide alone and combined with common antibiotics against drug‐resistant isolates of Brucella melitensis. Methodology. A total of 50 pathogenic samples of B. melitensis were isolated from patients and their antibiotic susceptibility pattern was evaluated by E‐test. Then, the synergistic reaction of the peptide with selected antibiotics was evaluated using a chequerboard procedure. Results. Based on the susceptibility pattern of isolates, ciprofloxacin, rifampin, streptomycin and co‐trimoxazole were used for synergistic study. According to the results, synergic effect was observed for streptomycin and co‐trimoxazole in combination with the peptide while ciprofloxacin and rifampin showed partial synergy and additive effect, respectively. Consistent with these results, in the time‐killing assay, a decrease in colony counts for streptomycin‐peptide and co‐trimoxazole‐peptide was >2 Log10 while for ciprofloxacin‐peptide and rifampin‐peptide it was about 1.5 Log10 and <2 Log10, which represents synergy, partial synergy and additive interaction, respectively. Conclusion. These results showed that by antibiotic‐CM11 combination, their effective dose can be reduced particularly for drug‐resistant isolates. In conclusion, considering the importance of brucellosis caused by B. melitensis in the Middle East beside reports on antibiotic resistance strains, especially against rifampin, which may literally lead to an increase in resistant strains of Mycobacterium tuberculosis in endemic areas, our findings can be used to develop a suitable alternative treatment for brucellosis, and with less risk.