Supplementation of zebrafish (Danio rerio) diet using a short antimicrobial peptide: Evaluation of growth performance, immunomodulatory function, antioxidant activity, and disease resistance.

Short-chain bioactive peptides are new and promising antimicrobial, immune moderating, and antioxidant agents. Therefore, the present study was conducted to evaluate in vitro antibacterial activity of CM11, a short antimicrobial peptide (AMP), against Streptococcus iniae and Yersinia ruckeri as fish pathogenic bacteria using standard disk diffusion and microdilution assays. In addition, in vivo effects of CM11 on fish growth, immunity, antioxidant activity, and disease resistance were evaluated using zebrafish (Danio rerio) as an animal model. For in vivo study, based on in vitro susceptibility results, four diets were designed to include zero (as control), 10, 20, and 50 µg of CM11 per g diet referred to as control, P1, P2, and P3 treatments, respectively. After eight weeks of dietary trial, fish were challenged with Streptococcus iniae, and the survival rate was calculated for a period of two weeks. Results showed that CM11 effectively inhibited the growth of S. iniae and Y. ruckeri on agar plates at a concentration of eight µg/ml. Minimum inhibitory and minimum bactericidal concentrations of CM11 were measured at 8 and 32 µg/ml for S. iniae and 16 and 64 µg/ml Y. ruckeri, respectively. In vivo results showed no noticeable effects on fish growth parameters, however, feed conversion ratio (FCR) was found lower in P3 and P2 compared to control (P < 0.05). Immunological and antioxidant responses were found strongly affected by CM11 in all treatment groups in which the highest values were found in the P3 treated group. Key immune and antioxidant genes were up-regulated particularly in fish receiving the highest level of CM11 (P3). Fish receiving the CM11 peptide showed better survival when challenged with S. iniae. These findings suggest the potential of CM11 for use in aquaculture as an antibacterial and immunostimulant agent.

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.