Objective To investigate the in vitro antibacterial activity of lycosin-Ⅰ against clinical isolates of Pseudomonas aeruginosa.Methods The clinical isolates of Pseudomonas aeruginosa,including 10 strains of multidrug resistant Pseudomonas aeruginosa and 10 strains of non-multidrug resistant Pseudomonas aeruginosa,were randomly collected from the Second Xiangya Hospital of Central South University.The in vitro minimum inhibitory concentrations (MIC) of lycosin-Ⅰ against Pseudomonas aeruginosa were detected by the broth microdilution method.The representative isolates of multidrug resistant (Isolate 8,MIC =8 μg/mL) and non-multidrug resistant (Isolate 12,MIC =8 μg/mL) Pseudomonas aeruginosa were selected,and the bactericidal kinetics of 4 × MIC concentration of lycosin-Ⅰ against them were determined.After the representative isolates were cultured with suitable medium for 24 hours,the absorbency of 600 nm was detected and the growth curve was drawn.Five mmol/L of Ca2+ or Mg2+ were added into the medium to investigate the salt tolerance of lycosin-Ⅰ.Results Lycosin-Ⅰ in vitro showed good antibacterial activity against both multidrug resistant and non-multidrug resistant Pseudomonas aeruginosa.The MICs (median [P25,P75]) of lycosin-Ⅰ against multidrug-resistant and non-multidrug-resistant Pseudomonas aeruginosa were 12 (8,32) μg/mL,and there was no statistical difference between them (U =42,P ＞0.05).About 50％ of multidrug resistant and non-multidrug resistant Pseudomonas aeruginosa could be killed by 4 × MIC of lycosin-Ⅰ during 60 minutes.When the concentration of lycosin-Ⅰ below MIC,such as 0,2 or 4 μg/mL,was co-cultured with multidrug resistant and non-multidrug resistant Pseudomonas aeruginosa for 24 hours,the growth of bacteria was fast during 6 and 16 hours,and then tended to be slow after 18 hours.When the concentration of lycosin-Ⅰ increased to 8 μg/mL,the bacteria were hard to grow.The in vitro antibacterial activity of lycosin-Ⅰ could be reduced by the addition of 5 mmol/L of Ca2+ or Mg2+,and the MICs of lycosin-Ⅰ against multidrug resistant and non-multidrug resistant Pseudomonas aeruginosa increased from 8 μg/mL to 64 μg/mL for 5 mmol/L of Ca2+,and from 8 μg/mL to 32 μg/mL for 5 mmol/L of Mg2+,respectively.But high concentrations of lycosin-Ⅰ (＞ 64 μg/mL or ＞ 32 μg/mL) could still maintain its antibacterial activity against multidrug resistant and non-multidrug resistant Pseudomonas aeruginosa.Conclusion Lyosin-Ⅰ can effectively inhibit the growth of Pseudomonas aeruginosa in vitro,and has certain salt tolerance,which may be developed into a new type of antibacterial drug.