Linear Thanatin Is an Effective Antimicrobial Peptide against Colistin-Resistant Escherichia coli in Vitro

Colistin has been regarded as the last line antibiotic for treatment of infections caused by multidrug resistant gram-negative bacteria. Therefore, the increasing emergence of colistin resistance among gram-negative bacteria represents a serious problem. The objective of this study was to characterize the effectiveness of the chemically synthesized thanatin in linear form against colistin-resistant E. coli isolated from a pig farm in China. Agar diffusion assay and broth microdilution test were employed to analyze the susceptibility of colistin-sensitive E. coli (ATCC25922) and colistin-resistant E. coli (SHP45) to linear thanatin (L-thanatin). Combinatory effect of linear thanatin and colistin against E. coli was also determined by fractional inhibition concentration index (FICI) analysis. The results showed that L-thanatin at a concentration of 1 mg/ml produced larger inhibition zone on agar against ATCC25922 than SHP45. In the quantitative microdilution test, L-thanatin had the same MIC of 3.2 μg/ml for ATCC25922 and SHP45. Based on the FICI analysis, additive effect was obtained with 1.56 μg/ml of L-thanatin and 0.125 μg/ml of colistin for ATCC25922; but with 1.56 μg/ml of L-thanatin and 0.25 μg/ml of colistin or with 2 μg/ml of colistin and 0.39 μg/ml of L-thanatin for SHP45. These data proved that L-thanatin is an effective antimicrobial peptide against colistin-resistant E. coli.