Activity of colistin against two induced multidrug-resistant Gram-negative bacteria in vitro

To evaluate the antibacterial activity of colistin on the multidrug-resistant Pseudomonas aeruginosa and Escherichia coli strains occurred frequently in veterinary clinic,minimal inhibitory concentration(MIC) and minimum bactericidal concentration(MBC) to the two induced multidrug-resistant strains were determined by broth microdilution method. Time-kill curves were prepared according to colony count. Results showed the MIC value of colistin to E.coli and P.aeruginosa was 0.25 and 2μg/mL,and the MBC value was 0.5 and 2μg/mL,respectively.At the concentration of 0.5×,2×,4×,8× and 16×MIC,the antibacterial activity of colistin concentration showed concentration dependent to E.coli,and partly concentration-dependent to P.aeruginosa.At the concentration of 2×MIC,the concentration of E.coli and P.aeruginosa of multidrug resistant strains decreased significantly in 2h. At the concentration of 4×MIC,all of the bacteria can be killed within 8h(E.coli),4h(P.aeruginosa). Results indicated that colistin exerted strong and prompt bactericidal effect on the multidrug-resistant P.aeruginosa and E.coli strains.