Action of Polymyxin of Cholera Vibrios. Techniques of Determination of Polymyxin-Sensitivity

Summary

A comparative study has been made of 4 methods, namely, disc, well, plate and turbidimetric methods for testing polymyxin-sensitivity of the choleragenic vibrios. The plate method using 15 μg/ml of polymyxin B in agar medium is recommended as the method of choice for routine screening tests for differentiating V. cholerae and V. el tor strains. By this method all the 502 strains of classical cholera vibrios tested were found sensitive and 99.5% of the 544 strains of El Tor vibrios completely or partially resistant to polymyxin B. With the turbidimetric method, resting cells of both V. cholerae and V. el tor showed evidence of progressive lysis in the presence of polymyxin. But when vibrios were tested in their growing phase V. cholerae strains appeared sensitive to polymyxin while the V. el tor cultures proved resistant. El Tor strains, however, showed an initial phase of growth inhibition before their normal growth pattern was restored.