Inhibition of Corynebacterium michiganense subsp. nebraskense by Certain Lots of Polymyxin Used in the Selective Medium CNS. M. L. Smidt, Department of Plant Pathology, University of Nebraska, Lincoln 68583-0722. A. K. Vidaver, Department of Plant Pathology, University of Nebraska, Lincoln 68583-0722. Plant Dis. 68:536. Accepted for publication 19 March 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-536c.
CNS, a selective medium for Corynebacterium michiganense subsp. nebraskense contains several antibiotics, LiCI, and a fungicide in addition to the components of nutrient broth-yeast extract (NBY) medium. Contrary to previous experience, in the summer of 1983 we observed that C. M. subsp. nebraskense isolates grew on CNS only en masse, not in single colonies. The problem could be alleviated by omitting either LiCl or polymyxin b-sulfate (PMBS) from the medium; lowering the concentration of either component up to 10-fold did not alter the toxicity of the medium. Further investigation showed that the toxic effects were caused by a particular lot of PMBS (No. 13F-0397, Sigma). When equal units of PMBS from three other Sigma lots or from Cal Biochem lot 202997 were used, single colonies grew as expected. The role of LiCl and the reason its omission alleviated the problem are unknown. We suggest that frequent users of CNS pretest each batch of medium for growth of single colonies of known C. M. subsp. nebraskense strains whenever new supplies of the antibiotics, LiCI, or fungicide are used.