MECHANISM OF THE DEVELOPMENT OF RESISTANCE TO STREPTOMYCIN I

Abstract

A differential biol. procedure has been developed which demonstrates of the presence or absence of as few as one streptomycin-resistant cell in a large pop. of sensitive cells of Micrococcus pyogenes var. aureus. Attempts to obtain resistance to streptomycin failed in a glucose-free complete amino acid medium and in a medium containing glucose but deficient, for example, in aspartic acid or phenylalanine. Sensitive cells acquire resistance in complete amino acid medium containing glucose and streptomycin under aerobic conditions. The cells after acquiring resistance by contact with streptomycin are capable of growth under the foregoing deficient conditions. The blockage by streptomycin of the synthesis of phenylalanine and aspartic acid by sensitive cells blocks the emergence of streptomycin-resistant cells. In sensitive cells, the blockage of this synthesis is either antagonized or bypassed by the presence of these amino acids. In resistant cells, in the absence or presence of these amino acids, streptomycin is incapable of causing this blockage. Both the sensitive and resistant cells are capable of synthesizing these amino acids in the absence of streptomycin. After one contact with 10 [mu]g./ml. of streptomycin in complete medium, the cells show resistance to 100 [mu]g. and, after a 2d or 3d contact, to 2000 [mu]g./ml. of streptomycin. One 1-contact or one 3-contact streptomycin-resistant cell added to a large pop. of sensitive cells is capable of growth in the presence of streptomycin in a medium deficient in amino acids. Since no pop. or inoculum of sensitive cells used at various occasions and in multiple 95 tube systems was found to be capable of surviving 10 [mu]g. of streptomycin/ml. in the amino acid-deficient medium, it seems evident that there are no resistant cells at the beginning in a pop. of sensitive cells and that they arise in a complete medium from the sensitive cells after being acted upon by streptomycin.