derived from a Shigella strain (33) and conferred on its host resistance to tetracycline, chloramphenicol, streptomycin, sulfanilamide, ampicillin, spectinomycmn, and mercuric ions.Media and drugs. L-broth, bromothymol blue (BTB) lactose agar, Penassay broth, and Penassay broth agar were used. BTB agar consisted of nutrient broth (10 g of beef extract, 10 g of peptone, and 3 g of NaCl in 1 liter of distilled water) supplemented with 1% lactose, 0.008%' BTB, and 1.5% agar. Selective agar medium for conjugal transfer of the R plasmid from ML1410 R+ to W3630 was AG-agar. AG-agar medium consisted of medium A (7) with 1% glucose, 0.008'% BTB, and 1.5% agar. The selective concentration of drugs used for resistant bacteria were 12.5 pg of tetracycline, 12.5 pg of chloramphenicol, 6 pg of strepto-578 on August 1, 2020 by guest