During obligatory bacteriological examinations of persons occupied in food industry or distribution, we isolated from the stool specimens of 3 persons of a meat factory strains of Salmonella anatum resistant to antibiotics.Two atrains, S. anatum 5915 and 8. anatum 6100, respectively, were found to be resistant to tetracyclines (T) only, while the third strain showed an unusual type of resistance to colistine (colimycin).Our Antibiotic Reference Laboratory has just started a thorough examination of resistant Salmonella strains in Czechoslovakia for the presence of R-factors.Both T-resistent strains were therefore tested, in mixed cultures, with Escherichia coli K 12 185 resistant to nalidixic acid (Nx), for t&e transfer of their T-monoresistance. The procedure we are using for the detection of R-factors and for following up their further transfers, was already described (KRCMERY and J A N O U~K O V~ 1969, FRI~DERICQ et al. 1971). Briefly, 2 ml of 6 hrs. shaken cultures of the donor strains in nutrient broth (DIFCO) were mixed with equal volumes of 6 hrs. broth cultures of E. coli K 12 185 Nx, which is susceptible to T, but has a chromosomally located high-level resistance to Nx. The strains of S. anatum tested were all highly sensitive to Nx as seen in control plating on MACCONKEY Agar (DIFCO) containing 0.5 to 10.0 pglml of Nx. After overnight incubation, the mixture was diluted to lo-' in saline, and 0.01 ml of each dilution was plated on a segment of MACCONKEY Agar containing 20 pg/ml of T and the same amount of Nx. The plates were incubated 20 hrs. a t 37 "C.The growth with a considerable frequency on MACCONKEY Agar with T and Nx of E. coli K 12 185 Nx after the mixed incubation with 8. anaturn strain 5915suggests the transferable nature of the T-determinant in that Salmonella strain.The T-determinant of S.anatum 6100 was not transferable in the experimental system used, which, however, does not exclude its transferability t o another recipient strain. The frequencies of transfer are given as the fraction of resistant colonies of the recipient strain out of the total number of recipient cells, which was about 2.8 x lo8 cells/ml. I n the above instance, about los cells/ml of the recipient strain, present in the mixture with S. anatum 5915, received the Tdeterminant. The level of acquired T-resistance in the colonies of strain E. coli K 12 185 Nx on MACCONKEY Agar was between 50 and 75 pg/ml of T.The transferability of this T-determinant was reconfirmed by its further transfer from E. coli K 12 185 Nx T to S. typhimurium SH, susceptible to T
