[Effect of R-factors on the sensitivity of Shigella sonnei to nalidixic acid].

Bacteriostatic effect of nalidixic acid was studied by the method of serial dilutions in nutrient agar using 36 antibiotic sensitive strains of Shigella Sonnei and their conjugated substrains carrying separately the following R-factors: R1(TcCmSm) fi-, R2(TcCm) fi+, R3(CmSm) fi-. The above R-factors were isolated from clinical strains of Shigella Sonnei and transferred to the strains studied by means of an intermediate donor of E. coli MF-Nr. The bactericidal effect of nalidixic acid was studied on 9 antibiotic sensitive strains of Shigella Sonnei and their conjugated substrains carrying separately the following episomes: R1(TcCmSm) fi- and R2(TcCm) fi+. It was studied by determination of the number of alive bacteria 24, 48 and 72 hours after their interaction in Hottinger broth with nalidixic acid (400 gamma/ml). It was found that the R-factors did not change the sensitivity of Shigella Sonnei to the bacteriostatic effect of nalidixic acid: the MIC for the R- strains was 4+/- 0.14 gamma/ml, for the substrains with episomes R1(TcCmSm) fi-, R2(TcCm) fi+ and R3(CmSm) fi- it was 4.22 +/- 0.21, 3.88 +/- 0.25 and 4.16 +/- 0.27 gamma/ml respectively. The differences were not statistically reliable (P greater than 0.05) when estimated according to both Student criterion t and Wilcockson criterion for the wto-by-wto conjugated variants. The R-factors inreased the Shigella sensitivity to the bactericidal effect of nalidixic acid: the death rate of the R+-bacteria during the first day was 4--130 times higher than that of the two-by-two conjugated strains of the R-bacteria (P less than 0.01).