β-Lactamases of R Factors Derived from Shigella and Salmonella Strains

R. factors conferring resistance to penicillins and cephalosporins were transferred by conjugation from six strains of Shigella and from four strains of Salmonella typhimurium to a standard strain of S. typhimurium LT2. The beta-lactamases produced were then characterized by using cell-free extracts. The enzymes were of two types, I and II, with respect to specific activity against benzyl penicillin, substrate profile, K(m), pH optimum, temperature optimum, inhibition by chloride and nitrate ions, and heat inactivation. The six type I enzymes were associated with R factors from Shigella strains; five of these were R(f) factors. The four type II enzymes were associated with R factors from S. typhimurium; all these were R(i) factors.