Target for bacteriostatic and bactericidal activities of beta-lactam antibiotics against Escherichia coli resides in different penicillin-binding proteins

The relationship between cell-killing kinetics and penicillin-binding protein (PBP) saturation has been evaluated in the permeability mutant Escherichia coli DC2 in which the antimicrobial activity of beta-lactams has been described as being directly related to the extent of saturation of the PBP target(s). Saturation of a single PBP by cefsulodin (PBP 1s), mecillinam (PBP 2), and aztreonam (PBP 3) resulted in a slow rate of killing (2.5-, 1.5-, and 0.8-log-unit decreases in the number of CFU per milliliter, respectively, in 6 h). Saturation of two of the three essential PBPs resulted in a marked increase in the rate of killing, which reached the maximum value when PBPs 1s and 2 were simultaneously saturated by a combination of cefsulodin and mecillinam (4.7-log-unit decrease in the number of CFU per milliliter in 6 h). Inactivation of all three essential PBPs by the combination of cefsulodin, mecillinam, and aztreonam further increased the killing kinetics (5.5-log-unit decrease in the number of CFU per milliliter), and this was not significantly changed upon additional saturation of the nonessential PBPs 5 and 6 by cefoxitin. Similar relationships between PBP saturation and killing kinetics were obtained with imipenem and meropenem at concentrations which inhibited only one PBP (PBP 2), only two PBPs (PBP 1s and 2), or all three essential PBPs. Saturation of one or more PBPs also resulted in a different rate of bacteriolysis, the highest rate being obtained by the cefsulodin-mecillinam combination and by 5 micrograms of either imipenem or meropenem per ml. All of these conditions caused saturation of PBP 2 and saturation or extensive binding of PBP 1s. However, none of these conditions caused determined the fastest possible rate of killing, which occurred only when all three essential PBPs were saturated. It was concluded that the actual killing effect of beta-lactams is reflected by killing rates that approach the fastest possible rate for the given microorganism and that the targets for the bactericidal activity are precisely those PBPs whose saturation or binding occurs under conditions.