Iron-regulated outer membrane proteins of Escherichia coli K-12 and mechanism of action of catechol-substituted cephalosporins.

Selected aminothiazolyl-oxime cephalosporin congeners substituted at C-3' with a catechol moiety were used to probe the basis of the enhanced antibacterial activity against Escherichia coli K-12 often associated with chemical modifications of this type. Evidence is presented for a tonB-dependent illicit transport of the compounds across the outer membrane of E. coli K-12, the process involving jointly and specifically the Fiu and Cir iron-regulated outer membrane proteins. Thus, both tonB and fiu cir mutants showed a comparably reduced susceptibility to the probe compounds, whereas mutants singularly lacking any one of the six iron-regulated outer membrane proteins (Fiu, FepA, FecA, FhuA, FhuE, and Cir) or lacking any combination of any two of these proteins (except Fiu plus Cir) did not show this resistance. Mutants devoid of all six iron-regulated outer membrane proteins were no more resistant to the probe compounds than fiu cir or tonB strains. In addition to the latter genes, the products of the exbB and possibly the exbC loci were necessary for maximal antibacterial potency. A dependence of antibacterial activity on the level of expression of the uptake system components was noted. Comparison of penicillin-binding protein target affinity with antibacterial activity suggested a possible periplasmic accumulation of active compounds by E. coli K-12. Free vicinal hydroxyl groups of the catechol residue were a primary chemical requirement for recognition by the uptake pathway and thus for high antibacterial activity.

Properties of spontaneous Enterobacter cloacae mutants with temperature-conditional derepression of type I beta-lactamase synthesis.

Spontaneous mutants, with temperature-conditional derepression of chromosomally-encoded Type I beta-lactamase synthesis, were derived from two independent clinical isolates of Enterobacter cloacae. At the permissive temperature (28 degrees C) the mutants' beta-lactamase activity was equivalent to that of their respective parents but at restrictive temperatures (above 40 degrees C) the activity increased many hundred-fold. The increased beta-lactamase expression correlated with reduced beta-lactam susceptibility. In temperature shift-up experiments, the initial rate of beta-lactamase synthesis closely paralleled that of the parent strains induced with cefoxitin. Maximal beta-lactamase activity in the mutants was attained after about 3 h growth at restrictive temperatures and was significantly higher than that of the cefoxitin-induced parents. However, the level was not as high as that observed in isogenic temperature-stable derepressed mutants, under the same conditions. All temperature-conditional mutants showed hyper-induction of beta-lactamase synthesis at permissive temperatures. Our findings are discussed in relation to a positive control model for regulation of Type I beta-lactamase synthesis in Ent. cloacae.