%0 Journal Article %A Hechler, Ulrike %A Van Den Weghe, Mia %A Martin, Hans H. %A Frère, Jean-Marie %T Overproduced β-Lactamase and the Outer-membrane Barrier as Resistance Factors in Serratia marcescens Highly Resistant to β-Lactamase-stable β-Lactam Antibiotics %D 1989 %J Microbiology, %V 135 %N 5 %P 1275-1290 %@ 1465-2080 %R https://doi.org/10.1099/00221287-135-5-1275 %I Microbiology Society, %X In a clinical isolate of Serratia marcescens different states of low and high resistance to different β-lactam antibiotics considered to be β-lactamase-stable, viz. cefotaxime, ceftizoxime, ceftazidime, aztreonam, cefoxitin and imipenem, were found to be connected with the presence of constitutively overproduced, chromosomally encoded β-lactamase at concentrations in the bacterial periplasm of 0·4 and 0·9 mm, respectively. All the antibiotics were degraded by the β-lactamase. However, kinetic constants varied widely:K m from 92 to 0·012 µm, and k cat from 3·4 to 2 × 10−4 s−1. The relative contributions to resistance by the functioning of periplasmic β-lactamase, resynthesis of this enzyme, and limitation of antibiotic penetration by the bacterial outer membrane were analysed by computer simulations according to steady-state and non-steady-state models of interactions in the periplasm. Results for cefotaxime, ceftizoxime, ceftazidime, aztreonam and latamoxef revealed overproduced β-lactamase as the sole cause of the state of low resistance while antibiotic permeability was the same as in non-resistant S. marcescens strains. In contrast, high resistance was due to β-lactamase action and decreased permeability of antibiotics. For resistance to aztreonam, only, immobilization of the antibiotic as covalent acyl-enzyme by newly synthesized β-lactamase was essential. For cefoxitin, ampicillin and imipenem the analyses indicated that additional resistance factors may play a role, e.g. induction of β-lactamase. %U https://www.microbiologyresearch.org/content/journal/micro/10.1099/00221287-135-5-1275