Contribution of the cell-surface-associated enzyme in the Zimmermann-Rosselet assay of outer membrane permeability of beta-lactam antibiotics.

Analysis of beta-lactam hydrolysis by intact cells of several strains of enteric bacteria suggests that a significant fraction of beta-lactamase molecules is present at a location directly accessible for the substrates in the medium. We propose a method to correct for the error introduced by these "surface-exposed" enzymes in the Zimmermann-Rosselet assay of outer membrane permeability.