Antibiotics enhance binding by human lipid A-reactive monoclonal antibody HA-1A to smooth gram-negative bacteria

The effect of antibiotic exposure of phenotypically smooth gram-negative bacteria on binding by the human lipid A-reactive monoclonal antibody HA-1A (trademark of Centocor, Inc.) was examined by liquid-phase immunoassay and by dual-parameter flow cytometry (fluorescence-activated cell sorter [FACS]) analysis. HA-1A exhibited dose-dependent binding to untreated rough gram-negative bacteria such as the Escherichia coli D21F2 Re chemotype strain but little binding to untreated smooth strains such as E. coli O111:B4, or to gram-positive bacteria. However, overnight incubation of E. coli O111:B4 with inhibitory concentrations of ceftazidime produced dose-dependent enhancement of HA-1A binding. Similar augmentation of HA-1A binding was observed when other smooth strains were exposed to cell wall-active agents. Dual-parameter FACS analysis of E. coli O111:B4 exposed overnight to two times the MIC of ceftazidime revealed a decrease in forward light scatter, indicating a reduction in average cell size or bacterial fragmentation, accompanied by a striking increase in lipid A-inhibitable HA-1A binding. Moreover, ceftriaxone, but not gentamicin, produced a marked increase in propidium iodide uptake, indicating an increase in bacterial cell permeability, and a corresponding enhancement of HA-1A binding. Antibiotic-induced enhancement of HA-1A binding to smooth strains of gram-negative bacteria thus appears related to specific alterations in bacterial cell morphology resulting in exposure of the epitope recognized by HA-1A.