Effects of Sub minimal Inhibitory Concentrations of Ampicillin on Hemagglutination of Escherichia coli

The hemagglutination (HA) activity of Escherichia coli was enhanced by subminimal inhibitory concentrations (sub-MICs) of ampicillin. One half of the MIC of ampicillin caused a bacterial filamentation and diminished bacterial piliation (as observed by light and electron microscopies) as well as an increase of HA activity. HA activity, however, decreased after separation of ampicillin-treated bacteria. These results indicate that the increase in HA activity by ampicillin is mainly due to filament formation.