The Escherichia coli chromosome contains several operons encoding confirmed and predicted multidrug transporters. Among these transporters only the inactivation of components of the AcrAB-TolC complex leads to substantial changes in susceptibility to multiple drugs. This observation prompted a conclusion that other transporters are silent or expressed at levels insufficient to contribute to multidrug resistance phenotype. We found that increased expression of AcrA, the periplasmic membrane fusion protein, is toxic only in cells lacking the multidrug efflux transporter AcrEF. AcrEF-deficient cells with increased expression of AcrA have a severe cell division defect that results in cell filamentation (>50 microm). Similar defects were obtained in cells lacking the outer membrane channel TolC, which acts with AcrEF, suggesting that cell filamentation is caused by the loss of AcrEF function. Green fluorescent protein-AcrA fusion studies showed that in normal and filamentous cells AcrA is associated with membranes in a confined manner and that this localization is not affected by the lack of AcrEF. Similarly, the structure and composition of membranes were normal in filamentous cells. Fluorescence microscopy showed that the filamentous AcrEF-deficient E. coli cells are defective in chromosome condensation and segregation. Our results suggest that the E. coli AcrEF transporter is expressed under standard laboratory conditions and plays an important role in the normal maintenance of cell division.