Effect of erythromycin on Shigella infection of Caco-2 cells

Erythromycin (EM), one of the macrolides, shows a dose-dependent effect on Shigella flexneri invasion of Caco-2 cells even at concentrations less than the minimum inhibitory concentration (subMIC). LS13, a strain of S. flexneri 1b, invaded Caco-2 cells in vitro. When the strain was treated with subMIC of EM, the invasion efficiency decreased. The carrier rate of the invasion plasmid containing virulence genes was reduced by EM treatment, as determined by the colony pigmentation test on Congo red agar plates. Presence of the invasion plasmid was found to increase susceptibility of the organisms to EM, The growth of virulent organisms carrying the invasion plasmid was inhibited at 25 mu g ml(-1) of EM, whereas the growth of organisms without the plasmid was inhibited at 100 mu g ml(-1) of EM. This was supported by the finding that the MIC of EM for a virulent isolate of S. flexneri 2a YSH6000 (6.35 mu g ml(-1)) and for the mutant strain del-17 (50 mu g ml(-1)), carrying the type III apparatus, impaired plasmid. These findings suggested that EM passed through the type III apparatus and suppressed the growth of invasive organisms selectively. This mechanism may account for the clinical effect of EM on shigellosis. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.