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. flexne
ri 1b, invaded Caco-2 cells in vitro. When the strain was treated with subM
IC of EM, the invasion efficiency decreased. The carrier rate of the invasi
on plasmid containing virulence genes was reduced by EM treatment, as deter
mined 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 t
he finding that the MIC of EM for a virulent isolate of S. flexneri 2a YSH6
000 (6.35 mu g ml(-1)) and for the mutant strain del-17 (50 mu g ml(-1)), c
arrying 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 Microbiologic
al Societies. Published by Elsevier Science B.V. All rights reserved.