Comparison of antimicrobial susceptibility between invasive and non-invasive Shigella organisms

Antimicrobial susceptibility was determined for ten strains of Shigella spp. comparing invasive (invasion plasmid containing) and non-invasive members of each strain. The activity of the antimicrobial agents could be classified into three types from the differences between the minimum inhibitory concentration (MIC) for the invasive and non-invasive shigellae. For type 1, there was no difference between the MIC (an MIC gap) for invasive and non-invasive organisms. For type 2, the MIC for the invasive organisms of a strain was higher than that of non-invasive organisms of the strain. In the third type, macrolides taken in by shigellae through the type III secretion apparatus, more effectively inhibited the growth of invasive than non-invasive organisms.