(D) Microscopic examination of the corresponding strains under thiamine (repressing conditions) shows that, in most cases, the apical region of germlings is enlarged and growth is arrested. This morphological phenotype, taken as a strong indication of blocked secretion, is more evident in thiAp-sedV and thiAp-rabE but concerns all strains, except for thiAp-hypB. Scale bar: 5 μm. 