Spatio-temporal regulation of virulence by oxygen . The concentration of oxygen is low in the intestinal lumen but increases adjacent to the mucosal surface. A. In S. flexneri microaerobic condition of the lumen activates FNR [FNR(a)] which in turn represses Ipa secretion. Extended T3SS needles are formed. As the bacteria moves to the zone of higher oxygen concentration adjacent to the epithelial layer, FNR is inactivated [FNR(ia)] and T3SS effector genes are induced. B. In V. cholerae, the tcpA gene, but not ctxAB, is induced under the microaerobic condition of the lumen, when the bacteria reach the epithelial layer, the relatively higher oxygen concentration allows expression of both tcpA and ctxAB.