Examples of RNA-mediated regulation implicated in adaptation to the host environment in intracellular bacteria. (A) The regulatory cascade comprising CsrA and two sRNAs (RsmY and RsmZ) regulates the expression of secreted effectors of L. pneumophila that are released into the host cell cytosol. These effectors manipulate different processes of the secretory pathway to help in establishing the Legionella-containing vacuole (LVC). (B) The trans-encoded regulatory sRNAs RfrA and RfrB of Salmonella enterica serovar Typhi play a central role in iron homeostasis regulation, and are implicated in protecting the bacteria against oxidative stress, bactericidal antibiotics and acid resistance when replicating in the host cell. (C) Listeria monocytogenes virulence genes are under the control or PrfA, the master regulator of virulence gene expression. A thermosensor present in the 5^′ UTR of prfA senses the variation of the temperature when L. monocytogenes changes from the extracellular environment (lower temperature) to invade a mammalian host cell (37°C). The increase in the temperature changes its conformation, freeing the ribosome-binding site, which is in turn allows the translation of PrfA and subsequent virulence gene expression necessary for invasion of eukaryotic cells and the rapid escape from the vacuole to the cytosol.