Small RNA Regulation of Virulence in Pathogenic Escherichia coli

New Perspectives on Crosstalks Between Bacterial Regulatory RNAs from Outer Membrane Vesicles and Eukaryotic Cells

Regulatory small RNAs (sRNAs) help the bacteria to survive harsh environmental conditions by posttranscriptional regulation of genes involved in various biological pathways including stress responses, homeostasis, and virulence. These sRNAs can be found carried by different membrane-bound vesicles like extracellular vesicles (EVs), membrane vesicles (MVs), or outer membrane vesicles (OMVs). OMVs provide myriad functions in bacterial cells including carrying a cargo of proteins, lipids, and nucleic acids including sRNAs. A few interesting studies have shown that these sRNAs can be transported to the host cell by membrane vesicles and can regulate the host immune system. Although there is evidence that sRNAs can be exported to host cells and sometimes can even cross the blood-brain barrier, the exact mechanism is still unknown. In this review, we investigated the new techniques implemented in various studies, to elucidate the crosstalks between bacterial cells and human immune systems by membrane vesicles carrying bacterial regulatory sRNAs.

The phosphate-induced small RNA EsrL promotes E. coli virulence, biofilm formation, and intestinal colonization

Escherichia coli are part of the normal intestinal microbiome, but some enterohemorrhagic E. coli (EHEC) and enteropathogenic E. coli (EPEC) strains can cause potentially life-threatening gastroenteritis. Virulence factors underlying the ability of EHEC and EPEC to cause disease include those encoded in the locus of the enterocyte effacement (LEE) pathogenicity island. Here, we demonstrated that EsrL, a small RNA present in many E. coli strains, promoted pathogenicity, adhesion, and biofilm formation in EHEC and EPEC. PhoB, the response regulator of the two-component system that controls cellular responses to phosphate, directly repressed esrL expression under low-phosphate conditions. A phosphate-rich environment, similar to that of the human intestine, relieved PhoB-mediated repression of esrL. EsrL interacted with and stabilized the LEE-encoded regulator (ler) transcript, which encodes a transcription factor for LEE genes, leading to increased bacterial adhesion to cultured cells and colonization of the rabbit colon. EsrL also bound to and stabilized the fimC transcript, which encodes a chaperone that is required for the assembly of type 1 pili, resulting in enhanced cell adhesion in pathogenic E. coli and enhanced biofilm formation in pathogenic and nonpathogenic E. coli. Our findings demonstrate that EsrL stimulates the expression of virulence genes in both EHEC and EPEC under phosphate-rich conditions, thus promoting the pathogenicity of EHEC and EPEC in the nutrient-rich gut environment.

The stationary phase-specific sRNA FimR2 is a multifunctional regulator of bacterial motility, biofilm formation and virulence

Bacterial pathogens employ a plethora of virulence factors for host invasion, and their use is tightly regulated to maximize infection efficiency and manage resources in a nutrient-limited environment. Here we show that during Escherichia coli stationary phase the 3' UTR-derived small non-coding RNA FimR2 regulates fimbrial and flagellar biosynthesis at the post-transcriptional level, leading to biofilm formation as the dominant mode of survival under conditions of nutrient depletion. FimR2 interacts with the translational regulator CsrA, antagonizing its functions and firmly tightening control over motility and biofilm formation. Generated through RNase E cleavage, FimR2 regulates stationary phase biology by fine-tuning target mRNA levels independently of the chaperones Hfq and ProQ. The Salmonella enterica orthologue of FimR2 induces effector protein secretion by the type III secretion system and stimulates infection, thus linking the sRNA to virulence. This work reveals the importance of bacterial sRNAs in modulating various aspects of bacterial physiology including stationary phase and virulence.

Prokaryotic ncRNAs: Master regulators of gene expression

ncRNA plays a very pivotal role in various biological activities ranging from gene regulation to controlling important developmental networks. It is imperative to note that this small molecule is not only present in all three domains of cellular life, but is an important modulator of gene regulation too in all these domains. In this review, we discussed various aspects of ncRNA biology, especially their role in bacteria. The last two decades of scientific research have proved that this molecule plays an important role in the modulation of various regulatory pathways in bacteria including the adaptive immune system and gene regulation. It is also very surprising to note that this small molecule is also employed in various processes related to the pathogenicity of virulent microorganisms.