Enterotoxigenic E. coli virulence gene regulation in human infections

Abstract

Enterotoxigenic (ETEC) is a global diarrheal pathogen that utilizes adhesins and secreted enterotoxins to cause disease in mammalian hosts. Decades of research on virulence factor regulation in ETEC has revealed a variety of environmental factors that influence gene expression, including bile, pH, bicarbonate, osmolarity, and glucose. However, other hallmarks of the intestinal tract, such as low oxygen availability, have not been examined. Further, determining how ETEC integrates these signals in the complex host environment is challenging. To address this, we characterized ETEC's response to the human host using samples from a controlled human infection model. We found ETEC senses environmental oxygen to globally influence virulence factor expression via the oxygen-sensitive transcriptional regulator fumarate and nitrate reduction (FNR) regulator. In vitro anaerobic growth replicates the in vivo virulence factor expression profile, and deletion of in ETEC strain H10407 results in a significant increase in expression of all classical virulence factors, including the colonization factor antigen I (CFA/I) adhesin operon and both heat-stable and heat-labile enterotoxins. These data depict a model of ETEC infection where FNR activity can globally influence virulence gene expression, and therefore proximity to the oxygenated zone bordering intestinal epithelial cells likely influences ETEC virulence gene expression in vivo. Outside of the host, ETEC biofilms are associated with seasonal ETEC epidemics, and we find FNR is a regulator of biofilm production. Together these data suggest FNR-dependent oxygen sensing in ETEC has implications for human infection inside and outside of the host.

The sRNA DicF integrates oxygen sensing to enhance enterohemorrhagicEscherichia colivirulence via distinctive RNA control mechanisms

“…Although oxygen is appreciated as an environmental signal that modulates EHEC virulence (45,46), the underlying mechanisms are not fully understood, and the role of DicF in EHEC physiology and virulence was unknown. In addition to EHEC, other bacterial pathogens sense oxygen to coordinate virulence, including Shigella, enterotoxigenic E. coli, and Salmonella (47)(48)(49). In these examples, transcriptional adaptation through the regulatory factors FNR or ArcAB mediates changes in gene expression, including expression of sRNAs that modulate virulence (48).…”

Here, there, and everywhere: How pathogenicEscherichia colisense and respond to gastrointestinal biogeography

“…Under aerobic growth conditions, EHEC has been shown to increase expression of LEE genes as compared with anaerobic conditions, as regulated by transcription factors Cra, KdpE, and FusR (Carlson‐Banning & Sperandio, ). A recent study of virulence regulation in related pathogen, enterotoxigenic E. coli (ETEC), in a human model of infection revealed environmental oxygen as a trigger for virulence gene expression via the transcriptional regulator FNR (fumarate and nitrate reduction), which is oxygen sensitive (Crofts et al, ). FNR may be important for repressing virulence in the low oxygen lumen, and activating virulence in response to increasing oxygen gradients, signalling proximity to the epithelial layer which can seep oxygen, as demonstrated in Shigella pathogenesis (Marteyn et al, ).…”

Pathogens’ adaptation to the human host

“…ETEC's biofilms are more robust under anaerobiosis, leading to FNR regulation of biofilm-promoting genes. FNR acts by limiting biofilm formation under anaerobiosis (5). It is quite striking that one transcription factor dictates a pathogen's virulence during infection and contributes to the environmental reservoirs that lead to outbreaks.…”

“…several environmental cues (including bicarbonate, pH, bile, osmolality, and glucose) have been reported to regulate ETEC virulence gene expression (4), oxygen tension, which is a major feature of the gastrointestinal tract (GI), has not been examined in the context of ETEC pathogenesis. As described in PNAS, Crofts et al (5) conducted a controlled human infection study with strain H10407 and performed RNA sequencing from the stools of these volunteers to probe the patterns of ETEC gene expression within the human GI tract.…”

“…Moreover, the oxygen-sensing transcription factor fumarate nitrate reduction regulator (FNR) (16) is responsible for this regulation. Under anaerobic conditions, FNR directly binds to the regulatory regions of the genes encoding CFA, LT, ST1, and ST2 to repress their expression (5). Under aerobic conditions, the presence of oxygen inactivates FNR (16,17), relieving the repression of virulence gene expression in ETEC (5).…”