Friends or Frenemies?

This is a second paper from Host Cell and Microbe describing how mouse gut residing commensal microbe's metabolic end-product, succinate, fuels the virulence of pathogenic Citrobacter rodentium (C. rodentium).

This study, led by Vanessa Sperandio at UT Southwestern Medical Center, Dallas, analysed virulence factor expression in mouse C. rodentium, in presence of mouse gut commensal Bacteroides thetaiotaomicron (Bt). The authors have used C. rodentium disease model that mimics E. coli infection in humans.

It appears that the authors' initial objectives were to study the mechanisms by which gut commensals were driving virulence factor expression in pathogenic microbes and only later diverted their attention towards the role of succinate metabolism in this process.

First, the authors showed that presence of Bt enhances expression of several virulence factors in E. coli and in an in vitro culture.  

Similar results were observed with C. rodentium  culture.

Interestingly, this enhancement was not observed in cra mutant E. coli that is incapable of sensing sugar fermentation products, like succinate (however, the authors did not show similar results with cra mutant C. rodentium).

In vivo experiments with either wild-type C. rodentium or mutant strains showed that reconstitution of antibiotic treated mice with Bt could enhance C. rodentium pathogenicity. The authors reported that severity of infection with cra mutant C. rodentium  was attenuated, though not abolished, it appears (implying that C. rodentium showed cra-succinate independent virulence).

Still, analysis of sugar fermentation products in the day 2 post infected mice cecum showed that there was selective increase in succinate in presence of Bt or C. rodentium.

Finally, addition of succinate to E. coli culture could enhance expression of virulence factors in the wild-type but not in a cra mutant E. coli strain (again, the authors did not show similar results with cra mutant C. rodentium).

In summary, this study and another one from Justin Sonnenburg's lab, highlighted the potential detrimental role of gut commensals in fueling the virulence of pathogenic microbes. We are thinking that the term gut commensal implies friendly microbe, however occasionally those friendly microbes could involuntarily provide support to pathogenic ones.

David Usharauli

Fueling C. difficile growth

C. difficile is a spore-forming anaerobe, residing in the human gut. Usually it does not cause any pathology. However, antibiotic use (or over-use), or changes in gut motility can bring up the ugly side of this microbe. There are no vaccines or drugs available for the specific management of C. difficili infection. 

Curiously, in the past few years, several research studies indicated that C. difficile could be controlled by other gut microbes readily available in the healthy humans' feces.

If proven effective in humans, this will open a new chapter in medicine. Our physiology is heavily influenced by gut commensals and their role in human health and disease has just started to be unraveled.

In this respect, two new studies in journal Cell Host and Microbe provided additional clues about potential mechanisms of initiation of C. difficile disease. I will review both of them separately.

One study, led by Justin Sonnenburg at Stanford University of School of Medicine, has examined how fermentation end product, succinate, affect C. difficile growth pattern.

Initially, the authors observed that in germ-free mouse, C. difficile growth was accelerated in the presence of polysaccharide rich-diet and mouse gut flora commensal Bacteriodes Thetaiotaomicron, (Bt).

It is known Bt produces organic acid, succinate, as a fermentation end product. 

Using succinate-transporter mutant C. difficile, the authors confirmed that in vitro culture, WT C. difficile were able to utilize excess succinate for their growth enhancement.

In vivo experiments with succinate supplemented diet validated this observation.

Importantly, the authors showed that Bt-induced growth promotion was not observed with mutant C. difficile.  

To make their observation clinically relevant, the authors conducted the series of experiments with mice treated with antibiotics or gut hyper-motility (diarrhetic) drug.

As predicted, antibiotic treatment specifically increased succinate level.

Similarly, diarrhetic agent mimicked antibiotic's effect.

Finally, the authors showed that changes in gut motility fueled C. difficile growth.  

In summary, these results indicate that changes in gut flora or acceleration of gut motility could lead to specific increase in succinate level that is sensed and utilized by C. difficile for growth enhancement.

David Usharauli