The Short-Chain Fatty Acids Propionate and Butyrate Augment Adherent-Invasive Escherichia coli Virulence but Repress Inflammation in a Human Intestinal Enteroid Model of Infection.

Short-chain fatty acids (SCFAs), which consist of six or fewer carbons, are fermentation products of the bacterial community that inhabits the intestine. Due to an immunosuppressive effect on intestinal tissue, they have been touted as a therapeutic for inflammatory conditions of the bowel. Here, we study the impact of acetate, propionate, and butyrate, the three most abundant SCFAs in the intestine, on gene expression in the intestinal pathobiont adherent-invasive Escherichia coli. We pair this with adherence, invasion, and inflammation in Caco-2 and human intestinal enteroid (HIE)-derived monolayer models of the intestinal epithelium. We report that propionate and butyrate upregulate transcription of adherent-invasive Escherichia coli (AIEC) flagellar synthesis genes and decrease expression of capsule assembly and transport genes. These changes are predicted to augment AIEC invasiveness. In fact, SCFA supplementation increases AIEC adherence to and invasion of the Caco-2 monolayer but has no effect on these parameters in the HIE model. We attribute this to the anti-inflammatory effect of propionate and butyrate on HIEs but not on Caco-2 cells. We conclude that the potential of SCFAs to increase the virulence of intestinal pathogens should be considered in their use as anti-inflammatory agents. IMPORTANCE The human terminal ileum and colon are colonized by a community of microbes known as the microbiota. Short-chain fatty acids (SCFAs) excreted by bacterial members of the microbiota define the intestinal environment. These constitute an important line of communication within the microbiota and between the microbiota and the host epithelium. In inflammatory conditions of the bowel, SCFAs are often low and there is a preponderance of a conditionally virulent bacterium termed adherent-invasive Escherichia coli (AIEC). A connection between SCFA abundance and AIEC has been suggested. Here, we study AIEC in monoculture and in coculture with human intestinal enteroid-derived monolayers and show that the SCFAs propionate and butyrate increase expression of AIEC virulence genes while concurrently bolstering the intestinal epithelial barrier and reducing intestinal inflammation. While these SCFAs have been promoted as a therapy for inflammatory bowel conditions, our findings demonstrate that their effect on bacterial virulence must be considered.

Role for glycine betaine transport in Vibrio cholerae osmoadaptation and biofilm formation within microbial communities.

Vibrio cholerae is a halophilic facultative human pathogen found in marine and estuarine environments. Accumulation of compatible solutes is important for growth of V. cholerae at NaCl concentrations greater than 250 mM. We have identified and characterized two compatible solute transporters, OpuD and PutP, that are involved in uptake of glycine betaine and proline by V. cholerae. V. cholerae does not, however, possess the bet genes, suggesting that it is unable to synthesize glycine betaine. In contrast, many Vibrio species are able to synthesize glycine betaine from choline. It has been shown that many bacteria not only synthesize but also secrete glycine betaine. We hypothesized that sharing of compatible solutes might be a mechanism for cooperativity in microbial communities. In fact, we have demonstrated that, in high-osmolarity medium, V. cholerae growth and biofilm development are enhanced by supplementation with either glycine betaine or spent media from other bacterial species. Thus, we propose that compatible solutes provided by other microorganisms may contribute to survival of V. cholerae in the marine environment through facilitation of osmoadaptation and biofilm development.

Role of ectoine in Vibrio cholerae osmoadaptation.

Vibrio cholerae is both an intestinal pathogen and a microbe in the estuarine community. To persist in the estuarine environment, V. cholerae must adjust to changes in ionic composition and osmolarity. These changes in the aquatic environment have been correlated with cholera epidemics. In this work, we study the response of V. cholerae to increases in environmental osmolarity. Optimal growth of V. cholerae in minimal medium requires supplementation with 200 mM NaCl and KCl. However, when the NaCl concentration is increased beyond 200 mM, a proportionate delay in growth is observed. During this delay in growth, osmotic equilibrium is reached by cytoplasmic accumulation of small, uncharged solutes that are compatible with growth. We show that synthesis of the compatible solute ectoine and transport of the compatible solute glycine betaine impact the length of the osmoadaptive growth delay. We also demonstrate that high-osmolarity-adapted V. cholerae displays a growth advantage when competed against unadapted cells in high-osmolarity medium. In contrast, low-osmolarity-adapted V. cholerae displays no growth advantage when competed against high-osmolarity-adapted cells in low-osmolarity medium. These results may have implications for V. cholerae population dynamics when seawater and freshwater and their attendant microbes mix.