Investigation and Alteration of Organic Acid Synthesis Pathways in the Mammalian Gut Symbiont Bacteroides thetaiotaomicron.

Members of the gut-dwelling Bacteroides genus have remarkable abilities in degrading a diverse set of fiber polysaccharide structures, most of which are found in the mammalian diet. As part of their metabolism, they convert these fibers to organic acids that can in turn provide energy to their host. While many studies have identified and characterized the genes and corresponding proteins involved in polysaccharide degradation, relatively little is known about Bacteroides genes involved in downstream metabolic pathways. Bacteroides thetaiotaomicron is one of the most studied species from the genus and is representative of this group in producing multiple organic acids as part of its metabolism. We focused here on several organic acid synthesis pathways in B. thetaiotaomicron, including those involved in formate, lactate, propionate, and acetate production. We identified potential genes involved in each pathway and characterized these through gene deletions coupled to growth assays and organic acid quantification. In addition, we developed and employed a Golden Gate-compatible plasmid system to simplify alteration of native gene expression levels. Our work both validates and contradicts previous bioinformatic gene annotations, and we develop a model on which to base future efforts. A clearer understanding of Bacteroides metabolic pathways can inform and facilitate efforts to employ these bacteria for improved human health or other utilization strategies. IMPORTANCE Both humans and animals host a large community of bacteria and other microorganisms in their gastrointestinal tracts. This community breaks down dietary fiber and produces organic acids that are used as an energy source by the body and can also help the host resist infection by various pathogens. While the Bacteroides genus is one of the most common in the gut microbiota, it is only distantly related to bacteria with well-characterized metabolic pathways and it is therefore unclear whether research insights on organic acid production in those species can also be directly applied to the Bacteroides. By investigating multiple genetic pathways for organic acid production in Bacteroides thetaiotaomicron, we provide a basis for deeper understanding of these pathways. The work further enables greater understanding of Bacteroides-host relationships, as well as inter-species relationships in the microbiota, which are of importance for both human and animal gut health.

A child with recurrent otitis media due to Bacteroides thetaiotaomicron.

Brain abscess remains a life-threatening condition. Here, we are reporting a case of brain abscess due to Bacteroides thetaiotaomicron in a previously known case of recurrent otitis media. A 15 years old boy with a history of recurrent otitis media presented with the complaints of right otalgia, headache and fever. Computed Tomography (CT) brain and neck revealed fluid filled right middle ear cavity with bony destruction along the inner cortex of right temporal bone. The abscess was drained and culture showed growth of Bacteroides thetaiotaomicron. This report illustrates the importance of MALDI-TOF MS in the species level identification of anaerobes thereby facilitating the selection of appropriate and prompt adjuvant antibiotic therapy. This timely identification thus led to a favourable outcome in an era of increasing antimicrobial resistance.

A selective gut bacterial bile salt hydrolase alters host metabolism.

The human gut microbiota impacts host metabolism and has been implicated in the pathophysiology of obesity and metabolic syndromes. However, defining the roles of specific microbial activities and metabolites on host phenotypes has proven challenging due to the complexity of the microbiome-host ecosystem. Here, we identify strains from the abundant gut bacterial phylum Bacteroidetes that display selective bile salt hydrolase (BSH) activity. Using isogenic strains of wild-type and BSH-deleted Bacteroides thetaiotaomicron, we selectively modulated the levels of the bile acid tauro-ß-muricholic acid in monocolonized gnotobiotic mice. B. thetaiotaomicron BSH mutant-colonized mice displayed altered metabolism, including reduced weight gain and respiratory exchange ratios, as well as transcriptional changes in metabolic, circadian rhythm, and immune pathways in the gut and liver. Our results demonstrate that metabolites generated by a single microbial gene and enzymatic activity can profoundly alter host metabolism and gene expression at local and organism-level scales.

Frogs host faecal bacteria typically associated with humans.

Tree frogs commonly access drinking water tanks; this may have human health implications. Although amphibians might not be expected to host mammalian faecal indicator bacteria (FIB), it is possible that they may have human FIB on their skin after exposure to human waste. We collected faeces and skin wash from green tree frogs (Litoria caerulea) from a natural environment, a suburban site, and a suburban site near a creek occasionally contaminated with sewage effluent. We used molecular techniques to test for FIB that are routinely used to indicate human faecal contamination. Enterococci colonies were isolated from both faecal and skin wash samples, and specific markers (Enterococcus faecium and Bacteroides thetaiotaomicron) were found in frog faeces, demonstrating that these markers are not human- or mammalian-specific. Bacteroides thetaiotaomicron was detected in frogs from both natural and urban sites, but E. faecium was only associated with the sewage impacted site.